Redox titration is based on the use of different types of redox reactions.

Solutions of substances with oxidizing or reducing properties are used as titrants. In terms of its analytical characteristics, the method is similar to acid-base titration, although often the reaction rate is comparatively lower.

The principles for selecting indicators are also the same: the indicator should change color near the equivalence point. Organic substances that have weaker oxidizing or reducing properties than the reagents are used as indicators. The pH of the solution has a very strong influence on the performance of the indicator. The color transition interval of the redox indicator is determined by the formula:

ΔE = (Eº ind. – 0.059 m/n pH) ± 0.059/ n,

where m is the number of H +, n is the number of electrons participating in the half-reaction, Eº ind. – potential depending on the nature of the substance.

Redox indicators

Depending on the titrant used in the analysis, there are several variants of redox titration.

1. Permanganatometry uses potassium permanganate KMnO 4 as a titrant, which is a strong oxidizing agent.

This method is used to determine reducing agents: oxalic acid, Fe 2+, HNO 2, Mn 2+, Sn 2+, etc. (direct titration), some oxidizing agents: NO 3 -, K 2 Cr 2 O 7 (back titration), many metal cations (by titration of the substituent).
Typically titration is carried out in a strongly acidic sulfuric acid environment:
MnO 4 - + 8H + + 5e = Mn 2+ + 4H 2 O. For titration, a KMnO4 solution with a concentration of 0.02 to 0.1 mol/l is used. The titrant itself is used as an indicator, one excess drop of which turns the solution pink.

2. Iodometry uses a solution of iodine I 2 as a titrant, which is an oxidizing agent (this version of the method is called the iodometric method): I 2 + 2e =2I - , or KI solution, which is a reducing agent (iodometric method) 2I - - 2e = I 2.

The second option is the most common; it is used for the indirect determination of many KI-reducing substances. In this case, I 2 is released in an amount equivalent to the amount of the analyte. The released iodine is titrated with a standard solution of sodium thiosulfate Na 2 S 2 O 3

I 2 + 2 Na 2 S 2 O 3 = 2NaI + Na 2 S 4 O 6. In both analysis options, a starch solution is used as an indicator for iodine, which in the presence of free iodine acquires a blue color.

3. Chromatometry uses potassium bichromate K 2 Cr 2 O 7 as a titrant, which is an oxidizing agent: Cr 2 O 7 2- + 14H + + 6e = 2Cr 3+ + 7H 2 O.

Diphenylaminosulfonic acids can be indicators. The reaction is carried out in a strongly acidic environment, usually using sulfuric acid. This method determines Fe (II), Mn (II), Mn (IV), V (V), Mo (V), a number of anions, organic substances, etc.

Complexometric titration

In complexometric titration, substances capable of forming strong complexes with the analyte are used as titrants.

The most widely used in analytical practice is ethylenediaminetetraacetic acid - EDTA– (NOOC - CH 2) 2 – N- CH 2 – CH 2 – N – (CH 2 – COOH) 2 and its sodium salt - Trilon B.

This titrant is used primarily for the quantitative determination of metal cations (Fe 3+, Cr 3+, Ca 2+, Mg 2+, etc.). The EDTA molecule always reacts with 1 metal cation, i.e. the equivalence factor is 1.

Eriochrome black T, murexide and some other organic substances are often used as indicators.

The pH of the medium has a very strong influence on the results of complexometric titration, so the analysis is most often carried out in a buffer solution.

Precipitation titration

Precipitation titration is based on precipitation reactions that can occur at sufficient speed at low temperatures and are irreversible. Although there are quite a lot of such reactions, only a few were suitable for analysis.

A well-developed method of precipitation titration with silver nitrate AgNO 3 is argentometry. K 2 CrO 4 , FeCl 3 , and adsorption indicators are used as indicators in argentometric analysis.

The silver ion forms a fairly large amount of water-insoluble salts, which determines its analytical capabilities; Argentometry can be used to determine Cl -, Br -, I -, SCN -, AsO 4 3-, CO 3 2-, etc. However, the widespread use of the method is hampered by the high cost of silver nitrate.

Even less commonly used is the method of mercurymetry, in which very toxic mercury salts serve as titrants.

No. 10. Basic concepts: titrant, titration, equivalence point, end point of titration. Methods of analysis (direct, substitutive, reverse), methods of analysis (individual portions, aliquot samples (pipetting)).

Titrometry is the most widely used analysis method in analytical practice. In sensitivity it is close to gravimetry (detection limit 0.10%), in accuracy it is inferior to gravimetric analysis (accuracy 0.5%). However, it is much faster and easier to implement.

Titrimetric analysis is based on the precise measurement of the volume of the reagent solution that reacts with the analyzed component of the sample.

Titrant- a reagent solution with a precisely known concentration.

Titration- the process of gradually adding titrant to the solution being analyzed is called titration.

Equivalence point- the point of titration at which equality of equivalents of the reacted analyte and titrant is achieved. Before the equivalence point, there is practically no titrant in the solution, and after the equivalence point there is no analyte. Near the equivalence point, the properties of the system change sharply. This sudden change in the measured property of a solution near the equivalence point is called a titration jump.

End point of titration- the moment of titration when the end of the reaction is noticed by a change in the color of the indicator solution or other signs. Usually, by the end of the titration, the amount of titrant added is more or less than the equivalent amount. Titration will be more accurate the closer to the equivalence point the titration end point lies. The difference between the equivalence point and the end point of the titration causes the indicator error of the titration. Having reached the end point of the titration, the titrant addition is stopped. The results of the analysis are calculated based on the volume of titrant consumed and its concentration.

Standard titrant solutions are prepared by different methods:

1. by precise weighing (the substance must be chemically pure and stable);

2. by approximate weighing, followed by determination of the exact concentration of the titrant solution using a standard solution (standardization method);

3. from a fixanal, which is a strictly defined amount of a substance sealed in an ampoule. By carefully transferring this substance into a volumetric flask of a certain volume (usually 1 L), a solution of a given concentration is obtained.

Titration techniques.

In analytical practice, direct, reverse and indirect (substituent titration) titration is used.

If the quantitative determination is based on a direct reaction between the analyte and the titrant, the titration is called direct titration.

Sometimes, for a number of reasons, the reagent is added in excess, and the amount of reagent remaining after the reaction is titrated. Since the volume and concentration of the solution of the added reagent are known, the amount of reagent used for the reaction is determined from the titration results. This titration is called back titration.

In indirect titration, the reaction product of the analyte with a known amount of the reagent reacts with the titrant.

No. 11 Methods of expressing the concentration of titrated solutions, methods of their preparation. Standard (setting, starting) substances. Requirements for standard substances.

Ways to express solution concentrations:

1) Mass fraction of the substance
The mass fraction is denoted by the Greek letter "omega" and is equal to the ratio of the mass of the solute to the total mass of the solution

They are usually expressed in mass fractions or percentages (for this, the right side of the formula is multiplied by 100%).

2) Molar concentration- shows how many moles of a substance are contained in 1 liter (1000 ml) of solution. Designated C cm. Unit of measurement - [mol/l] (often written simply M)

where n is the amount of substance in moles, V is the volume of the solution, m is the mass of the substance, M r is the molar mass of the substance.

3) Molal concentration- the number of moles of dissolved substance in 1 kilogram (1000 g) of solvent. Unit of measurement - [mol/kg]

4) Normal concentration- this is the number of equivalents in 1 liter of solution. Denoted by the symbol C n

0.1 normal solution is decinormal.

5) Title- the amount of substance (in grams) dissolved in 1 ml. solution.

A distinction is made between the titer for the dissolved substance (for example, the titer of a hydrochloric acid solution - T HCl) or the titer for the substance being determined (for example, the titer of a hydrochloric acid solution for sodium hydroxide - T HCl / NaOH)

where T is the titer in g/ml, P is the mass of the sample, V is the volume of the volumetric flask.

Solutions with precise concentrations used in the analysis are called working or standard solutions.

Titrated solutions obtained from precise weighing of a substance are called cooked, A starting or setting substances called primary standards.

Solutions with a fixed titer are called secondary standards. The process of establishing the exact concentration is called standardization.

To standard (starting) substances present strict requirements. They can only be:

· chemically pure (impurities less than 0.01%),

chemically resistant

· highly soluble substances, the composition of which strictly corresponds to the chemical formula, with the largest possible molar mass with the smallest possible contribution of the molar mass of the reagent substance to it, in order to reduce the error in weighing.

· substances must meet the requirements for chemical reactions in quantitative chemical analysis. This method is used to prepare, for example, standard solutions of strong acids and alkalis, the substances of which, due to their aggressiveness, do not meet the requirements for starting substances.

Requirements for installation substances:

1. Have a crystalline structure and meet a certain chemical formula.

2. The chemical composition must correspond to its formula.

4. Methods for purifying the installation substance from accompanying impurities (crystallization, extraction, sublimation, etc.) must be available in the analytical laboratory.

5. A chemically pure installation substance should not be hygroscopic, but should be relatively well soluble in water.

6. Solutions of the setting substance should not change their titer during storage and contact with air.

7. The installation substance should have the highest possible equivalent weight. The greater the equivalent weight of the substance, the greater the accuracy of setting the titer of the solution, since when weighing a substance with a large molecular weight, weighing errors have a negligible effect.

No. 12. Theoretical foundations, the essence of alkalimetric, acidimetric, permanganatometric, iodometric titration. The titrants used, their concentration, methods for fixing the equivalence point, indicators.

Alkalimetry– determination of substances using standard alkali solutions.

Acidimetry- determination of substances using standard solutions of strong acids.

Permangonatometry is based on the oxidation reactions of various substances with potassium permanganate. Oxidation is carried out in an acidic environment, in which the MnO 4 - ion exhibits the strongest oxidizing properties and has the smallest equivalent (1/5).

Iodometry based on the titrometric reaction of iodine with sodium thiosulfate.

Indicators are organic acids and bases of complex structure, characterized by different colors of the molecular and ionized forms of the substance.

No. 13 Indicators of the acid-base titration method. Indicator color transition interval.

The titrants of the method are solutions of strong acids and bases: HCI, H 2 SO 4, NaOH, KOH. These substances do not meet the requirements for standard substances, therefore the concentration of titrants is established by standardizing their solutions. Borax Na 2 B 4 O 7 10H 2 O, anhydrous sodium carbonate Na 2 CO 3, oxalic acid dihydrate H 2 C 2 O 4 2H 2 O and some others are most often used as primary standards.

An acid-base indicator is itself an acid or base and during acid-base titration changes its color in or near the TE.

With the visual indicator method of fixing CTT in acid-base titration, the addition of titrant to the titrated solution is stopped when the color of the solution changes sharply due to a change in the color of the indicator introduced into the titrated solution.

The pH value to which a solution with a given indicator is titrated is called titration index of this indicator pT.

The most important indicators have the following transition areas and titration indicators:

Titration index pT pH transition region

Methyl orange……………4.0………………………….. 3.1 - 4.4

Methyl red……………….. 5.5…………………………. 4.4 - 6.2

Litmus……………….………………………7.0………………………….5.0 - 8.0

Phenolphthalein………………………..…9.0………………………….8.0 - 10.0

No. 14 Equipment for titrimetric analysis: burettes, volumetric flasks, Mohr and graduated pipettes, graduated cylinders, funnels, conical titration flasks. Rules for working with equipment.

Burettes – used for filling with titrane and performing titrations; glass tubes graduated in volume with 0 division at the top, designed for measuring solutions in small portions. V = 1-50 ml

Measuring pipettes - glass tubes for measuring and transferring solutions from one vessel to another. Mora's pipette has a spherical expansion in the middle part, and a ring mark is applied above the cat. On the expanded part indicate V,t when calibration was carried out (V = 5.00-50.0 ml) Graduated pipettes without expansion (V = 0.10-10.0 ml)

Conical flasks - used for titrimetric measurements (V = 25-250 ml) should not be filled more than 1/3

Volumetric flasks have a long narrow neck with an applied ring mark. Designed for preparing solutions of precisely specified concentration

Graduated cylinders are of secondary importance in titrimetric analysis. Used for approximate measurement of the volumes of some auxiliary solutions or water

A funnel is a device in the form of a cone ending in a tube for pouring liquids into narrow-necked vessels.

Glassware and instruments (including thermometers) should be handled with care, not placed on the edge of the table, or touched with elbows. Remove shards of broken dishes immediately.

No. 15. Calculation formulas used in titrimetric analysis according to the SI system

No. 16. The concept of physical and chemical methods of analysis.

Analysis methods based on observing changes in the physical properties of the analyzed system that occur as a result of certain chemical reactions are called physical and chemical methods.

Physico-chemical methods of analysis differ high sensitivity and speed of analytical determinations. The time required to complete an analysis using physical and physicochemical methods is sometimes measured in minutes.

Physico-chemical methods of quantitative analysis should not be confused with physico-chemical analysis according to N. S. Kurnakov, with the help of which the physical properties of systems are studied depending on their chemical composition.

No. 17. How many ml of sulfuric acid solution with a mass fraction of 98% (r = 1.84 g/ml) must be taken to prepare 200 ml of a solution with a mass fraction of 10% (r = 1.05 g/ml)?

No. 18. Calculate the mass fraction and molar concentration of a glucose solution containing 75 g of the substance in 500 g of water.

No. 19. What is the molar concentration of a 0.9% sodium chloride solution (r = 1.0 g/ml)?

No. 20. How to prepare a 5% glucose solution from a 20% solution?

No. 21. The concentration of glucose in the blood serum is 3.5 mmol/l, express the concentration in mg%.

No. 22. Hydroperite (contains hydrogen peroxide and urea) is used as an antiseptic. One tablet corresponds to 15 ml of 3% hydrogen peroxide solution. How many tablets must be dissolved in 100 ml of water to obtain a 1% solution?

No. 23. For the treatment of newly diagnosed patients with destructive tuberculosis, a 10% isoniazid solution is administered intravenously at a rate of 15 mg/kg body weight. Calculate the volume in ml of a 10% isoniazid solution (r = 1.0 g/ml) that must be administered to a patient weighing 75 kg.

No. 24 Taktivin is a polypeptide drug used in medical practice as an immunomodulatory agent. Release form: 0.01% solution in 1 ml bottles. For ophthalmoherpes, the drug is prescribed as a subcutaneous injection of 0.010-0.025 mg once a day. Calculate the volumes of 0.01% tactivin solution that correspond to the daily dose of the drug.

W%=(m(x) /m)*100%

m = V*p => m= 1*1=1 g

m(x)= (W*m(solution))/100%

m(x)= (0.01%*1g)/100% = 0.1 mg

No. 25 Ampicillin is a semi-synthetic antibiotic. Release form: tablets and capsules of 0.25 g. The daily dose for children is 100 mg/kg. The daily dose is divided into 4-6 doses. Calculate what part of the tablet should be given to a child weighing 10 kg per dose: a) with four doses; b) when taking the drug six times a day?

100 mg/kg *10 kg = 1000 mg = 1 g

1:4=0.25 – 1 tablet

1:6=0.166 – 2/3 tablets

Answer: a) 0.25 g b) 2/3 g

№26 To determine the total acidity of gastric juice, 5 ml of juice was titrated with an alkali solution with a concentration of 0.095 mol/l in the presence of phenolphthalein. 2.5 ml of alkali solution was consumed for the reaction. Calculate the acidity of the analyzed juice in mol/l.

C2 = (C1*V1)/V2

C2 = (0.095* 2.5)/5 = 0.0475 mol/l

Answer: 0.0475mol/l

№27 Calculate the mass of a sample of KMnO 4 required for the preparation of: a) 1 l of KMnO 4 solution with a molar concentration equivalent of 0.1 mol/l, b) 0.5 l of KMnO 4 solution with a molar concentration equivalent of 0.05 mol/l for work by permanganatometric titration.

M(KMnO4)=64+39+55=158

m(x)= (C(1/z x)*M(x)*V)/ z

a) m(KMnO4)=(0.1*158*1)/ 5 = 3.16

b) m(KMnO4)=(0.5*158*0.05)/ 5=0.79

Answer: a) 3.16 b)0.79

№29 Calculate the mass fraction (%) of acetic acid if 20 ml of 0.2 mol/l sodium hydroxide solution is consumed per 10 ml of its solution during titration.

С(NaOH)*V(NaOH) = С(specification)*V(specification)

C (unit) = (C (NaOH)*V (NaOH))/

/ V (uk. to-you)

C (specified values) = (0.2* 20) / 10 = 0.4 mol/l

m (UK) = C (UK)*M (UK)* B

m (unit size) = 0.4*60*0.01 = 0.24 g

р = 0.24/10 = 24 g/l

W = (C (specified kits)* M (specified kits)*v) / (p*10) = 0.1%

Answer: W = 0.1%

No. 31. Thermodynamics, basic concepts and tasks. State parameters (extensive and intensive) and system state functions.

Thermodynamics is a section of general chemistry that explains the processes of exchange of energy and matter between the body and the environment.

Thermodynamics studies processes associated with the transfer of energy (U) between bodies in the form of heat (Q) and work (W).

One of the most important concepts is the concept thermodynamic system – This is a body or group of bodies separated from the environment by an interface. Objects of nature that are not included in the system are environment . The system is characterized by mass (m) and internal energy (U).

There are three types of thermodynamic systems:

1. Isolated system – a system that does not exchange either matter or energy with the environment. ∆U = 0, ∆m = 0. (thermos).

2. Closed system – a system that does not exchange matter with the environment, but can exchange energy. ∆U ≠ 0, ∆m = 0. (vessel with water).

3. Open system – exchanges both matter and energy with the environment. ∆U ≠ 0, ∆m ≠ 0. (cell).

Systems are divided into:

1. Homogeneous – homogeneous - they have no phase boundaries (blood plasma, true solutions).

2. Heterogeneous – heterogeneous - there are phase boundaries (blood).

Any system can be characterized by a number of quantities called state parameters. They are distinguished into:

1. Extensive – summable upon addition (mass, volume, energy, entropy).

2. Intensive – that do not add up when added (pressure, temperature, density, concentration).

There are state parameters that depend only on the initial and final state of the system and do not depend on the process path; they are called state functions. For example:

X 1 is a thermodynamic quantity characterizing the initial state of the system.

X 2 is a thermodynamic quantity characterizing the final state of the system.

∆Х = Х 2 – Х 1 – change in thermodynamic quantity.

+∆X – profit (increment of variable X).

-∆Х – decrease.

The state functions are the following quantities: temperature - ∆T, pressure - ∆P, internal energy - ∆U, entropy - ∆S, enthalpy - ∆H, Gibbs energy - ∆G.

№32. The concept of internal energy. Work and heat are two forms of energy transfer. The first law of thermodynamics. Isochoric and isobaric processes.

Internal energy system is the sum of the energy of thermal motion of molecules, atoms and ions and the energy of interaction between them (attraction, repulsion, rotational and vibrational movements), with the exception of kinetic energy as a whole and potential energy of position. It is impossible to determine the absolute value of internal energy; you can only calculate the change in internal energy: ∆U = U 2 – U 1 . The change in internal energy is due to the work that is done during the interaction of the system with the environment, and the transfer of heat between the environment and the system. The relationship between the quantities U, Q, W, in thermodynamics is determined by the first law of thermodynamics.

In thermodynamics under work understand the work of expansion: W = p ∙ ∆V; Q = ∆U + p ∙ ∆V.

Heat in isobaric and isochoric processes it becomes a function of state and is called the thermal effect. This was established by Hess in 1840.

In total, there are six generally accepted formulations of the First Law of Thermodynamics.

1. In any isolated system, the energy supply remains constant. (Lomonosov)

2. Different forms of energy transform into each other in strictly equivalent quantities. (Joule)

3. A perpetual motion machine of the first kind is not possible, that is, it is not possible to build a machine that would produce mechanical work without spending energy on it.

4.

1. Isochoric process – characterized by a constant volume of the system, V – const.

Q v = ∆U + p ∙ ∆V;

The thermal effect of an isochoric process is equal to the change in internal energy.

2. Isobaric process

Q р = ∆U + p∙(V 2 – V 1) ;

Q р = U 2 – U 1 + p V 2 – p V 1 ;

Q р = H 2 – H 1 ;

∆H = ∆U + W.

№33. Thermal effect of a chemical reaction. Enthalpy as a function of the state of the system. Endothermic and exothermic processes.

N – enthalpy – a state function that shows the energy of the expanded system or the heat content of the system.

The thermal effect of an isobaric process is equal to the change in enthalpy.

Q р = ∆U + p∙(V 2 – V 1) ;

Q р = (U 2 + pV 2) – (U 1 + pV 1);

Q р = H 2 – H 1 ;

∆H = ∆U + W.

The nature of the process is judged by the enthalpy value:

1. Exothermic – process that occurs with the release of energy, ∆H< 0.

2. Endothermic - a process that occurs with energy absorption, ∆H > 0.

№34. The first law of thermodynamics for isobaric processes. Hess's law. Thermochemical calculations and their use for energy characterization of chemical and biochemical processes. Standard enthalpies of formation and combustion. Corollaries from Hess's law.

Isobaric process– characterized by constant system pressure, P – const.

Q р = ∆U + p∙(V 2 – V 1) ;

Q р = U 2 – U 1 + pV 2 – pV 1 ;

Q р = (U 2 + pV 2) – (U 1 + pV 1);

Q р = H 2 – H 1 ;

∆H = ∆U + W.

N – enthalpy – a state function that shows the energy of the expanded system or the heat content of the system. The heat of an isobaric process becomes a function of state and is called the thermal effect. The thermal effect of an isobaric process is equal to the change in enthalpy.

Hess's Law: The thermal effect of a reaction at constant volume and pressure does not depend on the process path, but depends on the initial and final state of the system.

Hess introduced the concept thermochemical equation – equation of a chemical reaction, which indicates the state of aggregation of the reacting substances and the thermal effect of the reaction. For example:

H 2 (g) + 1/2O 2 (g) = H 2 O (g), ∆H = -286 kJ/mol.

2H 2 (g) + O 2 (g) = 2H 2 O (g), ∆H = -572 kJ/mol.

The thermal effect of a reaction is determined in two ways:

Experimental (carried out in calorimeters);

Theoretical, calculated. It is based on two corollaries of Hess's law, which are associated with the concept of standard heats of formation and combustion.

Standard heat of formation – thermal effect of transformation from simple substances of 1 mole of compound under standard conditions - ∆H about 298 arr.

Standard terms – P = 1 atm = 760 mm Hg = 1.013·10 5 Pa (N/m 2) = 101 kPa; T = 25 o C = 298 o K.

Standard calorific value - the thermal effect of combustion of 1 mole of a substance under standard conditions is ∆H about 298 combustion. Most often used for organic substances.

The first corollary of Hess's law is the thermal effect of a chemical reaction is equal to the difference between the sum of the heats of formation of the reaction products and the sum of the heats of formation of the starting substances, taken with their stoichiometric coefficients.

∆H р = ∑ i n i ∆H о 298 formation of reaction products – ∑ i n i ∆H о 298 formation of starting materials of the reaction.

The heats of formation of simple substances are zero.

For example, for the reaction: C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O

∆H p = 6∆H o 298 (CO 2) + 6∆H o 298 (H 2 O) – ∆H o 298 (C 6 H 12 O 6) - 6∆H o 298 (O 2)

∆H р = 6(-393 kJ/mol) + 6(-296 kJ/mol) – (-1260 kJ/mol) – 6(0 kJ/mol) =

2874 kJ/mol

Second corollary of Hess's law– the thermal effect of a chemical reaction is equal to the difference between the sum of the heats of combustion of the starting substances of the reaction and the sum of the heats of combustion of the reaction products, taken with their stoichiometric coefficients.

∆H р = ∑ i n i ∆H о 298 combustion of the initial substances of the reaction – ∑ i n i ∆H о 298 combustion of the reaction products.

№35. Energy value of food products, rationale for diets, main tasks of bioenergy.

The mutual transformation of various types of energy also occurs in living organisms. The study of the laws and mechanisms of accumulation, storage and use of energy by living systems is the task of a special science: bioenergy, which allows us to get a correct idea of ​​the energy value of food and the organization of the diet. Each product has a certain energy or calorie content, so knowing the calorie content of the product and a person’s daily calorie need, you can correctly create a diet.

When compiling a diet, it is necessary to take into account not only the total energy supply, but also the need for proteins, fats, carbohydrates and vitamins.

Daily calorie needs:

For people with mental work (16-60 years old) - 2600-2800 cal;

For mechanized workers - 2800-3000 cal;

For persons of physical labor - 3400-3700 cal;

Students - 3000-3200 cal.

The daily protein requirement is 60-80g;

fats – 60-70g;

carbohydrates – 200-300g.

Knowing that 1g of protein provides 17 kJ (4.2 cal);

fat - 37 kJ (9 cal);

carbohydrates - 17 kJ (4.9 cal);

They make up the diet based on the total supply of calories and the qualitative composition.

Let's find out whether the first law of thermodynamics is true for open systems - living organisms: Q=ΔU+W.

If in a living organism t° = 37°С = const, then ΔU = 0, then the first law of thermodynamics for living organisms: Q = W.

In organisms, not only expansion work is performed, but also many other types of work - chemical (protein synthesis), mechanical (muscle contractions), electrical (conducting excitation through cells), osmotic (transfer of a substance across a membrane). The primary source of energy in the body for all types of work is the chemical energy of food substances. However, this energy is not used directly to perform all types of work; it is transformed into ATP energy.

The first law of thermodynamics for living organisms: All types of work in the body are performed due to an equivalent amount of energy released during the oxidation of nutrients.

No. 36. Second law of thermodynamics, contribution of S. Carnot and R. Clausius. Entropy as a function of the state of the system. Criteria for spontaneous processes in isolated systems. Relationship between entropy and the probability of the state of the system.

The second law of thermodynamics answers the question about the direction of the reaction. It is a generalized result of the work of many scientists.

S. Carnot (1840) is considered the discoverer of the second law of thermodynamics. He studied the conditions for converting heat into work and concluded that in heat engines the amount of heat received from the heat source cannot be completely converted into work; part of it is transferred to the refrigerator (dissipated).

Carnot derived the coefficient of performance - efficiency - η - the ratio of useful work to the initial one:

η = (Q 1 – Q 2) /Q 1 = W / Q 1.

The efficiency of a heat engine does not depend on the nature of the working fluid, but is determined only by the temperature range.

η = (T 1 – T 2) / T 1.

Statements of the second law of thermodynamics:

1. Clausius (1850): Heat cannot spontaneously transfer from a colder body to a hotter body.

2. Thomson (1851): A perpetual motion machine of the 2nd kind, in which all the heat imparted to the system is converted into work, is impossible.

Conclusion: The occurrence of spontaneous processes in an isolated system is accompanied by the dissipation of thermal energy.

In order to quantitatively characterize the process of energy dissipation, another thermodynamic function was required. It was introduced by Robert Clausius in 1865 - entropy – it is a state function, i.e. it does not depend on the process path and refers to extensive (summable) quantities. The magnitude of the entropy change is determined by the equation:

∆S = Q / T, J/K, where Q is the heat imparted to the system, T is the temperature of the system after the end of the process.

It follows from the equation that only part of the heat is used to perform work, and the other part is devalued or bound. Bound energy cannot be converted into work because it is dissipated. The amount of devalued energy is entropy .

The mathematical expression of the second law of thermodynamics is based on the fact that processes in isolated systems occur spontaneously with increasing entropy, i.e. ∆S > 0; for irreversible processes ∆S = 0. Thus, ∆S can be greater than or equal to zero in isolated processes.

The spontaneity of processes with increasing entropy follows from the Boltzmann equation:

S = K · lnω, where K – Boltzmann constant = 1.8 · 10 -23 J/K, ω – thermodynamic probability .

From the equation it is clear that as probability increases, entropy also increases.

Conclusion: although entropy is a measure of devalued energy, it is also the driving force of processes. If it were not in nature, all reactions would reach equilibrium, and for a living organism this is death; there would be no product output in production. The physical meaning can be defined as follows: entropy is a measure of disorder.

The change in entropy in a reaction can be calculated using 1 corollary of Hess's law: the change in entropy is equal to the difference between the sum of the standard entropies of the reaction products and the sum of the standard entropies of the starting substances taken with their stoichiometric coefficients.

∆S р = ∑ i n i ∆S о 298 reaction products – ∑ i n i ∆S о 298 initial substances of the reaction.

Usually four lessons (out of the six described) are devoted to studying the topic, including a brief examination of the main theoretical principles, solving problems on the topic and an experimental part.

Purpose of studying the topic

Based on knowledge of the theory of oxidation and reduction processes and skills in titrimetric analysis, it is reasonable to select and use the redox titration method (redox metry) to determine substances with oxidizing or reducing properties; learn to construct redox titration curves and select an indicator for titration; learn to perform quantitative calculations of the content of the analyte in the analyzed solution based on the results of titration.

Targets

1. Learn to quantitatively determine oxidizing agents and reducing agents in solution using various methods of redox titration.

2. Learn to calculate the redox potentials of solutions necessary to construct redox titration curves.

3. Learn to carry out calculations of the quantitative content of analytes in a solution based on titration results and statistical processing of analysis results.

4. Drawing up a protocol for laboratory work.

General characteristics of methods

Redox titration (redox measurement) is a method for determining oxidizing agents or reducing agents, based on the use of redox reactions occurring between the substance being determined and the titrant.

When titrating substances with oxidizing properties, a reducing titrant (reductometry) is used, and in solution

a reaction occurs, which in the general case (without taking into account stoichiometry and the participation of hydrogen and hydroxide ions) can be written as:

When titrating substances with reducing properties, a titrant-oxidizer (oxidimetry) is used, and a reaction occurs in the solution, which in general can be written as:

Redox metric methods are pharmacopoeial; they are included in the pharmacopoeias of all countries.

Lesson 4. Permanganometric titration

Permanganatometric titration is a method for the quantitative determination of reducing agents (less commonly, oxidizing agents and substances that do not have redox properties) using a solution of potassium permanganate in a sulfuric acid medium as a titrant. The direct titration of reducing agents is based on the reaction, which in the general case (without taking into account stoichiometry) can be written as:

The indicator of the method is the titrant itself, the first excess drop of which turns the titrated solution pink.

Purpose of the lesson

Quantitative determination of reducing agents hydrogen peroxide in solution and iron(II) in solution and in a sample by direct permanganometric titration.

Targets

1. Preparation of a solution of potassium permanganate with a given concentration by diluting a more concentrated solution.

2. Preparation of a standard solution of sodium oxalate by weighing with an accurately known mass.

3. Standardization of potassium permanganate solution to sodium oxalate solution.

4. Determination of the mass of hydrogen peroxide in solution.

5. Determination of the mass of iron(II) in solution and the mass fraction of iron(II) in the sample.

6. Statistical processing of analysis results.

Self-study assignment

Need to know before class

1. Redox reactions. Basic concepts. Redox potentials of redox systems, Nernst equation.

2. Methodology for constructing redox titration curves.

3. The essence and conditions of permanganatometric titration.

4. Examples of permanganatometric determination of reducing agents.

Be able to

1. Draw up equations of redox reactions.

2. Calculate the molar masses of the equivalent of oxidizing agents and reducing agents in oxidation-reduction reactions.

3. Carry out the necessary calculations based on the titration results.

Bibliography

1. Lectures: “Methods of redox titration. Method indicators. Construction of redox titration curves. Permanganometric titration".

2.Textbook. - Book 2, chapter 4. - P. 137-166.

3. Directory.

Self-test questions

1. What reactions are called redox?

2. Define the concepts: “oxidation”, “reduction”, “oxidizing agent”, “reducing agent”.

3. Which of the following reaction equations are redox? Identify the oxidizing agent and reducing agent in these reactions.

4. Complete the following reaction equations and equalize the coefficients using the electron-ion balance method:

5. Determine the molar mass of the equivalent of potassium permanganate, potassium dichromate, iron(II) sulfate, hydrogen peroxide, sodium nitrite, oxalic acid in the above reactions.

6. Why can’t a standard solution of potassium permanganate be prepared from a weighed portion with an accurately known mass?

7. What processes occur in a freshly prepared potassium permanganate solution?

8. Why are solutions of potassium permanganate stored in dark bottles?

9. What substances can be determined by direct permanganatometric titration?

10. On what factors does the real redox potential of the “permanganate-ion-manganese(II) ion” redox couple depend? Write the Nernst equation for the redox potential of this redox pair.

11. What quantities are used to assess the accuracy of analysis results? How are these values ​​calculated?

Reagents

1. Sodium oxalate (ch.d.a).

2. Potassium permanganate solution = 0.5 mol/l.

3. Sulfuric acid solution = 1 mol/l.

Study tables: titration curve of an oxidizing agent with a reducing agent.

1. Preparation of potassium permanganate solution

To complete the work, prepare 500 ml of potassium permanganate solution with an approximate molar concentration equivalent of 0.05 mol/l. The solution is prepared by diluting a concentrated solution of potassium permanganate with a molar concentration equivalent to 0.5 mol/l.

The required volume of concentrated potassium permanganate solution is calculated from relation (2.7), which in this case has the form:

where are the values ​​of the molar concentrations of equivalent

potassium permanganate ribbon in solution before and after dilution;

Volumes of initial and final solutions.

The calculated volume of a concentrated solution of potassium permanganate is measured using a graduated cylinder or graduated test tube and transferred to a dark bottle with a capacity of 0.5-1 liters, up to 500 ml of distilled water is added, mixed and capped. A label is placed on the bottle indicating the contents and student group number.

2. Preparation of standard sodium oxalate solution

To complete the work, prepare 250 ml of a standard sodium oxalate solution with a molar concentration equivalent to 0.05 mol/l. A solution of sodium oxalate is prepared by weighing a precisely known mass of crystalline sodium oxalate. The required theoretical mass of a sample of sodium oxalate is calculated using formula (2.10), which in this case has the form:

Where - molar concentration of sodium oxalate equivalent

in solution; - molar mass of sodium oxalate equivalent;

- volume of the sodium oxalate solution being prepared.

The molar mass of sodium oxalate equivalent is determined based on the half-reaction:

and using the Directory, table. 4.

Accurate weighing of sodium oxalate are selected for analysis

The concentration and titer of the prepared solution are calculated using formulas (2.2) and (2.3), which in this case have the form:

The flask label indicates its contents and group number.

3. Standardization of potassium permanganate solution to sodium oxalate

The essence of the technique

The standardization of the titrant method is based on direct permanganatometric titration. A standard solution of sodium oxalate is titrated with a solution of potassium permanganate in a sulfate solution while heating until a stable pink color appears. In this case, during the titration process, an oxidation-reduction reaction occurs in the solution:

The reaction is autocatalytic and accelerates in the presence of Mn 2 + cations. Based on the titration results, the titrant concentration is calculated.

3.1. Titration of sodium oxalate with potassium permanganate solution

Methodology

2 ml of a standard sodium oxalate solution are pipetted into a titration flask, 2 ml of a sulfuric acid solution with a concentration of 1 mol/l is added, the resulting mixture is heated to a temperature of 70-80 °C (until vapor appears) and titrated with a potassium permanganate solution. The first drops of titrant discolor slowly, and subsequent drops discolor instantly. Titration is carried out until a stable pink color of the solution appears from one drop of titrant, which does not disappear for about one minute. The titrant level in the burette is measured along the upper meniscus.

entered into the table. 2-9.

Table 2-9. Results of standardization of potassium permanganate solution

3.2. Calculation of the molar concentration of equivalent and titer of potassium permanganate solution

According to the table. 2-9 calculate the average volume of potassium permanganate solution , which went to titrate the standard

sodium oxalate solution as the arithmetic mean of the volumes of acid in three parallel titrations. The resulting value is entered into the table. 2-9.

Based on the titration results, the molar concentration of the equivalent and the titer of the potassium permanganate solution are calculated.

The law of equivalents (2.11) for the titration of sodium oxalate with a solution of potassium permanganate has the form:

Hence the concentration of the prepared potassium permanganate solution is equal to (2.12):

The titer of potassium permanganate solution is calculated using formula (2.3), which in this case has the form:

The molar mass of potassium permanganate equivalent is determined based on the half-reaction:

and using the table. 4 "Reference books". The obtained values ​​are entered into the table. 2-9.

4. Determination of the mass of hydrogen peroxide in solution

The essence of the technique

The quantitative determination of hydrogen peroxide is based on direct permanganometric titration. An aliquot of the analyzed hydrogen peroxide solution is titrated with a solution of potassium permanganate in a sulfate solution until a stable pink color appears. In this case, during the titration process, an oxidation-reduction reaction occurs in the solution:

Based on the titration results, the mass of hydrogen peroxide in the analyzed solution is calculated.

4.1. Titration of hydrogen peroxide with potassium permanganate solution

Methodology

The resulting control task in a 25 ml volumetric flask is brought to the mark with water and mixed thoroughly. An aliquot of the analyzed solution with a volume of 2 ml is pipetted into a titration flask, 2 ml of a sulfuric acid solution with a concentration of 1 mol/l is added and titrated with a solution of potassium permanganate until a stable pink color appears. zano-

are in the table. 2-10.

Table 2-10.

4.2. Calculation of the mass of hydrogen peroxide in the analyzed solution

According to the table. 2-10, calculate the mass of hydrogen peroxide in the analyzed solution for each titration using the formula for direct titration of the analyte (2.15), which in this case has the form:

The molar mass of hydrogen peroxide equivalent is determined based on the half-reaction:

According to the table. 2-10 carry out statistical processing of the obtained analysis results by mass of hydrogen peroxide.

Laboratory work “Determination of the mass of iron (II) in a salt solution. Determination of the mass fraction of iron(II) in a sample of iron(II) salt"

Reagents: Study tables: see previous lab. The essence of the technique

Determination of the mass of iron(II) in solutions of iron salts is carried out by direct titration of an aliquot of the analyzed salt solution with a standard solution of potassium permanganate in a sulfuric acid medium until a stable pink color appears. The addition of sulfuric acid is necessary to prevent the hydrolysis of iron(III) ions formed during the titration process and to reduce the rate of oxidation of iron(II) ions by atmospheric oxygen in the titrated solution. In this case, during the titration process, a redox reaction occurs in the solution:

Based on the titration results, the mass of salt in the analyzed solution and the mass fraction of iron(II) in the sample are calculated.

1. Titration of a solution of iron(II) salt with a solution of potassium permanganate

Methodology

To the control problem obtained in a 25 ml volumetric flask, add 10 ml of a sulfuric acid solution with a concentration of 1 mol/l using a graduated cylinder, bring the solution to the mark and mix. An aliquot of the prepared solution of iron(II) salt (2 ml) is pipetted into a titration flask and titrated with a solution of potassium permanganate until a stable pink color of the solution appears. Titration is carried out at least 5 times until reproducible results are obtained. The obtained values ​​of titrant volumes during parallel titrations entered into the table. 2-11.

Table 2-11. Results of titration of iron(II) salt solution

2. Calculation of the mass of iron(II) in the analyzed solution

According to the table. 2-11 calculate the mass of iron(II) in the analyzed solution for each titration using the formula for direct titration of the analyte (2.15), which in this case has the form:

The molar mass of iron(II) equivalent is determined based on the half-reaction:

and using the table. 4 "Reference books".

According to the table. 2-11 carry out statistical processing of the obtained analysis results.

3. Preparation of the analyzed solution of iron(II) salt

Methodology

To carry out the analysis, the student prepares 25 ml of a solution of the analyzed iron salt with an approximate molar concentration = = 0.05 mol/l. The theoretical mass of a sample of iron(II) salt required to prepare this solution is calculated using formula (2.9), which, taking into account the iron content in the sample, takes the form:

Mass fraction of iron , % in the analyzed salt samples

iron is usually 10-13%.

A sample of an iron(II) salt with an accurately known mass m (salt) is taken on an analytical balance by the difference of two weighings and transferred using a funnel into a 25 ml volumetric flask. To prevent hydrolysis of iron ions, add 10 ml of sulfuric acid solution with a molar concentration of 1 mol/l using a graduated cylinder. After dissolving the salt, the volume of the solution is adjusted to the mark with water and mixed.

4. Titration of the prepared solution of iron(II) salt with potassium permanganate solution

The prepared solution is titrated with a standard solution of potassium permanganate, as described above (step 1), and the data obtained are entered in the table. 2-12.

Table 2-12. Results of titration of iron salt solution (TT)

5. Calculation of the mass fraction of iron in the analyzed sample

According to the table. 2-12 calculate the mass fraction of iron (IF) in the analyzed solution for each titration based on the results of direct titration of the analyte using formula (2.15):

where m (salt) is the mass of the sample of salt being analyzed, g.

According to the table. 2-12 carry out statistical processing of the obtained analysis results by mass fraction , %.

ATTENTION! After completing the work, hand over the unused potassium permanganate solution to the laboratory assistant. Rinse the burette and fill it with water.

Control questions

1. How is the mass of a sample of sodium oxalate required to prepare a standard solution calculated?

2. How do you calculate the molar concentration of the equivalent and the titer of a standard solution of sodium oxalate prepared using an accurate sample?

3. How is the level of potassium permanganate solution in the burette measured during the titration process?

4. Why, when titrating a hot solution of sodium oxalate with potassium permanganate, do the first drops of the titrant discolor slowly, and the subsequent drops instantly?

5. What acid is used to acidify the analyzed solution during this titration?

6. Is it possible to acidify the analyzed solution with nitric or hydrochloric acid when performing permanganatometric titration?

7. What formulas are used to calculate the molar concentration and titer of potassium permanganate based on the titration results?

8. What formula is used to calculate the mass of hydrogen peroxide in solution based on the results of direct permanganometric titration?

9. How is the analyzed solution prepared to determine the mass of iron(II) salt in a sample by permanganometric titration?

10. Why, when dissolving a weighed amount of iron(II) salt in water, is the solution acidified with sulfuric acid?

11. How can oxidizing agents be determined by permanganatometric titration?

Lesson 5. Iodometric titration

Iodometric titration is a method for determining oxidizing agents, based on the use of indirect (substitution) titration and a standard sodium thiosulfate solution as a titrant. A known excess of solution is added to the substance being determined.

potassium iodide; a redox reaction occurs in solution, which in the general case (without taking into account stoichiometry) can be written as:

In this case, iodine is released in the solution in an amount equivalent to the reacted analyte. The released iodine is titrated with a standard sodium thiosulfate solution in the presence of a starch indicator:

Calculations for determining oxidizing agents are based on the fact that the amount of the oxidizing agent being determined is equivalent to the amount of reacted sodium thiosulfate.

Purpose of the lesson

Quantitative determination of oxidizing agents in the analyzed solution by indirect iodometric titration (substituent titration).

Targets

1. Preparation of a standard solution of potassium dichromate by weighing it with an accurately known mass.

2. Preparation of a sodium thiosulfate solution with a given concentration by diluting a more concentrated solution.

3. Standardization of sodium thiosulfate solution according to potassium dichromate.

4. Determination of the mass of hydrogen peroxide and the mass of copper (TT) in the analyzed solution by indirect iodometric titration.

5. Statistical processing of analysis results.

Self-study assignment

Need to know before class

1. The essence of the method of indirect iodometric titration.

2. Equations of reactions used in indirect iodometric titration.

4. Conditions for the iodometric determination of oxidizing agents.

Be able to

1. Draw up equations for the reactions of iodine with sodium thiosulfate, copper(II) salts, dichromate ions and hydrogen peroxide with iodidiones in ionic and molecular form.

2. Calculate the molar masses of the equivalent of sodium thiosulfate, potassium dichromate and copper(II) in the reactions used in the indirect iodometric titration method.

3. Calculate the mass of the determined oxidizing agents in the analyzed solution based on the results of indirect iodometric titration.

Bibliography

3. Directory.

Self-test questions

1. Name the titrant used in the method of iodometric determination of oxidizing agents.

2. How is sodium thiosulfate solution prepared and standardized?

3. What indicator is used for iodometric determinations of oxidizing agents?

4. What is the essence of the indirect iodometric method? What substances can be determined by this method? Give examples.

5. Write in ionic form the equations for the reaction between:

Iodine and sodium thiosulfate;

Potassium dichromate and potassium iodine house;

Copper(II) sulfate and potassium iodide;

Hydrogen peroxide and potassium iodide.

6. Determine the molar masses of the equivalent of iodine, sodium thiosulfate, hydrogen peroxide, potassium dichromate and copper(II) in the above reactions.

Laboratory work “Determination of the mass of hydrogen peroxide in solution”

Reagents

Potassium dichromate (h.h.).

Sodium thiosulfate solution: = 0.5 mol/l.

Potassium iodide solution (10%, free of KIO 3 impurities).

A solution of sulfuric acid with a molar concentration of 1 mol/l.

Ammonium molybdate solution 10%.

1. Preparation of sodium thiosulfate solution

To complete the work, prepare 500 ml of sodium thiosulfate solution with an approximate molar concentration equivalent of 0.05 mol/l. The solution is prepared by diluting a concentrated solution of sodium thiosulfate with a molar concentration of 0.5 mol/l.

The required volume of concentrated sodium thiosulfate solution is calculated from relation (2.7), which in this case has the form:

where are the values ​​of the molar concentrations of sodium thio-

sulfate in solution before and after dilution; - volumes

initial and final solution.

The calculated volume of concentrated sodium thiosulfate solution is measured using a graduated cylinder or graduated test tube and transferred to a bottle with a capacity of 0.5-1 liters, up to 500 ml of distilled water is added, mixed and capped. A label is placed on the bottle indicating the contents and student group number.

2. Preparation of a standard solution of potassium dichromate

To complete the work, prepare 250 ml of a standard solution of potassium dichromate with a molar concentration equivalent to 0.05 mol/l. A solution of potassium dichromate is prepared by weighing a precisely known mass of crystalline potassium dichromate. The required theoretical mass of a sample of potassium dichromate is calculated using formula (2.10), which in this case has the form:

Where - molar concentration of potassium dichromate equivalent

in solution; - molar mass of potassium dichromate equivalent;

- volume of the prepared potassium dichromate solution.

The molar mass of potassium dichromate equivalent is calculated based on the half-reaction:

and using the table. 4 "Reference books".

Accurate weighing of potassium dichromate are selected for analysis

on a commercial scale based on the difference between the results of two weighings and carefully transfer it through a dry funnel into a 250 ml volumetric flask. The contents of the flask are stirred with a rotational motion until the salt is completely dissolved and water is added to the mark (the last drops are added using a pipette). The flask is capped and the solution is thoroughly mixed, turning it upside down several times.

The concentration of the prepared salt solution is calculated using formulas (2.2) and (2.3), which in this case have the form:

3. Standardization of sodium thiosulfate solution to potassium dichromate

The essence of the technique

The standardization of sodium thiosulfate to potassium dichromate is based on indirect iodometric titration. A known excess of potassium iodide solution in an acidic medium is added to a standard solution of potassium dichromate. In this case, a redox reaction occurs in the solution:

as a result of which a mass of iodine is released, equivalent to potassium dichromate. The released iodine is titrated with a standardized solution of sodium thiosulfate:

in the presence of a starch indicator, introducing it at the end of the titration, since starch forms a fairly strong adsorption compound with iodine. Based on the titration results, the molar concentration of sodium thiosulfate is calculated.

3.1. Indirect titration of a solution of potassium dichromate with a solution of sodium thiosulfate

Methodology

In a titration flask, take 2 ml of a standard solution of potassium dichromate with a measuring pipette, add 1 ml of sulfuric acid with a concentration of 1 mol/l, 1 ml of a 10% solution of potassium iodide using a cylinder with stirring, cover the flask with glass and place in a dark place for 10 minutes. . During this time, the dichromate ion reacts quantitatively with the iodide ion to release an equivalent amount of iodine.

The released iodine is titrated with sodium thiosulfate solution to a greenish-yellow color. Then the contents of the flask are diluted with water approximately 2 times, 10 drops of starch solution are added and the solution is continued to be titrated with thorough stirring until the blue color of the starch disappears. At the end point of the titration, the solution has a pale green color, which is due to the presence of trivalent chromium cations.

Titration is carried out at least three times until reproducible results are obtained. The obtained values ​​of titrant volumes during parallel titrations entered into the table. 2-13.

Table 2-13. Results of standardization of sodium thiosulfate solution

3.2. Calculation of the molar concentration and titer of sodium thiosulfate solution

According to the table. 2-13 calculate the average volume of sodium thiosulfate solution who went to titrate the standard

potassium dichromate solution as the arithmetic mean of titrant volumes in three parallel titrations. The resulting value is entered into the table. 2-13.

Based on the titration results, the molar concentration and titer of the sodium thiosulfate solution are calculated.

The law of equivalents (2.11) for this titration has the form:

Sodium thiosulfate equivalence factor according to oxidation half-reaction:

is equal to 1. The concentration of the prepared sodium thiosulfate solution is equal to (2.12):

The titer of sodium thiosulfate solution is calculated using formula (2.3), which in this case has the form:

4. Indirect (substitution) titration of a hydrogen peroxide solution with a sodium thiosulfate solution

The essence of the technique

A known excess of potassium iodide solution is added to an aliquot of hydrogen peroxide. In this case, an oxidation-reduction reaction occurs in the solution with the release of iodine:

The released iodine, the amount of which is equivalent to the oxidizing agent, is titrated with a solution of sodium thiosulfate in the presence of a starch indicator:

Based on the titration results, the mass of the oxidizing agent in the analyzed solution is calculated. Methodology

The resulting control task in a 25 ml volumetric flask is diluted with distilled water, adjusted to the mark and mixed. An aliquot of the analyzed solution with a volume of 2 ml is taken into the titration flask with a pipette, 2 ml of a sulfuric acid solution with a concentration of 1 mol/l, 2 ml of a 10% potassium iodide solution and 1 drop of ammonium molybdate (catalyst) are added using a cylinder with stirring. The flask is covered with glass and left in a dark place for 10 minutes.

The released iodine is titrated with sodium thiosulfate solution to a straw-yellow color, after which 5 drops of starch solution are added and titration continues until the blue color disappears. Titration is carried out at least 5 times until reproducible results are obtained. The obtained values ​​of titrant volumes during parallel titrations entered into the table. 2-14.

Table 2-14. Results of titration of hydrogen peroxide solution

5. Calculation of the mass of hydrogen peroxide in the analyzed solution

According to the table. 2-14 calculate the mass of hydrogen peroxide in the analyzed solution for each titration using the formula for direct titration of the analyte (2.15), which in this case has the form:

The molar mass of hydrogen peroxide equivalent is determined based on the half-reaction

and using the table. 4 "Reference books".

According to the table. 2-14 carry out statistical processing of the obtained analysis results.

Laboratory work “Determination of the mass of copper(II) in solution”

1. Indirect (substitution) titration of copper(II) with sodium thiosulfate solution

The essence of the technique

The determination of copper(II) in solution is based on indirect iodometric titration. An aliquot of the analyzed solution

copper(II) salts are treated with an excess of potassium iodide solution in an acidic medium. In this case, a redox reaction occurs in the solution

as a result of which a mass of iodine equivalent to copper (II) is released. The released iodine is titrated with a standardized solution of sodium thiosulfate

in the presence of a starch indicator, introducing it at the end of the titration, since starch forms a fairly strong adsorption compound with iodine. Based on the titration results, the mass of copper in the analyzed solution is calculated. Methodology

The resulting control task in a 25 ml volumetric flask is diluted with distilled water, adjusted to the mark and mixed. An aliquot of the analyzed solution with a volume of 2 ml is pipetted into a titration flask, 2 ml of a 10% potassium iodide solution and 1 ml of a sulfuric acid solution with a concentration of 1 mol/l are added, covered with glass and left in a dark place for 10 minutes.

The released iodine is titrated with sodium thiosulfate solution to a straw-yellow color, after which 5 drops of starch solution are added and titration continues until the blue color disappears. Titration is carried out at least 5 times until reproducible results are obtained. The obtained values ​​of titrant volumes during five parallel titrations entered into the table. 2-15.

Table 2-15. Results of titration of copper(II) solution

2. Calculation of the mass of copper (M) in the analyzed solution

According to the table. 2-15 calculate the mass of copper (II) in the analyzed solution for each titration using the formula for direct titration of the analyte (2.15), which in this case has the form:

The molar mass of copper(II) equivalent in this reaction is determined based on the half-reaction:

and using the table. 4 "Reference books".

According to the table. 2-15 carry out statistical processing of the obtained analysis results.

ATTENTION! The sodium thiosulfate solution is saved until the next lesson.

Control questions

1. Why can’t a standard solution of sodium thiosulfate be prepared from a sample with an accurately known mass?

2. What processes occur in a sodium thiosulfate solution during storage? Write the corresponding reaction equations.

3. Why are solutions of sodium thiosulfate and potassium iodide stored in dark bottles?

4. How is a sodium thiosulfate solution standardized against potassium dichromate? Write the corresponding reaction equations.

5. For what purpose, when titrating sodium iodine with thiosulfate, is the solution diluted with distilled water before reaching the end point of titration?

6. Write the reaction equations underlying the iodometric determination of oxidizing agents: hydrogen peroxide, copper(II).

7. Why, when determining oxidizing agents by the iodometric method, the test solutions, after adding excess potassium iodide, are covered with glass and left for some time in a dark place? Why is potassium iodide added in excess?

8. How is the mass of hydrogen peroxide and copper(II) in the analyzed solution calculated when determining them by indirect iodometric titration?

9. Why can’t potassium iodide solution be used as a titrant for the determination of oxidizing agents?

Lesson 6. Iodimetric titration

Iodimetric titration is a method for the quantitative determination of reducing agents based on the use of direct titration

reducing agents titrant - a solution of iodine in potassium iodide. During the titration process, triiodide ion is converted to iodide ion according to the reduction half-reaction:

Formally, it is believed that molecular iodine serves as an oxidizing agent:

That is why, when composing chemical equations, for simplicity, we write not the formula of the triiodide ion, but the formula of molecular iodine.

The iodine titration method is based on a redox reaction, which in the general case (without taking into account stoichiometry) can be written as:

Purpose of the lesson

Quantitative determination of the mass of arsenic (III) in solution by direct iodine titration.

Targets

1. Preparation of an iodine solution with a given concentration by diluting a more concentrated solution.

2. Standardization of iodine solution with sodium thiosulfate solution.

3. Determination of the mass of arsenic (III) in solution by direct iodine titration.

Self-study assignment

Need to know before class

1. The essence of the direct iodine titration method.

2. Equations of reactions used in direct iodine titration.

3. The procedure for preparing and standardizing the method titrant.

4. Conditions for the iodinemetric determination of reducing agents.

Be able to

1. Draw up equations for the oxidation reactions of compounds with reducing properties in ionic and molecular form with iodine.

2. Calculate the molar masses of iodine equivalent and arsenic (III) compounds in the reactions used in the direct iodine titration method.

3. Calculate the mass of the determined reducing agents in the analyzed solution based on the results of direct iodine titration.

Bibliography

1. Lecture “Iodine and iodometric titration. Iodatemetric titration. Chloriodimetry".

2.Textbook. - Book 2, chapter 4. - P. 168-178.

3. Directory.

Self-test questions

1. Name the titrant used in the iodinemetric determination of reducing agents.

2. How is the iodine solution prepared and standardized?

3. What is the essence of direct iodine titration? What substances can be determined by this method? Give examples.

4. Write the equation for the reaction of iodine with arsenithione and calculate the molar mass of the equivalent of arsenic(III) and arsenite ion in this reaction.

5. Why should iodinemetric determination of reducing agents be carried out in acidic, neutral and slightly alkaline media?

6. Why can’t iodinemetric determination of reducing agents be carried out at pH >9.0? Write the corresponding reaction equations.

7. Why is it necessary to use burettes with a glass stopcock for iodine determinations using iodine solution?

8. How do you measure the level of iodine solution and other intensely colored solutions using a burette?

Reagents

Sodium thiosulfate solution

Solution of iodine in potassium iodide = 0.5 mol/l.

Freshly prepared starch solution (0.2%).

1. Preparation of iodine solution

Methodology

To complete the work, prepare 200 ml of iodine solution with an approximate molar concentration equivalent of 0.05 mol/l. The solution is prepared by diluting a concentrated solution with a molar concentration of iodine equivalent of 0.5 mol/l.

The required volume of concentrated iodine solution is calculated from relation (2.7), which in this case has the form:

where are the values ​​of the molar concentrations of iodine equivalent

in solution until and after dilution; - volumes of initial and final

no solution.

The calculated volume of concentrated iodine solution is measured using a graduated cylinder or graduated test tube and transferred to a 0.25 liter bottle, up to 200 ml of distilled water is added, mixed and capped. A label is placed on the bottle indicating the contents and student group number.

2. Standardization of iodine solution with sodium thiosulfate

The essence of the technique

The standardization of the titrant is based on the direct titration of an aliquot of a standardized sodium thiosulfate solution with an iodine solution in the presence of a starch indicator until a blue color appears in the solution. In this case, during the titration process, an oxidation-reduction reaction occurs in the solution:

Based on the titration results, the titrant concentration is calculated.

2.1. Titration of sodium thiosulfate solution with iodine solution

Methodology

2 ml of a standardized sodium thiosulfate solution is taken into a titration flask with a measuring pipette, 4-5 drops of freshly prepared starch solution are added and titrated from a burette with an iodine solution until a stable blue color appears from 1 drop of titrant. The titrant level in the burette is measured along the upper meniscus.

Titration is carried out at least three times until reproducible results are obtained. The obtained values ​​of titrant volumes during parallel titrations are entered in the table. 2-16.

Table 2-16. Results of standardization of iodine solution

2.2. Calculation of molar concentration equivalent and titer of iodine solution

According to the table. 2-16 calculate the average volume of iodine solution, which was used for titration of an aliquot of sodium thiosulfate, as the arithmetic average of the titrant volumes in three parallel titrations. The resulting value is entered into the table. 2-16.

Based on the titration results, the molar concentration of the equivalent and the titer of the iodine solution are calculated.

The law of equivalents (2.11) for the titration of sodium thiosulfate with iodine solution has the form:

Hence the concentration of the prepared iodine solution is:

The titer of the iodine solution is calculated using formula (2.3), which in this case has the form:

The molar mass of iodine equivalent is determined based on the half-reaction:

and using the table. 4 "Reference books". The obtained values ​​are entered into the table. 2-16.

3. Determination of the mass of arsenic (III) in solution

ATTENTION! Solutions containing arsenic are toxic; working with arsenic solutions requires special care and careful compliance with safety regulations!

The essence of the technique

Determination of the mass of arsenic (III) in compounds is carried out by direct iodine titration of arsenites with an iodine solution in the presence of a starch indicator until a blue color of the solution appears. The definition is based on a reversible chemical reaction:

To ensure the necessary completeness of this reaction and shift the equilibrium towards the reaction products (to the right), titration is carried out in a weakly alkaline medium (pH ~ 8) in the presence of sodium bicarbonate, which binds the released H+ ions into carbonic acid:

Based on the titration results, the mass of arsenic (III) in the analyzed solution is calculated.

3.1. Titration of sodium arsenite solution with iodine solution

Methodology

The resulting control problem is diluted with distilled water in a 25 ml volumetric flask, adjusted to the mark and mixed. The control task contains the required amount of sodium bicarbonate. Using a rubber bulb, pipette 2 ml of the solution to be analyzed into the titration flask. (a solution containing arsenic should not be drawn into the pipette with your mouth!), add 1 ml of starch solution and titrate with iodine solution until the solution turns blue. Titration is carried out at least 5 times until reproducible results are obtained. The obtained values ​​of titrant volumes during parallel titrations entered into the table. 2-17.

Table 2-17. Results of titration of hydrogen peroxide solution

3.2. Calculation of the mass of arsenic(III) in the analyzed solution

According to the table. 2-17 calculate the mass of arsenic(III) in the analyzed solution for each titration using the formula for direct titration of the analyte (2.15), which in this case has the form:

The molar mass of arsenic(III) equivalent is determined based on the half-reaction:

and using the table. 4 "Reference books".

According to the table. 2-17 carry out statistical processing of the obtained analysis results.

ATTENTION! After finishing work, hand over the unused iodine solution to the laboratory assistant. Rinse the burette and fill it with water.

Control questions

1. Why can’t a standard iodine solution be prepared from a sample with an accurately known mass?

2. Why is a standard iodine solution prepared by dissolving crystalline iodine in a solution of potassium iodide? Write the appropriate reaction equation.

3. What reaction is the basis for the iodinemetric determination of arsenic(III)? Write its equation. What is special about this reaction?

4. Why is sodium bicarbonate added to the solution before titrating arsenic(III) with iodine? Can it be replaced with sodium carbonate?

5. Why, when titrating sodium thiosulfate with iodine, starch solution is added at the end of the titration, and when titrating sodium thiosulfate and arsenic (III) with iodine - at the beginning of the titration?

Lesson 7. Bromatometric titration

Bromatometric titration is a method for determining reducing agents by direct titration with a standard solution of potassium bromate The method is based on the half-reaction:

Purpose of the lesson

Quantitative determination of the mass of arsenic (III) in solution by direct bromatometric titration.

Targets

1. Preparation of a standard solution of potassium bromate.

2. Titration of the analyzed arsenic (III) solution with a solution of potassium bromate in the presence of the methyl orange indicator.

3. Calculation of arsenic (III) content in the analyzed solution based on the titration results.

4. Statistical processing of analysis results.

Self-study assignment

Need to know before class

1. The essence of the direct bromometric titration method.

2. Equations of reactions used in direct bromatometric titration.

3. The procedure for preparing the titrant of the method.

4. Conditions for the bromatometric determination of reducing agents.

Be able to

1. Draw up equations for the oxidation of potassium bromate with compounds with reducing properties in ionic and molecular form.

2. Calculate the molar masses of the titrant equivalent and reducing agents in the reactions used in the direct bromometric titration method.

3. Calculate the mass of the determined reducing agents in the analyzed solution based on the results of direct bromometric titration.

Bibliography

2.Textbook. - Book 2, chapter 4. - P. 186-193.

3. Directory.

Self-test questions

1. What substances can be determined by direct bromatometric titration? Give examples.

2. Write the reaction equations for the interaction of sodium arsenite with potassium bromate in ionic form.

3. What is the molar mass of arsenic(III) equivalent in this reaction?

4. What indicators are used in the direct bromometric titration method? Give examples of reversible and irreversible redox indicators.

5. What is the purpose of blank titration?

6. Write the Nernst equation to calculate the real potential of the “bromate-bromide” redox couple.

Laboratory work “Determination of the mass of arsenic(III) in solution”

Reagents

Potassium bromate (analytical grade).

Hydrochloric acid: = 3 mol/l.

Aqueous solution of methyl orange 0.1%.

1.

Methodology

To complete the work, prepare 250 ml of a standard solution of potassium bromate with a molar concentration equivalent to 0.01 mol/l. A solution of potassium bromate is prepared by weighing it with a precisely known mass of crystalline potassium bromate. The required theoretical mass of a sample of potassium bromate is calculated using formula (2.10), which in this case has the form:

Where ) - molar concentration of potassium bromate equivalent in dis-

creation; I- molar mass of potassium bromate equivalent; -

volume of prepared potassium bromate solution.

The molar mass of potassium bromate equivalent is calculated based on the half-reaction:

and using the table. 4 "Reference books".

Accurate weighing of potassium bromate m(КВгО 3) is selected on an analytical balance based on the difference between the results of two weighings and carefully transferred through a dry funnel into a 250 ml volumetric flask. The contents of the flask are stirred with a rotational motion until the salt is completely dissolved and water is added to the mark (the last drops are added using a pipette). The flask is capped and the solution is thoroughly mixed, turning it upside down several times.

The concentration of the prepared solution is calculated using formulas (2.2) and (2.3), which in this case have the form:

2. Determination of the mass of arsenic(III) in solution

The essence of the technique

An aliquot of a strongly acidic arsenic solution heated to boiling (ATT) is titrated in the presence of the methyl orange indicator with a standard solution of potassium bromate. In this case, a redox reaction occurs in the solution:

The first excess drop of titrant discolors the indicator and the titration is completed. To account for the volume of titrant used to oxidize the indicator, a blank titration is performed.

2.1. Titration of sodium arsenite solution with potassium bromate solution

Methodology

The resulting control task in a 25 ml volumetric flask is diluted with water, adjusted to the mark and mixed. 2 ml of the resulting solution is taken into a titration flask with a measuring pipette, 1 ml of a 3 mol/l hydrochloric acid solution, one drop of a methyl orange indicator solution are added with stirring using a graduated cylinder, and heated on an asbestos grid with the flame of a gas burner almost to a boil. The hot solution is titrated with a standard solution of potassium bromate until the solution becomes colorless.

To determine the volume of potassium bromate solution used for the oxidation of the indicator, a blank titration is carried out in parallel. To do this, place 2 ml of water, 1 ml of 3 mol/l hydrochloric acid, 1 drop of indicator into a titration flask, heat to boiling and titrate with a solution of potassium bromate until the solution becomes colorless.

Titration is carried out at least 5 times until reproducible results are obtained. The obtained values ​​of titrant volumes during parallel titrations entered into the table. 2-18.

Table 2-18. Results of titration of arsenic(III) solution

2.2. Calculation of the mass of arsenic(III) in the analyzed solution

According to the table. 2-18 calculate the mass of arsenic(III) in the analyzed solution for each titration using the formula for direct titration of the analyte (2.15), which in this case has the form:

Where - volume of titrant that went into blank titration.

The molar mass of arsenic (III) equivalent is calculated based on the half-reaction:

and using the table. 4 "Reference books".

According to the table. 2-18 carry out statistical processing of the obtained analysis results.

ATTENTION! Save the standard solution of potassium bromate until the next lesson.

Control questions

1. How is the titration end point determined for the bromatometric determination of arsenic?

2. Why does the solution become discolored at the end of the titration when titrating potassium bromate with methyl orange?

3. Why is it necessary to introduce a correction for blank titration when performing direct bromatometric titration? Should this be done in the acid-base titration method?

4. Why is bromatometric titration carried out in a strongly acidic environment?

5. Why is a solution of arsenic (III) heated to boiling and titrated while hot?

6. Is it possible to titrate a solution of arsenic (III) potassium with bromate without an indicator?

7. How is the mass of arsenic in a solution calculated when determining it by direct bromatometric titration?

Lesson 8. Bromometric titration

Bromometric titration is a method for the quantitative determination of reducing agents (usually organic compounds) using bromine solution as a reagent that interacts with the substance being determined. The method is based on the half-reaction:

Purpose of the lesson

Quantitative determination of the mass fraction of sodium salicylate in the preparation by bromometric titration.

Targets

1. Conducting a quantitative determination of sodium salicylate, including the following stages:

Bromination of salicylic acid;

Replacement of excess bromine with iodine;

Titration of the released iodine with a standard solution of sodium thiosulfate;

Calculation of the mass fraction of sodium salicylate in the sample based on the titration results.

2. Statistical processing of analysis results.

Self-study assignment

Need to know before class

1. The essence of the bromometric titration method.

2. Equations of reactions used in bromometric titration.

3. Conditions for the bromometric determination of reducing agents.

4. Method for the bromometric determination of sodium salicylate in the preparation.

Be able to

1. Draw up equations for the reactions underlying bromometric titration.

2. Calculate equivalence factors and molar masses of the equivalent of all substances involved in reactions in the bromometric determination of sodium salicylate.

3. Calculate the mass of the determined reducing agents in the analyzed solution based on the results of bromometric titration.

Bibliography

1.Lecture “Bromo- and bromatometric titration.”

2.Textbook. - Book 2, chapter 4. - P. 189-193.

3. Directory.

Self-test questions

1. Write in general form the reaction equation underlying the bromometric method of analysis.

2. What is the titrant of the method and how is it prepared?

3. Does the redox potential of the titrant of the method depend on pH? Give a reasoned answer.

4. What is the equivalence factor of potassium bromate when carrying out bromometric determinations?

5. What substances can be determined by bromometric titration?

6. What indicators can be used in bromometric titration?

7. How are the equivalence factor and mass of the determined organic substances in solution calculated during bromometric titration?

Laboratory work “Determination of the mass fraction of sodium salicylate in the preparation”

Reagents

Potassium bromate standard solution = 0.05 mol/l, prepared in the previous lesson.

Potassium bromide, potassium iodide (analytical grade).

Sodium thiosulfate solution = 0.05 mol/l, standardized in the previous lesson.

Hydrochloric acid = 3 mol/l.

Chloroform.

1. Preparation of a standard solution of potassium bromate

See previous lesson.

2. Bromometric titration of sodium salicylate in solution

The essence of the technique

The mass of sodium salicylate is determined by reverse bromometric titration with iodometric termination. In this case, a number of chemical reactions occur in the solution.

The titration is carried out in a strongly acidic environment, so the salicylate ion is converted to salicylic acid (see step 1 below).

When a precisely known excess of potassium bromate is added to an aliquot of the analyzed solution in the bromide-bromate mixture, bromine is formed in an amount equivalent to potassium bromate (stage 2). The released bromine reacts with salicylic acid (step 3). The unreacted bromine is then replaced with an equivalent amount

iodine by adding excess potassium iodide solution (step 4). The released iodine is titrated with a standard solution of sodium thiosulfate (step 5) in the presence of chloroform, since the indicator starch is hydrolyzed in a strongly acidic environment.

Thus, when determining sodium salicylate by bromometric titration, the following reactions occur in solution:

According to the reaction equations occurring in steps (1) and (3), one molecule of sodium salicylate is equivalent to one molecule of salicylic acid, and one molecule of salicylic acid reacts with 6 bromine atoms and donates 6 electrons. That is why the equivalence factor of sodium salicylate is equal. Based on the titration results, the mass fraction of sodium salicylate in the drug is calculated.

2.1. Bromination of salicylic acidMethodology

A weighed portion of the sodium salicylate (NaSal) preparation, taken on an analytical balance by the difference of two weighings, with an accurately known mass about 0.2 g is transferred to a 100 ml volumetric flask, dissolved in water, the volume of the solution is adjusted to the mark with distilled water and mixed thoroughly. An aliquot of the prepared solution with a volume of 2 ml is taken into the titration flask with a measuring pipette, sequentially added with another measuring pipette while stirring, 5 ml of a standard solution of potassium bromate, 0.2 g of dry potassium bromide taken on a pharmaceutical scale, and using a measuring cylinder - 2 ml 3 mol/l hydrochloric acid. The flask is covered with glass and kept in a dark place for 10 minutes.

2.2. Replacement of excess bromine with iodine and titration of iodine with sodium thiosulfate

Add 0.2 g of dry potassium iodide, taken on a pharmacy scale, to the resulting solution, mix the contents of the flask thoroughly, cover the flask with a watch glass and leave in a dark place. After approx.

Add 1 ml of chloroform to the resulting solution using a graduated cylinder over a period of 10 minutes; the released iodine is slowly titrated with vigorous stirring with a standard solution of sodium thiosulfate until the chloroform beads at the bottom of the flask become discolored.

Titration is carried out at least 5 times until reproducible results are obtained. The obtained values ​​of titrant volumes during parallel titrations entered into the table. 2-18.

Table 2-18. Results of titration of sodium salicylate solution

2.3. Calculation of the mass fraction of sodium salicylate in the preparation

Let us derive a formula for calculating the mass fraction of sodium salicylate in the drug based on the obtained titration results.

Let's write down the law of equivalents for all 5 stages of titration. The amount of the total bromine equivalent substance formed in step (2) is equal to the amount of the equivalent substance taken in a precisely known excess of potassium bromate:

The released bromine partially reacts with salicylic acid, the remaining bromine is converted into an equivalent amount of iodine, therefore the law of equivalents for reactions occurring in stages (3) and (4) has the form:

The iodine released in step (4) is equivalent to sodium thiosulfate:

Let us combine all the above expressions into one and obtain the law of equivalents for this titration:

which can be rewritten as:

Hence the molar concentration of sodium salicylate equivalent in the analyzed solution is equal to:

We substitute the resulting expression into relation (2.15) and obtain a formula for calculating the mass of sodium salicylate in the analyzed solution:

The mass fraction of sodium salicylate in the sample is equal to:

where = 26.69 g/mol; - volume of ana-

lysed sodium salicylate solution.

According to the table. 2-18 calculate the values ​​of the mass fraction of sodium salicylate in the preparation for each of 5 parallel titrations and carry out statistical processing of the obtained analysis results for the mass fraction of sodium salicylate in the preparation.

Control questions

1. Write the reaction equations underlying the bromometric determination of sodium salicylate.

2. Write the formula by which the mass fraction of sodium salicylate in the drug is calculated and explain it.

4. What indicator is used in the bromometric determination of sodium salicylate?

5. List the main stages of the method for determining sodium salicylate by the bromometric method.

6. What standard solutions are used to perform this determination?

7. Why is the acidified solution of sodium salicylate, after adding the bromide-bromate mixture, covered with glass and kept for 10 minutes? Write the corresponding reaction equations.

8. Explain why, in the bromometric determination of sodium salicylate with iodometric termination, the sodium thiosulfate solution must be added slowly and the titrated solution must be vigorously stirred.

9. Is it possible to use a starch solution as an indicator in this titration?

10. How to check the absence of impurities of potassium iodate and potassium bromate in solutions of potassium iodide and potassium bromide, respectively?

Lesson 9. Nitritometric titration

Nitritometric titration is a method for the quantitative determination of inorganic and organic substances using a titrant - a standard solution of sodium nitrite. The method is based on the use of a half-reaction occurring in an acidic environment:

The titrant equivalence factor for this titration is 1.

When titrating organic substances containing a primary aromatic amino group, a diazotization reaction occurs in solution, which in general can be written as:

where R is an aromatic radical.

Purpose of the lesson

Quantitative determination of an organic compound containing a primary aromatic amino group by nitritometric titration.

Targets

1. Preparation of titrant - sodium nitrite solution.

2. Standardization of sodium nitrite solution using a standard potassium permanganate solution.

3. Determination of the mass fraction of novocaine in the drug.

4. Statistical processing of analysis results.

Self-study assignment

Need to know before class

1. The essence of the method for nitritometric determination of inorganic substances with reducing properties.

2. The essence of the method for nitritometric determination of organic substances containing a primary aromatic amino group.

3. The procedure for preparing and standardizing the method titrant.

4. Conditions for carrying out nitritometric titration. Be able to

1. Draw up equations for the oxidation reactions of various inorganic reducing agents with sodium nitrite.

2. Draw up a general equation for the reaction underlying the nitritometric determination of organic substances containing a primary amino group.

3. Calculate the mass fraction of the analyte in the preparation based on the titration results.

Bibliography

1. Lecture “Nitritometric titration”.

2.Textbook. - Book 2, chapter 4. - pp. 193-198.

3. Directory.

Self-test questions

1. What redox properties can the nitrite ion exhibit depending on the nature of the substance reacting with it?

2. What reaction underlies the nitritometric titration of novocaine? Write the reaction equation in general form. What are the optimal conditions for its occurrence?

3. What indicators are used to establish the equivalence point during nitritometric titration?

4. Write and explain the formula by which the mass fraction of the analyte in the preparation is calculated based on the results of nitritometric titration.

5. How to prepare a 0.1 mol/l solution of sodium nitrite in the presence of dry salt?

6. What substances can be used as primary or secondary standards to standardize a sodium nitrite solution?

7. Write the reaction equations underlying the standardization of sodium nitrite solution using a secondary standard by reverse permanganatometric titration and iodometric termination. What are the equivalence factors of sodium nitrite, potassium permanganate and sodium thiosulfate in the corresponding reactions?

Laboratory work “Determination of the mass fraction of novocaine in the drug”

Reagents

Novocaine drug.

Sodium nitrite, potassium bromide (reagent grade).

Standard solution of potassium permanganate = = 0.05 mol/l.

Standard sodium thiosulfate solution = = 0.05 mol/l.

Sulfuric acid solution i = 1 mol/l.

Hydrochloric acid (= 1.175 g/ml).

Hydrochloric acid. i = 2 mol/l.

Potassium iodide solution (10%).

Freshly prepared starch solution (0.2%).

Aqueous solution of tropeolin 00 (0.03%).

Aqueous solution of methylene blue (0.02%).

1. Preparation of sodium nitrite titrant solution

To complete the work, prepare 250 ml of sodium nitrite solution with a molar concentration of 0.1 mol/l. The required theoretical mass of a sample of sodium nitrite is calculated using formula (2.10), which in this case has the form:

Where - molar concentration of sodium nitrite in solution; -

molar mass of sodium nitrite; - volume of prepared solution -

ra sodium nitrite.

The molar mass of sodium nitrite is determined from the table. 4 "Reference books".

The calculated portion of sodium nitrite is weighed on an analytical balance, carefully transferred through a dry funnel into a 250 ml flask and dissolved in a small volume of water, then bringing the final volume of the solution with water to 250 ml.

2. Standardization of sodium nitrite solution according to standard potassium permanganate solution

The essence of the technique

Standardization of sodium nitrite solution is carried out by reverse permanganatometric titration with iodometric termination.

To a precisely known volume of a standard solution of potassium permanganate (titrant 1), taken in excess of sodium nitrite, is added a solution of sulfuric acid, and then a known volume of a standardized solution of sodium nitrite. In this case, a redox reaction occurs in the solution:

The reagents should not be added in the reverse order, since nitrithione, in the absence of potassium permanganate, is easily decomposed by strong acids, releasing nitrogen oxides.

After completion of the reaction, excess potassium iodide is added to the solution containing unreacted potassium permanganate (substitution step). As a result of the reaction:

iodine is released in the solution in an amount equivalent to unreacted potassium permanganate. The released iodine is titrated with a standardized solution of sodium thiosulfate (titrant 2) in the presence of starch:

Let us write the law of equivalents for the above stages that occur during standardization of a sodium nitrite solution:

Sodium nitrite equivalence factor according to the oxidation half-reaction:

equals

The molar concentration of sodium nitrite equivalent is calculated using the formula for back titration (2.19), which, taking into account the above law of equivalents for this titration, has the form:

Based on the titration results, the molar concentration of the equivalent and the molar concentration of the prepared sodium nitrite solution are calculated.

2.1. Carrying out titration

Methodology

Using a measuring pipette, take 2 ml of a standard solution of potassium permanganate into a 50 ml titration flask. = 0.05 mol/l,

add 1 ml of a 1 mol/l sulfuric acid solution, the solution is heated in a water bath to ~40 °C and 1 ml of the standardized solution is slowly added to it from a pipette with continuous stirring. After 15-20 minutes, 1 ml of 10% potassium iodide solution is added to the resulting solution, the flask is covered with glass and kept in a dark place for ~10 minutes.

Then add 10 ml of water to the solution and titrate with standard sodium thiosulfate solution = 0.05 mol/l to a faint yellow color, add 1 ml of 0.2% starch solution and continue titrating until the color of the solution changes from blue to colorless. Titration is carried out at least three times until reproducible results are obtained.

The obtained values ​​of titrant volumes during parallel titrations entered into the table. 2-19.

2.2. Calculation of the molar concentration of sodium nitrite

According to the table. 2-19 calculate the average volume of sodium thiosulfate solution, which was used for titration during the standardization process, as the arithmetic mean of the titrant volumes in three parallel titrations. The resulting value is entered into the table. 2-19.

Based on the data obtained, the molar concentration of sodium nitrite equivalent and the molar concentration of sodium nitrite in solution are calculated taking into account its equivalence factor.

Table 2-19. Results of standardization of sodium nitrite solution

3. Determination of the mass fraction of novocaine in the drug

The essence of the technique

Novocaine is p-aminobenzoic acid diethylaminoethyl ester hydrochloride with molar

weighing 272.8 g/mol. The novocaine molecule contains a primary amino group, and its quantitative determination is based on the diazotization reaction occurring during the titration process.

Novocaine is titrated with a standardized solution of sodium nitrite at a temperature not exceeding 18-20 ° C (at higher temperatures, decomposition of the reaction product, a diazo compound, is possible). Titration is carried out in a hydrochloric acid medium in the presence of potassium bromide, which is added to speed up the reaction, and an internal indicator - tropeolin 00 mixed with methylene blue - until the red-violet color of the solution changes to blue. The diazotization reaction does not proceed quickly enough, so the titration is carried out slowly, especially near the equivalence point.

The determination of novocaine based on the results of nitritometric titration is based on the fact that, in accordance with the diazotization reaction equation, novocaine and titrant react in equal molar ratios:

3.1. Titration of novocaine solution

Methodology

To perform the work, prepare 250 ml of novocaine solution with a molar concentration of 0.1 mol/l. A solution of novocaine is prepared by weighing it with a precisely known mass. Required theoretical weight of the sample

(the mass fraction of novocaine in the drug is about 100%) is calculated using formula (2.10), which in this case has the form:

Where With(new) - molar concentration of novocaine in solution; M(new) - molar mass of novocaine; - volume of the prepared novocaine solution.

Accurate weighing of novocaine are selected on analytical instruments

the difference between the results of the two weighings and carefully transfer it through a dry funnel into a 250-ml volumetric flask. The contents of the flask are stirred with a rotational motion until novocaine is completely dissolved and water is added to the mark (the last drops are added using a pipette). The flask is capped and the solution is thoroughly mixed, turning it upside down several times.

An aliquot of the prepared novocaine solution with a volume of 2 ml is pipetted into the titration flask, 1 ml of hydrochloric acid, 5 ml of water, 0.1 g of potassium bromide, 1 drop of tropeolin 00 solution and 2 drops of methylene blue solution are added. The mixture is titrated with constant stirring at a temperature not exceeding 18-20 °C with a standardized solution of sodium nitrite, adding it slowly during the titration process (1 drop per minute) until the color of the solution changes from red-violet to blue.

Titration is carried out at least five times until reproducible results are obtained. The obtained values ​​of titrant volumes during parallel titrations entered into the table. 2-20.

Table 2-20. Results of titration of novocaine solution
3.2. Calculation of the mass fraction of novocaine in the drug

According to the table. 2-20 calculate the mass fraction of novocaine in the sample for each titration in two ways.

3.2.1. Using titrimetric titrant conversion factor

Calculate the theoretical titrimetric factor for the conversion of sodium nitrite solution to novocaine using formula (2.4), which for this titration has the form:

Where = 0.1000 mol/l.

Calculate the titrant correction factor (2.5):

Where - practical molar concentration of sodium nitrite in dis-

creation used in the analysis.

The mass fraction of novocaine in the drug is calculated taking into account relation (2.18) using the formula, which for this case has the form:

Where ) - average volume of sodium nitrite titrant.

3.2.2. Using molar titrant concentration The mass fraction of novocaine in the drug using the molar concentration of the titrant is calculated taking into account relation (2.15) according to the formula, which for this titration has the form:

According to the table. 2-20 calculate the values ​​of the mass fractions of novocaine in the drug based on the results of five parallel titrations and carry out statistical processing of the obtained analysis results.

Control questions

1. What formula is used to calculate the mass of a sample of sodium nitrite required for the preparation of the titrant?

2. Explain the expediency of using back titration when standardizing a solution of sodium nitrite with potassium permanganate.

3. Why, when standardizing a sodium nitrite solution by reverse permanganatometric titration, is a sodium nitrite solution added to an acidified potassium permanganate solution, and not vice versa?

4. Write and explain the formula for calculating the molar concentration of the equivalent of sodium nitrite in the calculation

solution during its standardization by reverse permanganatometric titration with iodometric termination.

5. Why is titration carried out during the nitritometric determination of novocaine:

a) in a hydrochloric acid environment;

b) in the presence of potassium bromide;

c) at a temperature not higher than 18-20 °C;

d) adding titrant slowly, especially near the equivalence point?

6. What formula is used to calculate the mass of a sample of novocaine preparation necessary for preparing the analyzed solution?

7. Write and explain the formulas used to calculate the mass of novocaine in an aliquot of the analyzed solution according to nitritometric titration data:

a) using the correction factor of sodium nitrite solution and the titrimetric conversion factor of 0.1000 mol/l sodium nitrite solution for novocaine;

b) using the molar concentration of sodium nitrite solution.

Examples of current control test points

on topic II

I. Test items with one correct answer

Instructions. From the following answers, choose the correct one.

1. The titrant equivalence factor for iodometric titration is equal to:

A) 1/6;

B) 1/5;

AT 12 ;

d) 1/4;

e) 1.

Answer: d.

2. To determine the CTT in the permanganatometric determination of iron(III) ions, titration is carried out:

a) in the presence of methyl orange indicator;

b) in the presence of a starch indicator;

c) without indicator;

d) in the presence of the indicator tropeolin 00;

e) in the presence of diphenylamine indicator. Answer: V.

3. The titrimetric conversion factor of 0.050 mol/l potassium permanganate solution for iron is equal to:

a) 1.8. 10 -3 g/ml;

b) 2.8. 10 -3 g/ml;

c) 5.6. 10 -3 g/ml;

d) 6.4. 10 -3 g/ml;

e) 5.6. 10 -4 g/ml. Answer: b.

4. The color of the methyl orange indicator during the bromatometric determination of arsenic(III) changes in the CTT:

a) from colorless to yellow;

b) from yellow to colorless;

c) from pink to yellow;

d) from yellow to pink;

d) from pink to colorless. Answer: d.

II.Compliance task

Instructions. Match. Each answer can be used once, several times, or not at all.

Exercise. Select the appropriate titrant for each redox titration method.

Answers: 5 - b, 6 - a, 7 - d, 8 - c.

Exercise. Choose a method of quantitative determination for each substance. Write in ionic form the equation of the reaction that occurs during the quantitative determination of sodium thiosulfate.

Answers: 9 - a, 10 - b, 11 - d, 12 - d.

III.Tasks with the choice of one or more correct answers

Instructions. For each incomplete statement, one or more answers are correct. Choose the numbers of the correct answers.

13. When used in redox titrations, the following indicators are irreversible:

a) methyl orange;

b) ferroin;

c) methyl red;

d) phenylanthranilic acid. Answer: a, c.

14. The magnitude of the jump during redox titration depends on:

a) from the nature of the reacting substances;

b) concentrations of reacting substances;

c) pH of the titrated solution;

d) temperature. Answer: a B C D.

15. Indicate substances that can be quantitatively determined by permanganometric titration:

a) sodium oxalate;

b) potassium bromide;

c) hydrogen peroxide;

d) novocaine.

Write the corresponding reaction equations in ionic form. Answer: a, c.

16. Indicate the standard substances or titrated solutions used to standardize the titrant in the nitritometric titration method:

a) potassium bromate;

b) sulfanilic acid;

c) sodium oxalate;

d) potassium permanganate solution. Answer: b, d.

IV. Test items for determining cause-and-effect relationships

Instructions. The question consists of two statements connected by the conjunction “because.” Check whether each statement is true or false, and then the relationship between them. Use the chart below to select your answers.

17. In bromatometric titrations, methyl orange is used as an indicator because its color depends on the pH of the solution.

Answer: b.

18. To create an acidic environment during permanganatometric titration, nitric acid is added to the titrated solution, because with increasing acidity of the environment, the oxidizing properties of permanganate ions increase.

Answer: G.

19. With iodinemetric determination Sodium bicarbonate is added to the titrated solution to create a slightly alkaline environment, because iodine is disproportionate in an acidic environment.

Answer: V.

on topic II

1. Theoretical basis of redox titration

The essence of redox methods of titrimetric analysis. The redox potential of the redox pair and its dependence on the nature of the substances, pH of the environment, concentration, temperature. Molar mass equivalent of oxidizing and reducing agents. Requirements for reactions used in redox titration methods. Equilibrium constant of a redox reaction, its mathematical expression and relationship with standard redox pair potentials. Catalytic and autocatalytic reactions in analytical chemistry.

Classification of redox analysis methods.

Indicators of redox titration methods, their classification and mechanism of action. Indicator color transition interval.

Calculation of redox potentials of solutions when constructing redox titration curves.

2. Redox titration methods

Permanganometric titration. The essence and basic equation of the method. Conditions for permanganatometric titration (pH, temperature). Determination of the titration end point. Titrant method, its preparation, stability, standardization. Determination of reducing agents: iron(II), hydrogen peroxide, sodium nitrite. Determination of oxidizing agents by back titration.

Iodimetric titration. The essence of the method, the basic reaction equation. Titrant method, preparation, stability, standardization. Conditions for carrying out iodine measurements. Determination of the titration end point. Determination of reducing agents by iodine titration (arsenic(III), sodium thiosulfate, sodium sulfite).

Iodometric titration. The essence of the method, the basic reaction equation. Titrant method, preparation, stability, standardization. Conditions for carrying out iodometric determinations. Determination of the titration end point. Determination of oxidizing agents by iodometric titration [hydrogen peroxide, copper(II) salts, potassium dichromate, sodium arsenate].

Chlorodimetric titration. The essence of the method. Basic reaction equation. Method titrant, preparation and standardization. Conditions for chloriodimetric determinations. Determination of the titration end point. Determination of reducing agents by direct titration [tin(II) salts, iron(II) salts, arsenic(III) and antimony(III) compounds, hydroquinone, ascorbic acid].

Iodatemetric titration. The essence of the method. Basic reaction equation. Titrant method, its preparation, stability, standardization. Conditions for carrying out iodate metric determinations. Determination of the titration end point. Determination of compounds of arsenic(III), antimony(III), iron(II), sodium thiosulfate, ascorbic acid.

Bromatometric titration. The essence of the method, the basic reaction equation. Titrant method, its preparation, standardization. Determination of the titration end point. Conditions for carrying out bromatometric determinations. Determination of reducing agents: compounds of arsenic(III), antimony(III), iron salts(II).

Bromometric titration. The essence of the method and the basic reaction equation. Titrant of the method and its preparation. Determination of the titration end point. Determination of organic substances (resorcinol, sodium salicylate).

Dichromatometric titration. The essence of the method, the basic reaction equation. Titrant method, its preparation. Determination of the titration end point. Determination of reducing agents by direct titration [iron(II) salts, sulfites, arsenites, ascorbic acid]. Determination of oxidizing agents by back titration (nitrates, chlorates).

Nitritometric titration. The essence of the method, the basic reaction equation. Titrant method, its preparation and standardization. Determination of titration end point; external and internal indicators. Determination of inorganic reducing agents (tin(II) salts, iron(II) salts, arsenic(III) in compounds) and organic substances containing an amino group (novocaine, streptocide).

Perimetric titration. The essence of the method, the basic reaction equation. Titrant method, its preparation and standardization. Conditions for cerimetric determinations. Determination of tin(II) salts, iron(II) salts, arsenic(III) compounds, hydrogen peroxide.

3. Solving typical problems

3.1. Calculation of points on redox titration curves.

3.2. Calculations for the preparation and standardization of titrants.

3.3. Calculations based on titration results.

Example of a written test ticket (full-time)

1. Construct a titration curve for a solution of iron(II) sulfate with a concentration of 0.1 mol/L with a solution of cerium (IV) sulfate with a concentration of 0.1 mol/L.

2. Iodometric titration. Basic reaction equation, method titrant, its preparation and standardization.

3. Task. Calculate the mass of arsenic in a solution of sodium arsenite if 18.40 ml of a solution of potassium bromate with a molar concentration of 0.0500 mol/l was used for its titration.

4. Task. To what volume should 835.0 ml of a 0.1179 mol/l solution of cerium (IV) sulfate be diluted to obtain a solution with a titrimetric conversion factor of cerium (IV) sulfate to iron (II) equal to 4.381. 10 -3 g/ml?

Example of a written test ticket (evening department)

1. Redox potential of a redox couple and its dependence on the nature of substances, pH of the environment, concentration, temperature.

2. Bromatometric titration. The essence of the method, the basic reaction equation. Titrant method, its preparation. Determination of the titration end point.

3. Calculate the mass of iodine in the sample if 16.00 ml of sodium thiosulfate solution with a molar concentration of 0.0800 mol/l was used for its titration.

Bibliography

Methods redox titration, or redox methods, are based on the use of electron transfer reactions - redox (OR) reactions. In other words, redox titration, or redoxmetry, - This is a titration accompanied by the transfer of one or more electrons from a donor ion or molecule (reducing agent) Red 1 to an acceptor (oxidizing agent) Ox 2:

Red 1 + Ox 2 = Ox 1 + Red 2

The reduced form of one substance Red 1, donating electrons, goes into the oxidized form Ox 1 of the same substance. Both of these forms form one redox pair Ox l  Red l.

The oxidized form Ox 2 of the second substance participating in the OB reaction, accepting electrons, goes into the reduced form Red 2 of the same substance. Both of these forms also form a redox couple Ox 2 Red 2.

Any redox reaction involves at least two redox pairs.

The higher the RH potential of the redox couple Ox 2 Red 2, the oxidized form of which plays the role of an oxidizing agent in this reaction, the greater the number of reducing agents Red 1 can be titrated and determined using this oxidizing agent Ox 2. Therefore, in redoxmetry, oxidizing agents are most often used as titrants, the standard OB potentials of redox pairs of which have the highest possible values, for example (at room temperature):

Se 4+, E°(Ce 4+ Ce 3+) = 1.44 V; МnО 4 - , E°(МnО 4 ‑, Н + Мn 2+) = 1.51 V,

Cr 2 O 7 2‑, E°(Cr 2 O 7 2‑, H + Сr 3+) = 1.33 V, etc.

On the contrary, if the substances to be determined are Ox 2 oxidizers, then for their titration it is advisable to use reducing agents whose standard redox vapor RH is as minimal as possible, for example

Jֿ E°(J 2 J⁻) = 0.54 V; S 2 O 3 2‑, E°(S 4 O 6 2‑ S 2 O 3 2‑) = 0.09 V, etc.

Redox methods are the most important pharmacopoeial methods of quantitative analysis.

4.2. Classification of redox methods

Several dozen different methods of OM titration are known. They are usually classified as follows.

Classification according to the nature of the titrant. In this case, RH titration methods are divided into two groups:

oxidimetry - methods for determining reducing agents using an oxidizing titrant;

reductometry - methods for determining oxidizing agents using a reducing titrant.

Classification according to the nature of the reagent, interacting with the analyte. Below, after the name of the corresponding method, the main active ingredient of this method is indicated in parentheses: bromatometry(potassium bromate KBrO 3, bromometry(bromoBr 2), dichromatometry(potassium dichromate K 2 Cr 2 O 7), iodotometry(potassium iodate KJO 3), iodymetry(iodJ 2), iodometry(potassium iodide KJ, sodium thiosulfate Na 2 S 2 O 3, nitritometry(sodium nitrite NaNO 2), permanganatometry(potassium permanganate KMnO 4). chloriodimetry(iodine chloride JC1), cerimetry(cerium(IV) sulfate).

Some other RH titration methods are less commonly used, such as: ascorbinometry(ascorbic acid), titanometry(titanium(III) salts), vanadatometry(ammonium vanadate NH 4 VO 3), etc.

4.3. Conditions for redox titration

Reactions used in RH titration methods must meet a number of requirements, the most important of which are the following:

The reactions should proceed almost to completion. The higher the equilibrium constant, the more complete the OB reaction is. TO, which is determined by the relation

lg K =n( E 1°‑ E 2°)/0.059

at room temperature, where E 1° and E 2 ° - respectively, standard OB potentials of redox pairs participating in a given OB reaction, P - the number of electrons given up by a reducing agent to an oxidizing agent. Therefore, the greater the difference E° =E 1 ° - E 2 °, the higher the equilibrium constant, the more complete the reaction proceeds. For reactions like

A + B = Reaction products

at n =1 and TO 10 8 (with this value TO the reaction proceeds no less than 99.99%) we obtain for E°:

E°0.059lg10 8 0.47 V.

The reaction must proceed quickly enough so that equilibrium, in which the real OB potentials of both redox pairs are equal, is established almost instantly. Typically, RH titrations are carried out at room temperature. However, in the case of slow OM reactions, solutions are sometimes heated to speed up the reaction. Thus, the oxidation reaction of antimony(III) with bromate ions in an acidic medium at room temperature proceeds slowly. However, at 70-80 °C it proceeds quite quickly and becomes suitable for the bromatometric determination of antimony.

To speed up the achievement of equilibrium, homogeneous catalysts are also used. Consider, for example, the reaction

HAsO 2 + 2Ce 4+ + 2H 2 O=H 3 AsO 4 + 2Ce 3+ + 2H +

Standard OB potentials of redox pairs participating in the reaction are equal at room temperature E°(Ce 4+ Ce 3+) = 1.44 V, Eº (H 3 AsO 4 HAsO 2 = 0.56 V. Hence, for the equilibrium constant of this reaction we obtain (n = 2)

lg K = (1,44 ‑ 0,56)/0,059≈30;TO≈ 10 30

The equilibrium constant is large, so the reaction proceeds with a very high degree of completeness. However, under normal conditions it proceeds slowly. To speed it up, catalysts are introduced into the solution.

Sometimes the catalysts are the reaction products themselves. Thus, during the permanganatometric titration of oxalates in an acidic medium according to the scheme

5C 2 O 4 2‑ + 2МnО 4 ‾ + 16Н + = 2Mn 2+ + 10CO 2 + 8H 2 O

Manganese(II) cations Mn 2+ act as a catalyst. Therefore, at first, when a titrant solution - potassium permanganate - is added to a titrated solution containing oxalate ions, the reaction proceeds slowly. Therefore, the titrated solution is heated. As manganese(II) cations are formed, the achievement of equilibrium accelerates and titration is carried out without difficulty.

The reaction must proceed stoichiometrically , side processes must be excluded.

The end point of the titration must be determined accurately and unambiguously either with indicators or without indicators.

oxidation states

For example:

For example:

Methods for establishing T.E.

To determine the equivalence point during redox titration, use:

a) non-indicator methods. In the case where the solution of the titrated substance or titrant is colored, TE can be determined by the disappearance or appearance of this color, respectively;

b) specific indicators - changing color when the titrant appears or the substance being determined disappears. For example, for the J 2 /2J - system, the specific indicator is starch, which colors solutions containing J 2 blue, and for Fe 3+ ions the specific indicator is SCN - ions (thiocyanate ions), the resulting complex is colored blood-red ;

c) RH (redox) indicators – changing color when the RH potential of the system changes. Single-color indicators are diphenylamine, two-color indicators are ferroin.

Redox indicators exist in two forms - oxidized (Ind ok) and reduced (Ind rec), and the color of one form is different from the other. The transition of an indicator from one form to another and a change in its color occurs at a certain transition potential, which is observed when the concentrations of the oxidized and reduced forms of the indicator are equal and according to the Nernst-Peters equation:

The transition interval of redox indicators is very short, unlike acid-base indicators.

RH titration curves

RH titration curves depict the change in the RH potential of the system as the titrant solution is added.

Reductometry, when a solution of an oxidizing agent is titrated with a standard solution of a reducing agent

In reductometry, titration curves are calculated:

2)

3)

Oxidimetry, when a reducing agent solution is titrated with a standard oxidizing agent solution

In oxidimetry, titration curves are calculated:

2)

3)

Example. Let's calculate the titration curve of a 100 cm 3 solution of FeSO 4 with a molar concentration equivalent to 0.1 mol/dm 3 with a KMnO 4 solution of the same concentration.

Reaction equation:

The equilibrium constant of this reaction is

A large numerical value of the equilibrium constant indicates that the equilibrium of the reaction is almost entirely shifted to the right. After adding the first drops of titrant, two OM pairs are formed in the solution: , the potential of each of which can be calculated using the Nernst equation:

In this case, the reducing agent solution is titrated with an oxidizing agent solution, i.e. Titration refers to the oxidimetry method; the titration curve is calculated according to the appropriate scheme.

3) After T.E.

Calculation data for constructing a titration curve

No.	τ	Calculation formula	E, B
	0,10		0,71
0,50	0,77
0,90	0,83
0,99	0,89
0,999	0,95
			1,39
	1,001		1,47
1,01	1,49
1,10	1,50
1,50	1,505

Using the table data, we construct a titration curve:

For titration error ±0.1% titration jump

∆E = E τ =1.001 - E τ =0.999 = 1.47 – 0.95 = 0.52.

For titration error ± 1.0% titration jump

∆E = E τ =1.01 - E τ =0.99 = 1.49 – 0.89 = 0.60.

In the region of TE, when moving from a solution undertitrated by 0.1% to a solution overtitrated by 0.1%, the potential changes by more than 0.5 V. The potential jump makes it possible to use directly potentiometric measurements or RH indicators, the color of which changes with change in potential. In addition, in this case, a colored solution is used as a titrant, therefore T.E. can be determined by the appearance of a faint pink color from one excess drop of potassium permanganate.

PERMANGANOMETRY

The method is based on the oxidation of solutions of reducing agents with potassium permanganate KMnO 4. The oxidation of reducing agents can be carried out in various environments, and manganese (VII) is reduced in an acidic environment to Mn 2+ ions, in a neutral environment to manganese (IV) and in an alkaline environment to manganese (VI). Typically, in the permanganatometry method, the reaction is carried out in an acidic environment. In this case, a half-reaction occurs

A titrated solution cannot be prepared using an exact weighing, because it contains . Therefore, first prepare a solution of approximately the required concentration, leave it in a dark bottle for 7-10 days, filter off the precipitate, and then set the exact concentration of the resulting solution. Standardization of the solution is carried out using a titrated solution of oxalic acid ( ) or sodium oxalate ().

The indicator is the permanganate itself, colored red-violet. The end of the reaction is easily determined by the change in color from one excess drop of permanganate. In an acidic environment, the titrated solution turns pink due to excess MnO 4 - ions. A big disadvantage of redox reactions is their low speed, which complicates the titration process. Heat is used to speed up slow reactions. As a rule, with every 10° increase in temperature, the reaction rate increases by 2-3 times. The oxidation reaction with oxalic acid permanganate is carried out at a temperature of 70-80 °C. Under these conditions, titration proceeds normally, since the reaction rate increases significantly.

If heating cannot be used (volatilization of one of the substances, decomposition, etc.), the concentrations of the reacting substances are increased to speed up the reaction. The reaction rate can be affected by the introduction of a catalyst into the solution.

The oxidation reaction of oxalic acid permanganate can be catalytically accelerated by the addition of MnSO 4, the role of which is as follows:

The resulting manganese dioxide oxidizes oxalic acid, reducing to manganese (III):

Thus, manganese (II) added to the solution is completely regenerated and is not consumed in the reaction, but greatly accelerates the reaction. In permanganatometry, one of the products of the oxalic acid oxidation reaction is Mn 2+ ions, which, as they form in solution, accelerate the reaction process. Such reactions are called autocatalytic. The first drops of permanganate during the titration of a hot acidified solution of oxalic acid become discolored slowly. As a small amount of Mn 2+ ions is formed, further discoloration of the permanganate occurs almost instantly, since the formed Mn 2+ ions play the role of a catalyst.

Redox titration

Redox processes include chemical processes that are accompanied by changes oxidation states atoms of substances participating in the reaction.

Substances whose atoms during a reaction reduce their oxidation state due to the addition of electrons are called oxidizing agents, i.e. they are electron acceptors. In this case, the oxidizing agents themselves are reduced. Reducing agents, being electron donors, are oxidized.

The product of reduction of an oxidizing agent is called the reduced form, and the product of oxidation of a reducing agent is its oxidized form. The oxidizing agent with its reduced form constitutes a half-pair of the redox system, and the other half-pair is the reducing agent with its oxidized form. Thus, a reducing agent with an oxidized form and an oxidizing agent with its reduced form constitute two semi-pairs (redox pairs) of the redox system.

All OM processes (redox reactions) can be divided into three types

a) intermolecular, when during the OB reaction the transfer of electrons occurs between particles of different substances. For example

In this reaction, the role of the oxidizing agent in the presence of H 3 O + is played by ions, and the ions act as a reducing agent

b) dismutation (disproportionation), during which the transfer of electrons occurs between particles of the same substance. As a result of disproportionation, the oxidation state of one part of the atoms decreases at the expense of another part of the same atoms, the oxidation state of which becomes greater.

For example:

c) intramolecular, in which the transfer of electrons occurs between two atoms that are part of the same particle of a substance, leading to the decomposition of the substance into simpler ones.

Titrimetric analysis. Basic concepts (aliquot, titrant, equivalence point, indicator, titration curve). Requirements for reactions in titrimetry. Reagents used in titrimetry. Standard substances, titrants.

A method of quantitative analysis based on measuring the volume of a solution with a precisely known concentration of the reagent required to react with a given amount of the analyte. Aliquot-an accurately measured multiple of the sample (volume of solution) taken for analysis, which retains the properties of the main sample. Titrant or working solution is the solution with which the titration is carried out. Equivalence point point in titration when the amount of titrant added is chemically equivalent to the amount of the substance being titrated. TE can also be called the stoichiometric point, the theoretical end point. Indicator- a substance that changes its color in TE is characterized by low concentration and transition interval. Titration curve-shows a graphical dependence of the logarithm of the concentration of a participant in the reaction occurring during titration, or some type of solution, on the volume of the added titrant (or on the degree of titration). For example, for an acid-base reaction, titer curves. The pH-volume of the titrant is plotted in coordinates.

Requirements for reactions in titrimetry: 1. The interaction of the titrant with the analyte must occur in strict accordance with the stoichiometric reaction equation, and the titrant must be spent only on the reaction with the analyte. At the same time, the analyte must react only with the titrant and not interact, for example, with atmospheric oxygen, as can, in principle, be the case when titrating reducing agents.

2. The titration reaction must proceed quantitatively, that is, the equilibrium constant of the titration reaction must be sufficiently large.

3. The interaction of the analyte with the titrant must occur at high speed.

4. There must be a way to determine the end of the titration.

5. The titrant solution must be standardized.
Reagents: Based on the properties of substances and the method of their preparation, titrants are of two types: standard, with a prepared titer, standardized or with a set titer. Standard solutions or prepared titres are called primary standard solutions. It is prepared by dissolving a precise amount of pure chemical in a specific volume of solvent. Primary standard substances include: Na2CO3, Na2B4O7*10H2O, Na2SO4, CaCO3, CaCI2, MgSO4, MgCI2, H2C2O4*2H2O, Na2C2O4, K2Cr2O7, sodium bicarbonate, potassium bromate, potassium iodate and others.

The first type of titrants (with a prepared titer) are used in titrimetry for quantitative determinations of certain substances and for establishing titers of the second type - secondary standard solutions.

A secondary standard solution is a solution of such substances, the concentration of which is established (standardized) by the concentration of the primary standard solutions or prepared by the known mass of the secondary standard substance.

The second type of titrants includes solutions of substances that do not meet the requirements for primary standard substances. These include: alkalis, acid solutions HCI, H2SO4, HNO3, CH3COOH, KMnO4, AgNO3, Na2S2O3 and others.

Typical calculations in titrimetry. Methods of expressing concentrations in titrimetry (molar concentration, molar concentration equivalent, titer, correction factor. Calculation of the mass of a standard sample for the preparation of titrant, calculation of titrant concentration

Molar concentration c(A) is the amount of dissolved substance A in moles contained in one liter of solution: mol/l. c(A) = n(A)/V(A) = m(A)/M/(A)V(A), where p(A)- amount of dissolved substance A, mol; V(A)- volume of solution, l; t(A)- mass of dissolved substance A, g; M/(A) - molar mass of solute A, g/mol. Molar concentration of equivalent c(1/zA),, - the amount of dissolved substance A in moles, corresponding to the equivalent of A contained in one liter of solution: mol/l c(1/z A) = n(1/z A)/V(A)= t(A)/M(1/z A)V(A), where 1/z is the equivalence factor; calculated for each substance based on the stoichiometry of the reaction; n(1/zA)- amount of substance equivalent to A in solution, mol; M(1/zA)- molar mass of equivalent solute A, g/mol. Titre T(A) dissolved substance A is the mass of dissolved substance A contained in one ml of solution: measured in ml T(A)= m(A)/V(A) = с(1/z А)М(1/z A)/1000. Titer of the solution for the analyte X, or titrimetric conversion factor t(T/X), is the mass of the titrated substance X interacting with one ml of titrant T: t(T/X) = T(T)M(1/zX) /M(1/zT) = c(1/zT) M(1/zX)/1000. Measured in g/ml. Correction factor F (or K)- a number expressing the ratio of the actual (practical) concentration c(1/zA) pr of substance A in solution to its given (theoretical) concentration c(1/z A) theor: F = c(1/zA) pr/c(1/zA) theor. Calculation of the mass of a sample of a standard substance. Hitch weight t(A) standard substance A, necessary to obtain a solution with a given molar concentration of the equivalent с(1/zА), calculated using the formula: m(A) = с(1/z А)М(1/z A)VA), where M(1/z A) is the molar mass of the equivalent of substance A. If the molar concentration c(A) is specified, then the mass of the sample is calculated similarly using the formula: t(A) = c(A)M(A)V(A), Where M/(A) is the molar mass of substance A. The mass of the sample is usually weighed on an analytical balance with a weighing error of ±0.0002 g. The concentration of the titrant T when standardized against a standard solution of substance A is calculated as follows. Let the reaction T + A = B occur during standardization. According to the law of equivalents, equivalent amounts of substances T, A And B are equal to n (1/z T) = n (1/z A) = n (1/z V), the equivalent amount of a substance is equal to the product of the molar concentration of the equivalent of this substance by the volume of its solution: c(1/z T)= c(1/z A)V(A)/V(T) = c( 1/z IN) V(B)/V(T).

Classification of titrimetric analysis methods - acid-base, oxidation-reduction, precipitation, complexometric. Types of titration (direct, reverse, indirect). Methods for establishing the titration point.

1) Acid-base titration (neutralization method)- tit
ation based on the proton transfer reaction from one reacting
particles to another in solution. There are acidimetry and alkalimetry.

Acidimetry (acidimetric titration)- determination of substances by titration with a standard acid solution.

Alkalimetry (alkalimetric titration)- determination of substances by titration with a standard solution of a strong base.

2) Redox titration (redoxmetry)-
titration accompanied by the transition of one or more numbers

electrons from a donor ion or molecule (reducing agent) to an acceptor (oxidizing agent).

3) Precipitation titration- a titration when the titrated substance, upon interaction with the titrant, is released from the solution in the form of a precipitate

4) Compleximetric titration- titration of a substance with a solution [of a compound that forms a weakly dissociating soluble complex with the titrated substance.

A type of compleximetric titration is complexometric titration (complexometry)- such a titration when the titrated substance, when interacting with a titrant - a solution of complexones - forms metal complexonates.

Direct titration- this is a titration when the analyte is directly titrated with a standard titrant solution or vice versa. Back titration (residue titration)- titration of unreacted substance, which is added in excess to the analyzed solution in the form of a standard solution. Indirect titration (substitution titration)- titration, in which the substance being determined does not react directly with the titrant, but is determined indirectly as a result of the use of a stoichiometric reaction to form another substance that reacts with the titrant. Methods for establishing titration endpoints There are two groups of methods for fixing CTT: visual and instrumental.

Visual methods. The progress of the reaction is monitored visually, observing the change in color (or other property) of the specially introduced indicator | by neutralization, oxidation-reduction, precipitation or complexation. CTT is determined by a sharp change in the visible property of the system in the presence of an indicator or without it: the appearance, change, disappearance of color, the formation or dissolution of a precipitate. B indicator In visual methods, an indicator is added to the titrated solution. IN non-indicator visual methods use the color of the titrant or titrated substance. CTT is determined by the appearance of the titrant color or the disappearance of the color of the titrated substance.

Instrumental methods. CTT is determined by changes in the physicochemical properties of the solution - fluorescence, optical density, potential, specific electrical conductivity, current strength, radioactivity, etc. Changes in physicochemical properties are recorded on various instruments.

Acid–base titration. Basic reactions and titrants of the method. Types of acid-base titration (alkalimetry and acidimetry). Indicators, requirements for them. Ionic, chromophore, ion-chromophoric theories of acid-base titration indicators.

ACID-BASE TITRATION - this is a method for determining acids, bases, salts, based on the interaction reaction between proto-lites - acid NA and base B: NA + B = A "+ HB + In aqueous solutions - this is the neutralization reaction of H 3 0 + +0H = 2H 2 0 therefore, the acid-base titration method is also called the neutralization method. The titrants of the method are solutions of strong acids and bases: HC1, H 2 S0 4, NaOH, KOH. These substances do not meet the requirements for standard substances, therefore the concentration of titrants is established by standardization. their solutions, borax Na 2 B 4 0 7 10H 2 O, anhydrous sodium carbonate Na 2 C0 3, oxalic acid dihydrate H 2 C 2 0 4 2H 2 0 and some others are most often used as primary standards. Acidimetric titration (acidimetry)- a method for determining strong and weak bases, salts of weak acids, basic salts and other compounds with basic properties by titration with a standard solution of a strong acid. Alkalimetric titration (alkalimetry)- a method for determining strong and weak acids, acid salts, salts of weak bases by titration with a standard solution of a strong base. Indicator- is a substance that exhibits a visible change at or near its equivalence point.

The acid-base indicator is itself an acid or base and during acid-base titration changes its color in TE or

near her. (Methyl orange рТ=4 pH transition interval and indicator color 3.1–4.4 Red – orange-yellow; Phenolphthalein рТ=9.0 8.2–10 Colorless – violet).

Requirements for indicators:1) coloring d.b. intense, different in acidic and alkaline environments 2) color change d.b. clear in a narrow pH range of the solution 3) indicator d.b. sensitive 4) ind-r d.b. stable, does not decompose in air, in solution. Indicator theories:

1) ionic (Ostwald theory) - indicators are weak acids or bases that ionize in aqueous solutions

HInd↔H+ +Ind-. Disadvantages: 1) it only states the differences in color in acidic and alkaline. Wed, but does not explain the nature of the color 2) ion reaction occurs instantly, and the indicator changes color only over time

2) Chromophore - the presence of color is explained by the appearance of chromophore groups. Indices in the solution are present in the form of tautomeric forms. Disadvantages: does not explain why tautomeric transformations occur when the pH changes.

3) ion-chromophoric-acid-base indicators are weak acids and bases, with the neutral indicator molecule and its ionized form containing different chromophoric groups. Indicator molecules in an aqueous solution are capable of either donating hydrogen ions (the indicator is a weak acid) or accepting them (the indicator is a weak base), while undergoing tautomeric transformations.

REACTION (see notebook topic acid-base titration)

Acid-base titration curves. Calculation, construction and analysis of typical titration curves of a strong acid with an alkali and a strong and weak base with an acid. Selection of indicators based on the titration curve. Titration of polyprotic acids. Errors in acid-base titration, their calculation and elimination.

Acid-base titration curves graphically display the dependence of the change in pH of the titrated solution on the volume of added titrant or on the degree of titration f= V(T)/V, where V(T) and V are, respectively, the volume of the added titrant at a given moment and in the fuel cell. Most often (though not always) when constructing acid-base titration curves, the volume of the added titrant or the degree of titration is plotted along the abscissa axis, and along the axis ordinate - pH value of the titrated solution.

Calculation, construction and analysis of titration curves. To construct an acid-base titration curve, the pH values ​​of the titrated solution are calculated at various points in the titration, i.e. at different titration points: for the initial solution, for solutions before TE, in TE and after TE.

After the start of titration and before TE, the pH value of the solution is determined as pH = -1 8 s(X)

Calculation of pH at the equivalence point. When titrating a strong acid with a strong base, the medium in the fuel cell is neutral, pH = 7.

Calculation of pH after TE. determined by concentration c(T) alkali added in excess of the stoichiometric amount. Considering that pH + pOH = 14, we can write: pH = 14-pOH

The formulas are used to calculate the pH values ​​of the solution at different moments of titration, and based on the calculated data, a titration curve is constructed in pH-V coordinates (T).

Calculated titration curve for 20 ml of 0.1000 mol/l HC1 solution with 0.1000 mol/l NaOH solution

To determine the CTT in this case, you can use acid-base titration indicators such as methyl orange (pT = 4), methyl red (pT = 5.5), bromothymol blue (pT = 7.0), phenolphthalein (pT = 9) and others, for which the pT value lies in the range from 3 to 11. Methyl orange and phenolphthalein are most often used as the most accessible indicators of acid-base titration. Usually, one strives to choose an indicator so that, other things being equal, the pH value of the indicator would be as close as possible to the pH value of the solution in TE, since this reduces the titration error.

Titration of a strong base with a strong acid. When titrating a strong base with a strong acid, for example, a solution of Sodium hydroxide with a solution of hydrochloric acid, processes similar to those discussed in the previous section occur, but only in the opposite Direction: as the titrant is added, the pH value of the solution does not increase, but decreases. For the initial solution of a strong base and the titrated of a solution, the pH value before TE is determined by the concentration of alkali in the solution. In TE, the solution is neutral, pH = 7. After TE, the pH value of the solution is determined by the presence of excess titrant - a strong acid

Titration of polyacid bases. Solutions of polyacid bases are titrated with a solution of a strong acid sequentially, stepwise. At an acceptable level of titration, jumps in the titration curve are separated if differences in the values рК b, successive steps of base dissociation are at least 4 units, as in the case of titration of solutions of polybasic acids with a solution of a strong base.

Errors in main title: 1) measurement error (error of the burette, pipettes) If the solution is taken using a burette, then two measurements of the volume of the solution in the burette are carried out: before and after taking the solution. The random error of each such measurement when using conventional laboratory burettes is approximately ±(0.01-0.02) ml. If the volume of the sampled solution is equal to V, then the maximum random relative error e of measuring the volume taken for titration will be (in percent): έ = ±ν*100%/V, where ν = 0.02 + 0.02 = 0.04 ml. With the volume of the selected solution V= 20 ml, the maximum relative error in measuring the volume of a solution using a burette will be έ= ±0.04 100%/20 =0.2%.

The value of έ can be reduced by increasing the volume V sample solution.

2) methodological errors 3) systematic errors (incorrect indicator selection, discrepancy between the equivalence point and the end point of titration) a) indicator - the difference in the amount of titrant found at the end point of titration and the amount of titrant in t.eq.

a.1.) hydrogen error (X H3O+, XH+) - associated with overtitration of the solution with a strong acid (then the error is +) or undertitration (-) XH3O+ = a/a*100%

a-number of excess equivalents of H+ ions to total number of equivalents

а′=СН3о+ *V

a=СН3о+ * V(а+в)=СН3о+ * (Va+Vb)

C Н3o+=10(in step – рН)

Substitute into our enemy

X n3o+= +-(10 - pT)*(Va+Vb)/Cb*Vb)*100%

b-acid a-alkali.pT-display titre ind

a.2.) hydroxide error (main) - associated with an excess number of OH groups during titration with a strong base, or with under titration of a base solution

a.3.) acid error - caused by the presence of a certain amount of subtitric acid at the end point of the titration (weak acid)

Oxidation-reduction titration. The essence of the method. Classification of redox methods. Conditions for redox titration. Requirements for reactions. Types of redox titration (direct, reverse, substitution). Examples of redox indicators. Formulas, color transition at the equivalence point.

Oxidation-reduction titration(redoximetry, oxidimetry.)

Redox methods include a broad group of titrimetric analysis methods based on the occurrence of redox reactions. Redox titrations use various oxidizing and reducing agents. In this case, it is possible to determine reducing agents by titration with standard solutions of oxidizing agents, and vice versa, determining oxidizing agents with standard solutions of reducing agents. Due to the wide variety of redox reactions, this method makes it possible to determine a large number of different substances, including those that do not directly exhibit redox properties. In the latter case, back titration is used. For example, when determining calcium, its ions precipitate oxalate - an ion

Ca 2+ + C 2 O 4 2- ® CaC 2 O 4 ¯

The excess oxalate is then titrated with potassium permanganate.

Redox titration has a number of other advantages. Redox reactions occur quite quickly, allowing titration to be carried out in just a few minutes. Many of them occur in acidic, neutral and alkaline environments, which significantly expands the possibilities of using this method. In many cases, fixing the equivalence point is possible without the use of indicators, since the titrant solutions used are colored (KMnO 4, K 2 Cr 2 O 7) and at the equivalence point the color of the titrated solution changes from one drop of titrant. The main types of redox titrations are distinguished by the oxidizing agent used in the reaction.

Redox titration (redoximetry), depending on the nature of the reagent, is divided into permanganate, dichromate, cerium, iodo, bromato and iodotometry. They are based on the occurrence of a redox reaction, the essence of which is the transfer of an electron from a reducing agent to an oxidizing agent.

Types of OM titration:

Direct titration is that the solution of the analyte A titrate with standard titrant solution IN. The direct titration method is used to titrate solutions of acids, bases, carbonates, etc.

Back titration used in cases where direct titration is not applicable: for example, due to a very low content of the analyte, the inability to determine the equivalence point, when the reaction proceeds slowly, etc. During back titration to an aliquot of the analyte A pour in a precisely measured volume of a standard solution of the substance IN taken in excess. Unreacted excess substance IN determined by titration with a standard solution of the excipient WITH. Based on the difference in the initial amount of the substance IN and its amount remaining after the reaction, determine the amount of substance IN, which reacted with the substance A, on the basis of which the substance content is calculated A.

Indirect titration or titration by substituent. Based on the fact that it is not the substance being determined that is titrated, but the product of its reaction with the auxiliary substance WITH.

Substance D must be formed strictly quantitatively in relation to the substance A. Having determined the content of the reaction product D titration with a standard solution of the substance IN, Using the reaction equation, the content of the analyte is calculated A.

Redox titration curves, errors, their origin, calculation, elimination. Permanganatometry. The essence of the method, titration conditions, titrant, its preparation, standardization, establishment of the equivalence point. Application of permanganatometry.

Redox titration curves

Redox titration curves show the change in redox potential during the titration process: E = ƒ(V PB), (Fig. 2.7) Redox titration involves two redox systems - the titrated substance and the titrant. The potential of each of them can be calculated using the Nernst equation using the corresponding half-reaction. After adding each portion of titrant, equilibrium is established in the solution and the potential can be calculated using any of these pairs. It is more convenient to calculate the potential for the substance that is in excess in the titrated solution at the moment of titration, i.e. Before the equivalence point, the potential is calculated from the half-reaction involving the titrated substance, and after the equivalence point, from the half-reaction involving the titrant. Before titration begins, it is considered that for the titrated substance the concentrations of the oxidized and reduced forms differ by 1000 or 10,000 times. At the equivalence point, both conjugate forms of the oxidizing agent and the reducing agent are present in equal amounts, so the redox potential can be calculated as the sum of the potentials:

Transforming the equation, we get:

Where n 1, n 2 – the number of electrons participating in half-reactions of oxidation and reduction, respectively; E 0 1 , E 0 2 – standard redox potential for an oxidizing agent and a reducing agent, respectively.

Rice. Titration curves in the redoximetry method:

1 – the reducing agent is titrated with the oxidizing agent; 2 – the oxidizing agent is titrated with a reducing agent

Near the equivalence point on the titration curve there is a potential jump, the magnitude of which is greater, the greater the difference between E 0 ok and E 0 v-la. Indicator titration is possible if EMF = E 0 ok-la – E 0 v-la ≥ 0.4 V. If EMF = 0.4 - 0.2 V, you can use instrumental titration, where the equivalence point is fixed using instruments. If EMF< 0,2 IN direct redoximetric titration is not possible. The magnitude of the jump is significantly affected by a decrease in the concentration of one of the components of the redox pair. This is sometimes used to increase the jump in the titration curve, which is sometimes necessary when choosing an indicator.

For example, if Fe 2+ is titrated with any oxidizing agent, the redox couple Fe 3+ /Fe 2+ is used to calculate the redox potential to the equivalence point. The initial potential can be reduced by binding Fe 3+ ions into some low-dissociation complex, by adding, for example, fluorides or phosphoric acid. This is what is done when determining Fe 2+ by dichromatometry. The jump is observed in the range of 0.95 - 1.30 V. To carry out titration in the presence of the redox indicator diphenylamine ( E 0 = 0.76 V), it is necessary to shift the jump towards lower potential values. When adding the specified complexing agents, the jump is in the range of 0.68 – 1.30 V . The color transition potential of diphenylamine is within the jump range and can be used for Fe 2+ titration. The magnitude of the jump also depends on the pH of the medium in which the reaction is carried out. For example, for the half-reaction: MnO 4 - + 8H + + 5e – → Mn 2+ + 4H 2 O system potential

will increase with decreasing pH of the medium, which will affect the magnitude of the jump in the titration curve. Redox titration curves are asymmetrical around the equivalence point if the number of electrons involved in the oxidation and reduction half-reactions are not equal ( n 1 ≠ n 2). The equivalence point in such cases is shifted towards E 0 of the substance for which n more. When titrating mixtures of oxidizing or reducing agents, there may be several jumps in the titration curve if the difference between the redox potentials of the corresponding redox pairs is large enough, in which case separate determination of the components of the mixture is possible.

PERMANGANOMETRY

Permanganatometry- a method based on the use of potassium permanganate as a titrant for the determination of compounds that have reducing properties.

The reduction products of permanganate ions can be different depending on the pH of the environment:

Ø in a strongly acidic environment

+ 5e+ MnO 4 - + 8H + ↔ Mn 2+ + 4H 2 O E 0= 1.51 V

Ø slightly acidic or neutral environment

+ 3e+ MnO 4 - + 4H + ↔ MnO 2 ↓ + 2H 2 O E 0= 1.69 V

Ø slightly alkaline environment

+ 3e+ MnO 4 - + 2H 2 O ↔ MnO 2 ↓ + 4OH - E 0= 0.60 V

For analysis, the oxidative properties of MnO 4 - - ions in a strongly acidic environment are most often used, since the product of their reduction in this case is colorless ions Mn 2+ ( in contrast to the brown precipitate MnO 2), which do not interfere with observing a change in the color of the titrated solution from an excess drop of titrant. The required pH value of the medium is created using a solution of sulfuric acid. Other strong mineral acids are not used. Thus, nitric acid itself has oxidizing properties, and in its presence, side reactions become possible. In a solution of hydrochloric acid (in the presence of traces of Fe 2+), an oxidation reaction of chloride ions occurs. Titrant method- a solution of 0.1 * (0.05) mol/dm 3 potassium permanganate - prepared as a secondary standard solution and standardized according to standard substances: oxalic acid, sodium oxalate, arsenic (ΙΙΙ) oxide, Mohr's salt (NH 4) 2 Fe(SO 4) 2 ∙ 6H 2 O and etc.

It is impossible to prepare a titrated solution of potassium permanganate from an accurate weighing of the crystalline preparation, since it always contains a certain amount of MnO 2 and other decomposition products. Before establishing the exact concentration, the KMnO 4 solution is kept in a dark bottle for 7-10 days. During this time, oxidation of reducing agents occurs, the presence of which in distilled water cannot be completely eliminated (dust, traces of organic compounds, etc.). To speed up these processes, a solution of potassium permanganate is sometimes boiled. It must be taken into account that water has redox properties and can reduce permanganate. This reaction is slow, but MnO 2 and direct sunlight catalyze the decomposition process of KMnO 4, so after 7-10 days the MnO 2 precipitate must be removed. The KMnO 4 solution is usually carefully drained from the sediment or filtered through a glass filter. The KMnO 4 solution prepared in this way is not of too low a concentration (0.05 mol/dm 3 or higher) and does not change the titer for a long time. The titer of a potassium permanganate solution is most often determined by anhydrous sodium oxalate Na 2 C 2 O 4 or oxalic acid H 2 C 2 O 4 ∙ 2H 2 O:

МnО 4 - + 5НС 2 О 4 - + 11H + ↔ 2Мn 2+ + 10СО 2 + 8Н 2 О

The first drops of permanganate, even in a hot solution, discolor very slowly. During titration, the concentration of Mn 2+ ions increases and the reaction rate increases. The titer of potassium permanganate can also be determined by arsenic (II) oxide or metallic iron. The use of metallic iron to establish the titer is especially advisable if permanganatometric determination of this element is planned in the future.

In permaganatometry, solutions of reducing agents are also used - Fe (II) salts, oxalic acid and some others - to determine oxidizing agents by back titration. Fe(II) compounds slowly oxidize in air, especially in a neutral solution. Acidification slows down the oxidation process, but it is usually recommended to check its titer before using a Fe (II) solution in an analysis. Oxalates and oxalic acid in solution slowly decompose:

H 2 C 2 O 4 ↔ CO 2 + CO + H 2 O

This process accelerates in light, so it is recommended to store oxalate solutions in dark bottles. Acidified oxalate solutions are more stable than neutral or alkaline solutions.

In permanganatometry, the use of a special indicator is often dispensed with, since the permanganate itself has an intense color, and an excess drop of it causes the appearance of a pink color of the solution that does not disappear within 30 s. When titrating with dilute solutions, redox indicators are used, such as diphenylamine sulfonic acid or ferroin (a coordination compound of Fe (II) with 1,10-phenanthroline). Determination of the titration end point is also performed using potentiometric or amperometric methods.

The permanganometric method can be used to determine:

Ø reducing agents H 2 O 2, NO 2, C 2 O 4 2-, Fe 2+ etc.,

Ø Ca 2+, Ba 2+ and other cations in various preparations;

Ø MnO 2, PbO 2, K 2 Cr 2 O 7, persulfates and other oxidizing agents by back titration. The second standard solution in this case is a solution of a reducing agent (usually oxalic acid or Mohr’s salt). In this case, the oxidizing agents are reduced with a titrated solution of oxalic acid or Mohr's salt, the excess of which is titrated with a solution of potassium permanganate.

For example, when analyzing lead dioxide, the sample is dissolved in a sulfate solution of oxalic acid:

MnO 2 + HC 2 O 4 - + 3H + ↔ Mn 2+ + 2 CO 2 + 2H 2 O

and excess oxalic acid is titrated with potassium permanganate.

Permanganatometry can be used to determine ions that do not have redox properties (substituent titration). This method can be used to determine, for example, cations of calcium, strontium, barium, lead, zinc and others, which form poorly soluble oxalates.

Analysis of organic compounds. The oxidation of organic compounds by potassium permanganate occurs at a low rate, which hinders the practical application of this method for the analysis of organic substances. Nevertheless, some organic substances can be successfully determined by this method using the reduction of MnO 4 - in an alkaline medium. Organic compounds are usually oxidized to carbonate. At the end of the reduction reaction of the permanganate in an alkaline medium, the solution is acidified and titrated with MnO 4 - a solution of iron (II) or another suitable reducing agent. This is how, for example, methanol is determined, which in an alkaline environment is oxidized with potassium permagane according to the following scheme:

CH 3 OH + 6MnO 4 - + 8OH- ↔ CO 3 2- + 6MnO 4 2- + 6H 2 O

This method can also determine formic, tartaric, citric, salicylic and other acids, glycerin, phenol, formaldehyde and other organic compounds.

Permanganatometry is pharmacopoeial method of analysis.

Dichromatometry. The essence of the method, titration conditions, titrant, its preparation, establishment of the equivalence point. Iodine - Iodometric titration. The essence of the method, titration conditions, titrant, its preparation, establishment of the equivalence point.

Dichromatometry- determination method based on the oxidation of substances with dichromate ions. It is based on the half-reaction:

+ 6e+ Cr 2 O 7 2- + 14H + ↔ 2Cr 3+ + 7H 2 O E 0= 1.33 V;

f (K 2 Cr 2 O 7) = 1/6.

in an acidic environment, K 2 Cr 2 O 7 is a strong oxidizing agent, therefore, this method can determine a number of inorganic and organic reducing agents, for example Fe 2+, 4-, SO 3 2-,

Problems for independent solution. 1. The demand curves for peaches purchased by Andrey and Dmitry are represented by the following functions: and

The supply curves of a rational monopolist and the demand for its products tend to intersect or not. If so, at what point?

Firm's supply curves in the short and long run

Distribution curves of induction along the armature circumference and voltage Uк along the collector