Electrochemical methods of analysis- this is a set of methods of qualitative and quantitative analysis based on electrochemical phenomena occurring in the medium under study or at the interface and associated with a change in structure, chemical composition or analyte concentration.
Varieties of the method are electrogravimetric analysis (electroanalysis), internal electrolysis, contact metal exchange (cementation), polarographic analysis, coulometry, etc. In particular, electrogravimetric analysis is based on weighing a substance released on one of the electrodes. The method allows not only to carry out quantitative determinations of copper, nickel, lead, etc., but also to separate mixtures of substances.
In addition, electrochemical methods of analysis include methods based on measuring electrical conductivity (conductometry) or electrode potential (potentiometry). Some electrochemical methods are used to find the end point of a titration (amperometric titration, conductometric titration, potentiometric titration, coulometric titration).
There are direct and indirect electrochemical methods. In direct methods, the dependence of the current strength (potential, etc.) on the concentration of the analyte is used. In indirect methods, the current strength (potential, etc.) is measured in order to find the end point of the titration of the component to be determined with a suitable titrant, i.e. use the dependence of the measured parameter on the volume of the titrant.
Any kind of electrochemical measurement requires an electrochemical circuit or an electrochemical cell, integral part which is the analyzed solution.
Electrochemical methods are classified depending on the type of phenomena measured during the analysis. There are two groups of electrochemical methods:
1. Methods without superimposing an extraneous potential, based on measuring the potential difference that occurs in an electrochemical cell consisting of an electrode and a vessel with a test solution. This group of methods is called potentiometric. In potentiometric methods, the dependence of the equilibrium potential of the electrodes on the concentration of ions involved in the electrochemical reaction on the electrodes is used.
2. Methods with the imposition of an extraneous potential, based on the measurement of: a) the electrical conductivity of solutions - conductometry; b) the amount of electricity passed through the solution - coulometry; c) the dependence of the current on the applied potential - voltammetry; d) the time required for the passage of an electrochemical reaction - chronoelectrochemical methods(chronovoltammetry, chronoconductometry). In the methods of this group, an extraneous potential is applied to the electrodes of the electrochemical cell.
The main element of devices for electrochemical analysis is an electrochemical cell. In methods without the imposition of an extraneous potential, it is galvanic cell, in which, due to the occurrence of chemical redox reactions, electricity. In a cell of the galvanic cell type, two electrodes are in contact with the analyzed solution - an indicator electrode, the potential of which depends on the concentration of the substance, and an electrode with a constant potential - a reference electrode, relative to which the potential of the indicator electrode is measured. Measurement of the potential difference is carried out with special devices - potentiometers.
In methods with superimposed extraneous potential, electrochemical cell, so named because electrolysis occurs on the electrodes of the cell under the action of an applied potential - the oxidation or reduction of a substance. Conductometric analysis uses a conductometric cell in which the electrical conductivity of a solution is measured. According to the method of application, electrochemical methods can be classified into direct methods, in which the concentration of substances is measured according to the indication of the instrument, and electrochemical titration, where the indication of the equivalence point is fixed using electrochemical measurements. In accordance with this classification, there are potentiometry and potentiometric titration, conductometry and conductometric titration, etc.
Instruments for electrochemical determinations, in addition to the electrochemical cell, stirrer, load resistance, include devices for measuring the potential difference, current, solution resistance, and the amount of electricity. These measurements can be carried out by pointer instruments (voltmeter or microammeter), oscilloscopes, automatic recording potentiometers. If the electrical signal from the cell is very weak, then it is amplified with the help of radio amplifiers. In devices of methods with superimposed extraneous potential, an important part is the devices for supplying the cell with the appropriate potential of a stabilized direct or alternating current (depending on the type of method). The power supply unit for electrochemical analysis instruments usually includes a rectifier and a voltage stabilizer, which ensures the stability of the instrument.
Potentiometry combines methods based on measuring the emf of reversible electrochemical circuits when the potential of the working electrode is close to the equilibrium value.
Voltammetry is based on the study of the dependence of the polarization current on the voltage applied to the electrochemical cell, when the potential of the working electrode differs significantly from the equilibrium value. It is widely used to determine substances in solutions and melts (for example, polarography, amperometry).
Coulometry combines methods of analysis based on measuring the amount of a substance released at an electrode during an electrochemical reaction in accordance with Faraday's laws. In coulometry, the potential of the working electrode differs from the equilibrium value.
Conductometric analysis is based on a change in the concentration of a substance or the chemical composition of the medium in the interelectrode space; it is not related to the potential of the electrode, which is usually close to the equilibrium value.
Dielectrometry combines methods of analysis based on measuring the dielectric constant of a substance, due to the orientation of particles (molecules, ions) with a dipole moment in an electric field. Dielectrometric titration is used to analyze solutions.
Course work
«ELECTROCHEMICAL METHODS
RESEARCH"
Introduction
1. Theoretical basis electrochemical research methods
1.1 History of the method
1.2 Description of electrochemical research methods
1.3 Potentiometry
1.4 Conductometry
1.5 Coulometry
1.6 Voltammetry
1.7 Electrogravimetry
2. Experimental part of electrochemical research methods
2.1 Determination of the concentration of acids by conductometric titration
2.2 Potentiometric titration
2.3 Electrolysis
2.4 Determination of electrode potentials
2.5 Determination of the EMF of a galvanic cell
Conclusion
Bibliography
Introduction
AT modern world more and more influence scientific and technological progress to all areas of our life. In this regard, there is a need for more accurate and faster methods of analysis. Electrochemical research methods (ECMI) meet these requirements most strongly. They are the main physicochemical methods for studying substances.
ECMI are based on the processes taking place on the electrodes or the interelectrode space. They are one of the oldest physical and chemical research methods (some were described at the end of the 19th century). Their advantage is high accuracy and comparative simplicity. High accuracy is determined by very precise laws used in EMHI, for example, Faraday's law. A great convenience is that ECMI uses electrical effects, and the fact that the result of this effect (response) is also obtained in the form of an electrical signal. This provides high speed and accuracy of counting, opens up wide possibilities for automation. ECMI are distinguished by good sensitivity and selectivity, in some cases they can be attributed to microanalysis, since sometimes less than 1 ml of solution is sufficient for analysis.
Equipment designed for electrochemical analysis is relatively cheap, readily available, and easy to use. Therefore, these methods are widely used not only in specialized laboratories, but also in many industries.
The purpose of the work: the study of electrochemical methods for studying the composition of a substance.
To achieve this goal, it was necessary to solve the following tasks:
consider electrochemical research methods, their classification and essence;
to study potentiometric and conductometric titration, determination of electrode potentials and electromotive force (EMF) of a galvanic cell, as well as the electrolysis process in practice.
Object of study: application of electrochemical methods in the analysis of the properties and composition of matter.
Subject of study: mechanisms of electrochemical processes, potentiometry, conductometry, coulometry, voltammetry, electrogravimetry.
electrochemical titration galvanic
1.Theoretical foundations of electrochemical research methods
1 History of the origin of the method
Systematic electrochemical studies became possible only after the creation of a permanent sufficiently powerful source of electric current. Such a source appeared at the turn of the 18th-19th centuries. as a result of the work of L. Galvani and A. Volta. While studying the physiological functions of a frog, Galvani accidentally created an electrochemical circuit consisting of two different metals and the muscle of a prepared frog's leg. When the foot, fixed with a copper holder, was touched with an iron wire, also connected to the holder, the muscle contracted. Similar contractions occurred under the action of an electric discharge. Galvani explained this phenomenon by the existence of "animal electricity". A different interpretation of these experiments was given by Volta, who considered that electricity arises at the point of contact of two metals, and the contraction of the frog muscle is the result of the passage of an electric current through it. A current also arose when a spongy material (cloth or paper) impregnated with salt water was placed between two metal disks, for example, zinc and copper, and the circuit was closed. Consistently connecting 15-20 such "elements", Volta in 1800 created the first chemical source of current - the "volt column".
The effect of electricity on chemical systems immediately interested many scientists. Already in 1800, W. Nicholson and A. Carlyle reported that water decomposes into hydrogen and oxygen when an electric current is passed through it using platinum and gold wires connected to a "voltaic column". The most important of the early electrochemical studies were the work of the English chemist H. Davy. In 1807, he isolated the element potassium by passing a current through slightly moistened solid potassium hydroxide. A battery of 100 galvanic cells served as a voltage source. Metallic sodium was obtained in a similar way. Later, Davy, using a mercury electrode, isolated magnesium, calcium, strontium and barium by electrolysis.
Davy's assistant M. Faraday investigated the relationship between the amount of electricity (the product of current strength and time) flowing through an electrode/solution interface and the chemical changes it caused. An instrument (now known as a gas coulometer) was created to measure the amount of electricity from the volume of hydrogen and oxygen released in the electrolytic cell, and it was shown (1833) that the amount of electricity needed to obtain a given amount of substance does not depend on the size of the electrodes, the distance between them and the number of plates in the battery feeding the cell. In addition, Faraday found that the amount of a substance released during electrolysis is directly proportional to its chemical equivalent and the amount of electricity that has passed through the electrolyte. These two fundamental provisions are called Faraday's laws. Together with his friend W. Whewell, a specialist in classical philology, Faraday also developed a new terminology in electrochemistry. He called the conductors immersed in the solution electrodes (previously they were called poles); introduced the concept of "electrolysis" ( chemical changes associated with the passage of current), "electrolyte" (the conductive liquid in electrochemical cells), "anode" (the electrode at which the oxidation reaction occurs) and "cathode" (the electrode at which the reduction reaction occurs). He called charge carriers in liquids ions (from the Greek “wanderer”, “wanderer”), and the ions moving towards the anode (positive electrode) were called “anions”, and towards the cathode - “cations”. Faraday's research on electromagnetic induction led to the creation of electrical generators, which made it possible to carry out electrochemical processes on an industrial scale.
Faraday explained the ability of solutions to pass electric current by the presence of ions in them, however, he himself and other scientists, such as I. Gittorf and F. Kohlrausch, believed that ions appear under the influence of current. In 1884, S. Arrhenius suggested that in fact ions are formed simply by dissolving salt in water. The works of S. Arrhenius, J. van't Hoff and W. Ostwald were an important milestone in the development of the theory of electrolytes and ideas about the physicochemical properties of solutions and their thermodynamics. The agreement between theory and experimental data on ionic conductivity and equilibria in solution became more complete after P. Debye and E. Hückel took into account long-range electrostatic interactions between ions in 1923.
The first attempt to find out the causes of the potential difference between the solution and the metal was made in 1879 by G. Helmholtz, who showed that this potential difference is caused by a double electric layer, positive side which is located on the metal, negative - in the liquid. Thus, G. Helmholtz considered the double layer as a flat capacitor. This model of the double layer remained out of the view of electrochemists for a long time. The microworld at the metal-solution boundary, where electrochemical processes take place, was still “waiting” for its time.
The French physicist J. Gouy in 1910 and the English electrochemist D. Chapman in 1913 showed that electrolyte ions are not located in the same plane (as G. Helmholtz imagined), but form a certain “diffuse” region (as they move away from the surface metal, the concentration of ions gradually changes). The Gouy-Chapman double layer structure theory was further developed by the German scientist O. Stern. In 1924, he proposed taking into account the size of ions and the effect of adsorption of ions and dipole solvent molecules when describing the structure of the electrical double layer. The study of the differential capacitance of the double layer using new research methods allowed the Soviet scientist, academician A.N. Frumkin in 1934-1935. and the American scientist D. Graham in 1941 to establish the limits of applicability of the Gouy-Chapman-Stern theory. A.N. Frumkin suggested that the discrepancy between theory and experimental data is due to the discrete nature of the charge distribution in the double layer. This idea, first expressed in 1935, in the 1940s and 1950s received further development.
A serious contribution to electrochemical thermodynamics and specifically to the elucidation of the nature of the electric potential (voltage) in an electrochemical cell and the balance between electrical, chemical and thermal energy was made by J. Gibbs and W. Nernst. Yu. Tafel (1905), J. Butler (1924), M. Volmer (1930), A.N. Frumkin (1930-1933).
2 Description of electrochemical research methods
An electrochemical cell, which is a vessel with an electrolyte solution, into which at least two electrodes are immersed, serves as a tool for ECM. Depending on the problem being solved, the shape and material of the vessel, the number and nature of electrodes, solution, analysis conditions (applied voltage (current) and recorded analytical signal, temperature, mixing, purging with an inert gas, etc.) can be different. The substance to be determined can be included both in the composition of the electrolyte filling the cell and in the composition of one of the electrodes. If the redox reaction proceeds spontaneously on the electrodes of the cell, that is, without the application of voltage from an external source, but only due to the potential difference (EMF) of its electrodes, then such a cell is called a galvanic cell. If necessary, the cell can be connected to an external voltage source. In this case, by applying sufficient voltage, it is possible to change the direction of the redox reaction and the current to the opposite of what takes place in a galvanic cell. The redox reaction occurring on the electrodes under the action of an external voltage source is called electrolysis, and the electrochemical cell, which is the consumer of the energy necessary for the chemical reaction to proceed in it, is called an electrolytic cell.
ECMI is divided into:
) conductometry - measurement of the electrical conductivity of the test solution;
) potentiometry - measurement of the currentless equilibrium potential of the indicator electrode, for which the test substance is potentiodetermining;
) coulometry - measurement of the amount of electricity required for the complete transformation (oxidation or reduction) of the substance under study;
) voltammetry - measurement of stationary or non-stationary polarization characteristics of electrodes in reactions involving the test substance;
) electrogravimetry - measurement of the mass of a substance released from a solution during electrolysis.
ECMI can be subdivided according to the use of electrolysis. Coulometry, voltammetry and electrogravimetry are based on the principles of electrolysis; electrolysis is not used in conductometry and potentiometry.
ECMI are of independent importance for direct chemical analysis, but can be used as auxiliary in other methods of analysis. For example, it can be used in titrimetry to register the end of the titration not with the help of a chemical color-changing indicator, but by changing the potential, electrical conductivity of the current, etc.
Let us consider in more detail the processes occurring in electrochemical studies.
The electrode is a system, in the simplest case, consisting of two phases, of which the solid has electronic, and the other - liquid - ionic conductivity. The solid phase with electronic conductivity is considered to be a conductor of the first kind, and the liquid phase with ionic conductivity is considered to be of the second kind. When these two conductors come into contact, an electric double layer (DEL) is formed. It may result from the exchange of ions between the solid and liquid phases, or from the specific adsorption of cations or anions on the surface of the solid phase when it is immersed in water or solution.
At ionic mechanism formation of DES, for example, in the case when the chemical potential of atoms on the surface of a metal (solid phase) is greater than the chemical potential of ions in solution, then atoms from the metal surface will pass into solution in the form of cations: Me ? Me z+ +ze -. The released electrons in this case charge the surface of the solid phase negatively and, due to this, attract positively charged ions of the solution to the surface. As a result, two oppositely charged layers are formed at the phase boundary, which are, as it were, plates of a kind of capacitor. For the further transition of charged particles from one phase to another, they need to do work equal to the potential difference between the plates of this capacitor. If the chemical potential of the atoms on the surface of the solid phase is less than the chemical potential of the ions in the solution, then the cations from the solution pass to the surface of the solid phase, charging it positively: Me z+ +ze - ? me. Both in the first and in the second case, these processes do not proceed indefinitely, but until a dynamic equilibrium is established, which can be represented by a reversible redoxy transition of the Me - ze type -? Me z+ or in the general case Ox + I0 ? Red z+ .
The processes in which the return or attachment of electrons occurs at the electrodes are called electrode processes.
Nernst obtained a formula relating the difference between the internal potentials of the EDL and the activities (concentrations) of the particles involved in the reversible redoxy transition:
where ?(Me) is the potential of the charged layer of the solid phase;
?(solution) is the potential of the solution layer adjacent to the solid phase;
??0- standard electrode potential; - universal gas constant (8.31 J/K mol); - temperature, K; - Faraday number (96 488 C/mol); is the number of electrons participating in the redoxy transition;
a (Ox) and a (Red) are the activities of the oxidized (Ox) and reduced (Red) forms of the substance in the redoxy transition, mol/L.
Set the internal potentials of the individual phases ?(Me) and ?(p - p), unfortunately, experimentally impossible. Any attempt to connect the solution with a wire to the measuring device causes the appearance of a new metal-solution phase contact surface, that is, the appearance of a new electrode with its own potential difference that affects the measured one.
However, it is possible to measure the difference ?(Me)- ?(p - p) using a galvanic cell. A galvanic cell is a system composed of two different electrodes, which has the ability to spontaneously convert the chemical energy of the redox reaction occurring in it into electrical energy. The electrodes that make up a galvanic cell are called half cells. The redox reaction occurring in the galvanic cell is spatially separated. The oxidation half-reaction takes place on a half-cell called the anode (negatively charged electrode), and the reduction half-reaction takes place on the cathode.
The electromotive force (EMF) of a galvanic cell is algebraically composed of the differences in the internal potentials of its constituent electrodes. Therefore, if we take an electrode with a known value of the internal potential difference as one half-element ?(Me)- ?(solution), then the measured value of the EMF can be used to calculate the required potential difference of the electrode under study.
For this purpose, it is customary to use a standard (normal) hydrogen electrode (see Fig. 1). It consists of a platinum plate or wire coated with platinum black (fine platinum) immersed in an acid solution C=1 mol/l, hydrogen pressure over which is 0.1 MPa (1 atm). Under the catalytic influence of platinum black, a reversible redox transition takes place in the electrode. The difference in internal potentials for a hydrogen electrode in accordance with the Nernst formula is:
Figure 1. Scheme of a standard hydrogen electrode
since \u003d 1 mol / l, and p (H2 ) = 1 atm, then
?(Me)- ?(p - p) = ??0(2H+ /H 2).
The decision of the International Union of Pure and Applied Chemistry (IUPAC) is conventionally considered to be the value ??0(2H +/H 2) = 0.00 V. Obviously, in this case, the measured value of the EMF of the galvanic cell, which includes a hydrogen electrode, is equal to the difference in the internal potentials of the second electrode. This EMF is usually called the electrode potential or redoxy potential and is denoted by the letter E. The transition from internal potentials to redoxy potentials does not change the nature of the Nernst formula:
For most electrodes, the value of the electrode potential at single activities of the oxidized and reduced forms (E 0) measured and listed in reference books.
Under normal conditions and the transition from natural to decimal logarithms, the prelogarithmic factor becomes equal to 0.0591, and the formula becomes
It should be remembered that the Nernst formula relates the equilibrium potential to the activities (concentrations) of the redoxy pair, i.e. potential acquired by an insulated electrode. Therefore, for analytical circuits, the measurement of the electrode potential should be carried out under conditions as close as possible to equilibrium: in the absence of current in the external circuit of the galvanic cell and after a time sufficient to achieve equilibrium. However, in real conditions, current can flow through the electrodes. For example, a current flows through the electrodes in a galvanic cell, the operation of which is associated with the passage of charged particles through the “solution-solid phase” interface, and this directed movement of particles is a current. The current flows through the electrodes during electrolysis, which means a set of redox processes occurring on the electrodes in solutions and melts of the electrodes of electrolytes under the influence of an external electric current. During electrolysis, processes opposite to those occurring in a galvanic cell can be carried out.
When current (i) flows through the electrode, its potential changes and acquires a certain value Ei, different from the potential of the electrode in equilibrium (isolated) conditions Ep. The process of shifting the potential from Ep to Еi and the difference Еi-Ep is called polarization
E=Ei-Ep. (5)
Not all electrodes are subject to polarization processes. Electrodes, the potential of which does not change when current flows through them, are called non-polarizable, and electrodes, which are characterized by polarization, are called polarizable.
Non-polarizable electrodes include, for example, type II electrodes, and polarizable ones include all metal and amalgam electrodes.
1.3 Potentiometry
Potentiometry is an electrochemical method for the study and analysis of substances, based on the dependence of the equilibrium electrode potential on the activity of the concentrations of the ion being determined, described by the Nernst equation (1).
The dependence of electrode potentials on the nature of electrode processes and the activities of the substances involved in them makes it possible to use the measurement of EMF (potentiometric method) to find the activity coefficients of electrolytes, standard electrode potentials, equilibrium constants, solubility products, pH solutions, etc. The advantages of the potentiometric method are accuracy, objectivity and speed.
It is known that
is an important characteristic of the solution and determines the possibility and nature of many reactions.
The potentiometric determination of pH is based on the use of so-called indicator electrodes, in which hydrogen ions participate in the electrode reaction, and the potential depends on pH. By measuring the EMF of the element containing the indicator electrode with the test solution, it is possible to calculate the pH of this solution. The electrode with a known potential should be taken as the second electrode.
element emf
H 2| test solution || KCl, Hg2 Cl 2| hg
The potentiometric method for determining pH allows you to find the pH of turbid and colored media. When using a hydrogen electrode as an indicator, it is possible to determine the pH of solutions in a wide range (from pH 1 to pH 14). The disadvantage is the need for prolonged saturation of the electrode with hydrogen to achieve equilibrium. It cannot be used in the presence of surfactants and certain salts.
The scheme of the element used in this case is as follows:
| hg 2Cl 2, KC l || test solution + quinhydrone | Pt,
its emf is
(10)
The potentiometric method for determining the pH of a solution using a quinhydrone electrode is very simple. It is applicable for solutions with a pH of 1 to 8. In alkaline media, as well as in the presence of oxidizing or reducing agents, the quinhydrone electrode is unsuitable.
The so-called glass electrode is often used as an indicator electrode. It is a thin-walled glass ball, inside which is placed a reference electrode, such as silver chloride. Glass is a supercooled silicate solution containing alkali metal cations and type anions. The glass ball is preliminarily kept in a strong acid solution, where cations are exchanged between the glass and the solution, and the glass is saturated with hydrogen ions. When determining pH, a glass electrode and another reference electrode are lowered into the test solution. The result is the following chain:
The potential jump?1 at the interface between the glass and the potassium chloride solution included in the reference electrode is constant due to the constancy of the concentration of this solution. The potential jump?2 depends on the concentration of the test solution and can be written
(11)
Where ?o and m are constants for a given glass electrode. Taking into account the potential jumps on the glass surface, we obtain
(12)
(13)
where . From here
(14)
Constants for a given glass electrode ?° and m are determined by preliminary graduation. For this, a glass electrode is placed in several buffer solutions with a known pH and the EMF of the circuit is measured. Further, according to formula (14), the pH of the studied solutions is found.
Let's move on to the consideration of the activity coefficient of the electrolyte. Consider a double concentration chain without transfer containing two electrolyte solutions:
Pt, H 2| HCl, AgCl | Ag | AgCl, HCl | H2 , Pt
a1 a2
where a1 and a2 - medium ionic activities HCl solutions. It can be used to determine the activity coefficient of HCl. The emf of this circuit is
(15)
Substitution of the numerical values of R, F and T = 298 K and the transition to decimal logarithms gives
(16)
Substitute into the resulting equation
(17)
where m 1- average molality; ?1- average activity coefficient of the electrolyte.
We transfer to the left side of the equation the quantities determined empirically, and we get
(18)
In view of the fact that in the limiting case of an infinitely dilute solution, it should be close to ideal, and ?one ? 1, then B is
(19)
We build a dependence graph (or, which is more convenient, since it gives a line close to a straight line) and extrapolate to. Thus, we determine B graphically (Fig. 2).
Figure 2. Determination of the activity coefficient of the electrolyte
The activity coefficient is calculated by the equation
(20)
4 Conductometry
Conductometry- a set of electrochemical methods of analysis based on measuring the electrical conductivity of liquid electrolytes, which is proportional to their concentration.
Measurements of electrical conductivity (conductometry) allow solving a number of theoretical and practical problems. Such measurements can be carried out quickly and accurately. Using conductometry, one can determine the constant and degree of dissociation of a weak electrolyte, the solubility and solubility product of sparingly soluble substances, the ionic product of water, and other physicochemical quantities. In production, conductometric measurements are used to select electrolyte solutions with a sufficiently high conductivity to eliminate unproductive energy costs, to quickly and accurately determine the content of a dissolved substance, to automatically control the quality of various liquids, etc.
In a conductometric titration, the progress of the reaction is monitored by the change in electrical conductivity after each addition of a titrating reagent. It does not require the use of indicators and can be carried out in opaque media. In the process of conductometric titration, the ions of the titrated substance are replaced by ions of the added reagent. The equivalence point is determined by a sharp change in the electrical conductivity of the solution, which is explained by the different mobility of these ions.
On fig. 3 shows the dependence curves of the specific electrical conductivity (x) on the volume V of the added reagent. Titration strong acid strong base or a strong base with a strong acid (curve l), a minimum is formed on the titration curve, corresponding to the replacement of hydrogen or hydroxyl ions by less mobile ions of the resulting salt. When a weak acid is titrated with a strong base or a weak base with a strong acid (curve 2), the slope of the curve changes at the equivalence point, which is explained by a more significant dissociation of the resulting salt compared to the dissociation of the starting substance. In the case of titration of a mixture of strong (a) and weak (b) acids with a strong base (curve 3), two equivalence points are observed.
Figure 3. Conductometric titration curves.
With the help of tables of ionic electrical conductivities or by measurements ?at different concentrations of the solution and subsequent extrapolation to zero concentration, one can find ?°. If we measure the electrical conductivity of a solution of a given concentration, then according to the equation
(22)
we get the relation
(23)
Figure 4 Orientation polar molecules solvent near electrolyte ions
From the equations
(24) and , (25)
assuming , we get
(26)
(27)
It remains to take into account that the value ?is due only to this electrolyte and does not include the electrical conductivity of the solvent, i.e.
5 Coulometry
Coulometry- an electrochemical research method based on measuring the amount of electricity (Q) passed through the electrolyzer during electrochemical oxidation or reduction of a substance on the working electrode. According to Faraday's unified law, the mass of an electrochemically converted substance (P) in g is related to Q in C by the relation:
(28)
where M is molecular or atomic mass substances, n is the number of electrons involved in the electrochemical transformation of one molecule (atom) of the substance (M/n is the electrochemical equivalent of the substance), F is Faraday's constant.
Coulometry is the only physico-chemical method studies that do not require reference materials. A distinction is made between direct coulometry and coulometric titration. In the first case, an electrochemically active substance is determined, in the second case, regardless of the electrochemical activity of the analyte, an electrochemically active auxiliary reagent is introduced into the test solution, the product of the electrochemical transformation of which chemically interacts with the analyte at a high rate and quantitatively. Both variants of coulometry can be carried out at a constant potential E of the working electrode (potentiostatic mode) or at a constant electrolysis current I uh (galvanostatic mode). The most commonly used direct coulometry at constant E and coulometric titration at constant I uh . For a coulometric study, the following conditions must be met: the electrochemical transformation of a substance must proceed with a 100% current efficiency, i.e. there should be no side electrochemical and chemical processes; we need reliable ways to determine the amount of electricity and determine the moment of completion of an electrochemical or chemical reaction. In direct coulometry, a 100% current efficiency is ensured if the value of E is kept constant in the region of the limiting diffusion current I pr on the voltammogram of the analyte. In this case, the analyzed solution should be free of foreign substances capable of electrochemical transformation under the same conditions. The amount of electricity is usually determined using electronic current integrators. Sometimes they use less accurate instruments - coulometers various types, as well as planometric and calculation methods. In the last two cases, the completion of electrolysis is considered the moment when I uh drops to the value of the background current I f , so the amount of electricity required to complete the electrode reaction is equal to the difference Q about -Q f , where Q about - total electricity, Q f - amount of electricity measured under the same conditions for the same electrolysis time t uh , but in the absence of the analyte. If the electrochemical reaction is first order, then
(29)
(30)
where I t and I o - electrolysis current, respectively, at time t and at ?=0, - electrode surface area, - diffusion coefficient electrochemically active in-va,
?is the thickness of the diffusion layer, is the volume of the solution in the cell.
The duration of electrolysis does not depend on the initial concentration of the substance, but decreases markedly with an increase in the S/V ratio and with intensive stirring of the solution. We can consider electrolysis completed when I uh becomes equal to 0.1 I 0or 0.01 I 0(depending on the required accuracy of the analysis). In the planometric method, to establish Q, the area under the curve I is measured ? - ?, because
(31)
In the calculation method, the last equation is solved by substituting into it the expression for I ?. To find I 0and K" expression for I ?take logarithms and build a straight line lg I from several (5-7) points ?-?, the slope of which is equal to K", and the point of intersection with the y-axis corresponds to lg I 0, i.e. to determine Q, there is no need to carry out the electrolysis to the end and measure I 0, whose value is poorly reproduced.
Installations for coulometric research consist of a potentiostat or galvanostat, a recording potentiometer or current integrator, an electrolyzer and an indicator system (in the case of using physical and chemical methods to establish the end of a chemical reaction in coulometric titration).
Electrolyzers are, as a rule, glass vessels, in which the cathode and anode chambers are separated by a diaphragm (for example, made of porous glass). Noble metals (Pt, Au), electrodes of the second kind and, less often, carbon materials (graphite, glassy carbon, etc.) are used as working and auxiliary (closing the electrolysis circuit) electrodes. The solution in which the working electrode is immersed is usually stirred with a magnetic stirrer; if necessary, the experiment is carried out in an inert gas atmosphere.
Advantages of coulometric titration: no need to standardize titrant solutions; the titrant is added in very small portions (almost continuously); the solution is not diluted; it is possible to generate electrochemically inactive titrants, for example, complexon III, as well as unstable strong oxidizing and reducing agents, in particular, Mn(III), Pb(IV), Cr(II), V(II), Ti(III).
6 Voltammetry
Voltammetry- a set of electrochemical research and analysis methods based on studying the dependence of the current strength in an electrolytic cell on the potential of an indicator microelectrode immersed in the analyzed solution, on which the investigated electrochemically active (electroactive) substance reacts.
In addition to the indicator, an auxiliary electrode with a much higher sensitivity is placed in the cell so that when the current passes, its potential practically does not change (non-polarizable electrode). The potential difference between the indicator and auxiliary electrodes E is described by the equation
where U is the polarizing voltage, is the resistance of the solution.
An indifferent electrolyte (background) is introduced into the analyzed solution in a high concentration in order, firstly, to reduce the value of R and, secondly, to eliminate the migration current caused by the action electric field on electroactive substances (outdated - depolarizers). At low concentrations of these substances, the ohmic voltage drop IR in solution is very small. To fully compensate for the ohmic voltage drop, potentiostating and three-electrode cells containing an additional reference electrode are used. In these conditions
As indicator microelectrodes, stationary and rotating ones are used - from metal (mercury, silver, gold, platinum), carbon materials (for example, graphite), as well as dripping electrodes (from mercury, amalgam, gallium). The latter are capillaries from which liquid metal flows drop by drop. Voltammetry using dripping electrodes, the potential of which changes slowly and linearly, is called polarography (the method was proposed by J. Geyrovsky in 1922). Reference electrodes are usually electrodes of the second kind, such as calomel or silver chloride. Dependence curves I \u003d f (E) or I \u003d f (U) (voltammograms) are recorded with special devices - polarographs of various designs.
Figure 5. Voltammogram obtained using a rotating disk electrode
Voltamperograms obtained using a rotating or dripping electrode with a monotonous change (linear sweep) of the voltage have the form shown schematically in Figure 5. The section of the current increase is called a wave. Waves can be anodic if the electroactive substance is oxidized, or cathodic if it is reduced. When the oxidized (Ox) and reduced (Red) forms of the substance are present in the solution, reacting rather quickly (reversibly) on the microelectrode, a continuous cathode-anode wave is observed on the voltammogram, crossing the abscissa axis at a potential corresponding to the oxidizing-reducing. potential of the Ox/Red system in a given environment. If the electrochemical reaction on the microelectrode is slow (irreversible), the voltammogram shows an anodic wave of oxidation of the reduced form of the substance and a cathodic wave of reduction of the oxidized form (at a more negative potential). The formation of the limiting current area on the voltammogram is associated either with a limited mass transfer rate of the electroactive substance to the electrode surface by convective diffusion (limiting diffusion current, I d ), or with a limited rate of formation of an electroactive substance from the determined component in solution. Such a current is called limiting kinetic, and its strength is proportional to the concentration of this component.
The waveform for a reversible electrochemical reaction is described by the equation:
(33)
where R is the gas constant, T is the absolute temperature, is the half-wave potential, i.e. potential corresponding to half the wave height. The value is characteristic of a given electroactive substance and is used to identify it. When electrochemical reactions are preceded by the adsorption of the analyte on the electrode surface, the voltammograms show not waves, but peaks, which is due to the extreme dependence of adsorption on the electrode potential. The voltammograms recorded during a linear change (sweep) of the potential with a stationary electrode or on one drop of a dripping electrode also show peaks, the descending branch of which is determined by the depletion of the near-electrode layer of the solution with an electroactive substance. The height of the peak is proportional to the concentration of the electroactive substance. In polarography, the limiting diffusion current (in μA) averaged over the lifetime of a drop is described by the Ilkovich equation:
(34)
where n is the number of electrons involved in the electrochemical reaction, C is the concentration of the electroactive substance, D is its diffusion coefficient, the lifetime of a mercury drop, m is the rate of mercury outflow.
Voltammetry is used: for the quantitative analysis of inorganic and organic substances in a very wide range of contents - from 10 -10 % to tens of %; to study the kinetics and mechanism of electrode processes, including the stage of electron transfer, preceding and subsequent chemical reactions, adsorption of initial products and products of electrochemical reactions, etc.; to study the structure of the electrical double layer, the equilibrium of complex formation in solution, the formation and dissociation of intermetallic compounds in mercury and on the surface of solid electrodes; to select the conditions for amperometric titration, etc.
7 Electrogravimetry
Electrogravimetry is an electrochemical research method based on determining the increase in the mass of the working electrode due to the release of a determined component on it as a result of electrolysis. Typically, the analyte is deposited as a metal (or oxide) on a pre-weighed platinum cathode (or anode). The moment of completion of electrolysis is set using a specific sensitive qualitative reaction to the ion being determined. The working electrode is washed, dried and weighed. By the difference in the masses of the electrode before and after electrolysis, the mass of the precipitated metal or oxide is determined.
The theoretical potential of metal precipitation at the cathode can be calculated from the standard electrode potentials E 0. For example, when determining Cu(II) in acid solution the corresponding reactions take place on the platinum cathode and anode:
Under electrolysis conditions, the cathode potential at 25 °C is described by the Nernst equation:
(35)
At the beginning of electrolysis, when the cathode surface is not covered with copper, a (Cu) is an infinitesimal value; in the presence of a current sufficient to fill the cathode surface with copper, a (Cu) approaches unity. In practice, for an electrochemical reaction to proceed at a noticeable rate, more than high voltage than the theoretically calculated release potential E. This is due to the oxygen overvoltage at the platinum anode and the ohmic voltage drop in the cell.
Electrogravimetry is a selective method: if the initial concentrations of the components are equal, separate separation on the electrode is possible with a difference in their electrode potentials of the order of 0.3 V (for singly charged ions) or 0.1 V (for doubly charged ions).
The electrolysis can be carried out at a constant voltage between the electrodes, at a constant current, or at a controlled potential of the working electrode. In the case of electrogravimetry at a constant voltage, the potential of the working electrode shifts to a more negative region due to polarization. The consequence of this is a decrease in selectivity due to the occurrence of an additional reaction (isolation of other metals or gaseous H 2). This variant of electrogravimetry is suitable for the determination of easily reduced substances in the presence of impurities that are more difficult to reduce than H ions. +. At the end of the electrolysis, gaseous H can be released 2. Although, in contrast to coulometry, a 100% current efficiency of the analyte is not necessary, the release of H 2often leads to the formation of precipitates with unsatisfactory physical properties. Therefore, it is recommended to introduce substances into the analyzed solution that are more easily reduced than H ions. +(hydrazine, hydroxylamine) and thus preventing the release of H2 .
If electrolysis is carried out at a constant current strength, it is necessary to periodically increase the external voltage applied to the cell in order to compensate for the decrease in current caused by concentration polarization. As a result, the analysis becomes less selective. Sometimes, however, it is possible to bind interfering cations into stable complex compounds that are reduced at a more negative potential than the analyte, or to remove the interfering ion in the form of a poorly soluble compound beforehand. The method is used, for example, to determine Cd in an alkaline solution of its cyanide, Co and Ni in an ammonium sulfate solution, Cu in a mixture of sulfuric and nitric acids.
Electrogravimetry has been known since the 1860s. and was used to determine the metals used for minting coins in various alloys and ores. This is a standardless method, which can be considered as the simplest version of coulometry. In terms of accuracy and reproducibility of results, electrogravimetry surpasses other methods in the determination of such metals as Сu, Sn, Pb, Cd, Zn. Despite the relative duration of the experiment, electrogravimetry is still used to analyze alloys, metals, and solutions for electrolyte baths.
2.Experimental part of electrochemical research methods
1 Determination of acid concentration by conductometric titration
The purpose of the laboratory work:determination of the concentration of acetic and hydrochloric acids by conductometric titration.
Equipment and reagents:general laboratory module, computer, burette, Mora pipettes for 5 and 10 ml; solutions: 0.1 N NaOH, HCl and CH solutions 3COOH with unknown concentration.
Progress
Conductometric titration involves two experiments:
Experience #1
Install the burette and beaker. Pour 10 ml of solution into the glass located in the sensor of the device with a Mohr pipette. of hydrochloric acid. The solution level in the beaker should be 3-5 mm above the top electrode and sensor. Dilute the solution with water. Turn on the magnetic stirrer. Fill the burette with 0.1 N solution. NaOH. We make measurements using a general laboratory module connected to a personal computer.
Process chemistry
Results processing
1)During the measurement, the computer measures the electrical conductivity of a given solution, which are summarized in Table 1.
Table 1. Dependence of electrical conductivity on the volume of alkali used for titration of hydrochloric acid.
V(NaOH), ml0246891010.51112131415L, mS9.2929.329.2959.2899.2789.2719.269.259.2419.219.1359.2489.256
)We build a graph of the dependence of electrical conductivity on the volume of alkali used for titration of hydrochloric acid (Figure 6).
Figure 6. Dependence of electrical conductivity on the volume of alkali used for titration of hydrochloric acid.
Veq (NaOH) = 13 ml
4)Using the law of equivalents, we calculate the concentration of hydrochloric acid:
from here (37)
Experience #2
The experiment is carried out with 5 ml of solution acetic acid s. Further actions are the same as in the previous experiment.
Process chemistry
Results processing
1)During the measurement, the computer measures the electrical conductivity of a given solution, which are summarized in Table 2.
Table 2. Dependence of electrical conductivity on the volume of alkali used for the titration of acetic acid.
V(NaOH), ml012344.555.5678910L, mSm6.63.84.65.76.67.08.08.38.58.99.09.19.2
)We build a graph of the dependence of electrical conductivity on the volume of alkali used for titration of acetic acid (Figure 7).
Figure 7. The dependence of electrical conductivity on the volume of alkali used for the titration of acetic acid.
3)We find the equivalence point on the graph:
Veq (NaOH) = 5 ml
)Using the law of equivalents, we calculate the concentration of acetic acid:
Conclusion
In the course of this work, we determined the concentrations of hydrochloric and acetic acids by conductometric titration:
2 Potentiometric titration
Target: to get acquainted with the method of potentiometric titration. Set the equivalence points when titrating a strong acid with a strong base, a weak acid with a strong base.
Equipment: pH meter, glass electrode, silver chloride electrode, 100 ml beaker; 0.1 N HC1 solution; CH 3COOH; 0.5 n. KOH solution; burette, magnetic stirrer.
Progress
Experience #1
Pour 15 ml of a 0.1 N solution into a glass using a pipette. hydrochloric acid, lower the slider, place the glass on the magnetic stirrer and turn it on after lowering the electrodes (make sure that the glass electrode does not touch the slider).
Off position of the pH meter "-1-14" and "0-t" are pressed. To change, press the "pH" button and remove the value on the lower scale. Then add a solution of 0.1 N. alkali 1-3 ml and fix the pH value. We set the microburettes so that the alkali flows out in drops. When approaching the equivalence point, we add alkali in very small doses. The glass during the experiment is on a magnetic stirrer, and the solution is constantly stirred.
After a sharp change in the pH of the solution, we add a small amount of alkali and constantly fix the pH.
Process chemistry
Results processing
1)As a result of this experiment, we obtained the following results:
Table 3. Dependence pH from the volume of alkali used for titration of acetic acid.
V(KOH), ml12345678910pH4.004.154.154.004.204.304.294.945.004.91
Continuation of the table. 3
V(KOH), ml
)Based on the data obtained, we plot the dependence of pH on the volume of alkali used for titration (Figure 8).
Figure 8. Titration curve for hydrochloric acid
)According to the graph (Figure 8), we determine the equivalence point.
V eq (NaOH) = 16.5 ml
Experience #2
We carry out a similar titration with 0.1 N. CH3 COOH.
Chemistry
Results processing
1)As a result of this experiment, we obtained the following data:
Table 4. Dependence of the pH value on the volume of alkali used for titration of acetic acid.
V(KOH), ml
)Based on the data obtained, we plot the dependence of pH on the volume of alkali used for titration (Figure 9).
Figure 9. Acetic acid titration curve
)According to the graph (Figure 9), we determine the equivalence point. eq (NaOH) = 14.2 ml
Conclusion
In the course of this work, we determined the equivalence point of solutions of hydrochloric and acetic acids by the method of potentiometric titration.
Equivalence point for hydrochloric acid solution:
V eq (NaOH) = 16.5 ml
Equivalence point for acetic acid solution: eq (NaOH) = 14.2 ml
3 Electrolysis
Objective: determination of the electrochemical equivalent of copper.
Equipment: rectifier, ammeter, bath with electrolyte and two copper electrodes, stopwatch, analytical balance, 5% CuSO solution 4, wires for mounting the device.
Progress
Electrochemical equivalent - the amount of a substance that has undergone a chemical transformation at the electrode when a unit of electricity is passed, provided that all the electricity passed is spent only on the transformation of this substance.
(38)
where E is the electrochemical equivalent,
?- molar connection mass,
?q is the number of electrons required for the electrochemical transformation of one molecule of this compound.
Molar mass of the equivalent of a substance that has undergone a chemical transformation at the electrode (Meq ) is equal to:
(39)
where m is the mass of deposited matter,
F - Faraday's constant,
I - current strength,
t is the time during which the current flowed.
To determine the electrochemical equivalent E, we assemble a device where the current from the source is passed through a rectifier and an electrolyte bath, an ammeter connected in series. When turned on, copper is released on the copper electrode, which is the cathode. The anode, also made of copper, dissolves. In order for copper to be deposited on the cathode, form a dense layer and not peel off during the experiment, distorting the results, you should use a current not exceeding 0.05 A per 1 cm 2cathode surface. To do this, before the start of the experiment, using a millimeter ruler, determine the surface of the cathode and calculate the maximum allowable current.
Before starting the experiment, the cathode is immersed in a 20-30% solution for 1-2 seconds. nitric acid and then rinse thoroughly with distilled water.
During the work, it is important not to touch the surface of the cathode immersed in the electrolyte, because. even slight traces of fat impair the adhesion of the copper cathode deposit.
After that, we fix the cathode in a voltmeter, which we fill with a solution of CuSO 4. The cathode is removed from the electrolyte bath, washed with distilled water, dried and weighed on an analytical balance. After that, the cathode is again installed in a bath with electrolyte and proceed to the experiment. At the same time turn on the current and start the stopwatch. The experiment continues for 40-50 minutes. At the same time turn off the current and stop the stopwatch. The cathode is removed from the electrolyte, washed with distilled water, dried and weighed.
During electrolysis, the following chemical reactions took place:
)Dissociation of copper (II) sulfate solution:
2)Redox reactions on electrodes:
Results processing
1)As a result of this laboratory work, we received the following data (Table 5):
Table 5. Data on the conducted laboratory work.
Current strength (I), A1.8 Time during which the current flowed (t), s2527 Weight of the cathode before the experiment, expressed in mass, g24.42 Weight of the cathode after the experiment, expressed in mass, g25.81 Weight of the deposited substance, expressed in mass (m ), r1.39 2)Calculation of the electrochemical equivalent:
)Calculation of molar mass equivalent, absolute and relative error:
Conclusion.
In the course of this work, we determined the electrochemical equivalent of copper, molar mass copper equivalent, as well as absolute and relative error.
2.4 Determination of electrode potentials
Objective: measure the potential of copper and zinc electrodes in solutions of their salts of various activity. Compare the measured potential values with the calculations using the Nernst equation.
Equipment: pH meter, copper electrode, zinc electrode, silver chloride electrode, U-tube with saturated KCl solution, sandpaper, CuSO solutions 4and ZnSO 4with different concentration.
Progress
To measure potentials of the 1st kind, we assemble a circuit consisting of a measuring device, a measured electrode and a reference electrode. In fact, we measure the EMF of a galvanic cell
| AgCl, KCl || CuSO4 | Cu;
Zn | ZnSO4 || KCl, AgCl | Ag.
The potential of the silver chloride electrode (electrode of the 2nd kind) is constant, depends only on the activity of Cl ions and is equal to Ag | AgCl (saturated solution of KC1) = 0.2 V. It is a reference electrode.
To eliminate the diffuse potential, we use bridges filled with a saturated KCl solution.
We use a pH meter to measure potentials. We connect the silver chloride electrode to a special socket "reference electrode" (on the VSP instrument panel), and the measuring electrode through a special plug to the socket "meas - 1", "meas - 2".
Process Chemistry
For a galvanic cell Ag | AgCl, KCl || CuSO4 | Cu:
For a galvanic cell Zn | ZnSO4 || KCl, AgCl | Ag:
Results processing
1)As a result of measuring the potentials of the copper electrode at different activities of Cu ions 2+we got the following data:
¾ for copper electrode (table 6):
Table 6. Lab data for copper electrode.
?meas, VSn, mol * equiv-1 * l-1 ?lg a ?calc, B0.2100.10.38-1.72120.2862230.3510.20.36-1.44370.2944110.3600.50.25-1.20410.3014780.3611.00.23-0.93930, 309291
¾ for zinc electrode (table 7):
Table 6. Lab data for zinc electrode.
?meas, VSn, mol * equiv-1 * l-1 ?lg a ?calculated, V-0.0650.10.25-1.9031-0.81914-0.0650.20.28-1.5528-0.80881-0.0290.50.38-1.0223-0, 79316-0.0501.00.40-0.6990-0.78362
2) We plot the dependence of the electrode potential on lg a (Cu2+).
¾ for a copper electrode (Figure 10):
Figure 10. Dependence of the electrode potential on the logarithm of the activity of copper (II) ions
¾ for zinc electrode (Figure 11):
Figure 11. Dependence of the electrode potential on the logarithm of the activity of zinc ions
.We calculate the potentials of the electrodes according to the Nernst equation (1):
¾ for copper electrode:
¾ for zinc electrode:
Conclusion: in the course of this work, we measured the potentials of copper and zinc electrodes at various concentrations of CuSO 4and ZnSO 4respectively, and also calculated these electrode potentials according to the Nernst equation, as a result of which they concluded that with increasing concentration, the electrode potentials of the copper and zinc electrodes increase.
5 Determination of the EMF of a galvanic cell
Purpose: to determine the EMF of a galvanic cell.
Equipment: zinc and copper electrode, CuSO solutions 4and ZnSO 4, silver chloride electrode, pH meter, sandpaper, U-tube with saturated KC1 solution, 0.1N. and 1n. CuSO solution 4, 0.1n. and 1n. ZnSO4 solution ,
Progress
Pour half the solutions of CuSO into two glasses 4and ZnSO 4. In the first we place an electrode made of copper, in the second - of zinc.
The electrodes are pre-cleaned with sandpaper and washed. We connect the wires to the pH meter on the rear panel to the inputs "Change 1" and "El. compare." We close the external circuit with a U-shaped tube filled with a saturated solution of KCl in agar-agar.
Before measurement, the device warms up for 30 minutes. When the circuit is assembled, we proceed to measurements, press the "mV" button and look at the readings of the device on the lower scale "1-14". For a more accurate determination of the EMF, press the button for the desired range. To convert the measured values to volts, the numerator of the value is multiplied by 0.1.
To perform the work, we measure the EMF of elements in solutions with a concentration of 1N. and 0.1n. and compare these data with calculations. We find the absolute and relative error.
Process Chemistry
For a given galvanic cell
| ZnSO4 || KCl, AgCl | Ag
the following reactions are typical:
The overall equation of the reaction occurring in a copper-zinc galvanic cell:
Results processing
1)As a result of this work, we obtained the following results (Table 6):
Table 6. Data on the conducted laboratory work
Solutions ?ism, V ?calculated, VRelative error, %0.1n. CuSO4 and 0.1n. ZnSO41.0871.0991.0921n. CuSO4 and 0.1n. ZnSO41.0821.0931.0061n. CuSO4 and 1n. ZnSO41.0601.070.935
)We calculate the EMF:
Potentials are calculated according to the Nernst equation (1). Standard electrode potentials are taken from reference data.
For solutions of 0.1N. CuSO 4 and 0.1n. ZnSO 4:
For solutions of 1N. CuSO 4 and 0.1n. ZnSO 4:
For solutions of 1N. CuSO 4 and 1n. ZnSO 4:
Conclusion: in this work, we determined the EMF of a galvanic cell in solutions of various concentrations:
at a concentration of 0.1N. CuSO4 and 0.1n. ZnSO4,
at a concentration of 1N. CuSO4 and 0.1n. ZnSO4,
at a concentration of 1N. CuSO4 and 1n. ZnSO4;
and also determined the relative error: 1.092%, 1.006%, 0.935%, respectively. As a result, it was concluded that with an increase in the concentration of E.D.S. at the galvanic cell decreases.
Conclusion
In this paper, we considered the main methods of electrochemical research, analyzed their classification, the main electrochemical processes, and also proved the relevance of these methods. Most of the work was dedicated to the description of electrode processes. Potentiometry, conductometry, coulometry, voltammetry and electrogravimetry were studied in detail.
In the course of practical research, we carried out: determination of the concentration of unknown acids by conductometric titration, determination of the equivalence point of solutions of hydrochloric and acetic acids by potentiometric titration, determination of the electrochemical equivalent of copper, determination of the potentials of copper and zinc electrodes, and determination of the EMF of a galvanic cell.
We were convinced of the speed and accuracy of these methods, but at the same time, on our own experience, we revealed some significant shortcomings: to obtain accurate data, very precise adjustment and calibration of instruments is required, the results obtained depend on various external factors (pressure, temperature, etc.) and other conditions can vary significantly, as well as the fragility and high cost of devices.
And yet, these are far from all known methods of electrochemical research. All the above methods are only a small part of the electrochemical research methods used in science and technology. And they are used so widely in all industries that without them neither the existence nor the further development of civilization is possible. Despite its considerable age, electrochemical research methods are undergoing rapid development with great prospects for the future. According to the forecasts of a number of leading scientists, their role will rapidly increase.
It remains only to contribute in every possible way to development in this direction, and perhaps in the future we will discover such secrets and areas of application of electrochemical research methods that we could only dream of.
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2. ELECTROCHEMICAL METHODS OF ANALYSIS
Electrochemical methods of analysis and research are based on the study and use of processes occurring on the electrode surface or in the near-electrode space. Any electrical parameter (potential, current strength, resistance, etc.) that is functionally related to the concentration of the analyzed solution and can be correctly measured can serve as an analytical signal.
Distinguish direct and indirect electrochemical methods. In direct methods, the dependence of the current strength (potential, etc.) on the concentration of the analyte is used. In indirect methods, the current strength (potential, etc.) is measured in order to find the end point of the titration of the component to be determined with a suitable titrant, i.e. use the dependence of the measured parameter on the volume of the titrant.
For any kind of electrochemical measurements, an electrochemical circuit or an electrochemical cell is required, the component of which is the analyzed solution.
2.1. Potentiometric method of analysis
2.1.1. Basic laws and formulas
Potentiometric methods are based on measuring the potential difference between the indicator electrode and the reference electrode, or, more precisely, electromotive forces(EMF) of various circuits, since it is the EMF, which is the potential difference, that is measured experimentally.
Equilibrium potential of the indicator electrode associated with the activity and concentration of substances involved in the electrode process, Nernst equation:
E = E ° + R T /(n F ) ln (and oxide/and restore)
E = E ° + R T /(n F ) ln ([ oxide] ү oxide /( [ restore] ү restore)),
R - universal gas constant, equal to 8.31 J / (mol. K); T is the absolute temperature; F - Faraday's constant (96500 C/mol); n - the number of electrons involved in the electrode reaction; and oxide, and restore- activities of the oxidized and reduced forms of the redox system, respectively;[ oxide] and[ restore] - their molar concentrations; ү oxide, ү restore - activity coefficients; E ° is the standard potential of the redox system.
Substituting T= 298.15 K and the numerical values of the constants in the equation, we get:
E = E ° + (0.059 / n) lg (and oxide/and restore)
E = E ° + (0.059 / n) lg ([ oxide] ү oxide / ([ restore] ү restore))
Direct Potentiometry Methods are based on the application of the Nernst equation to find the activity or concentration of the electrode reaction participant from the experimentally measured EMF of the circuit or the electrode potential. The most widespread among direct potentiometric methods was the method for determining pH, but the creation in recent times reliably operating ion-selective electrodes has greatly expanded the practical possibilities of direct methods. The pH value is also measured by potentiometric titration.
A glass electrode is most often used to determine pH. The main advantages of the glass electrode are ease of operation, rapid equilibrium and the ability to determine pH in redox systems. The disadvantages include the fragility of the electrode material and the complexity of work in the transition to strongly alkaline and strongly acidic solutions.
In addition to the concentration of hydrogen ions, the content of several tens of different ions can be determined by the direct potentiometric method with ion selective electrodes.
Potentiometric titration based on the determination of the equivalence point from the results of potentiometric measurements. Near the equivalence point, there is a sharp change (jump) in the potential of the indicator electrode. Just like in others titrimetric methods, potentiometric titration reactions must proceed strictly stoichiometrically, have high speed and go to the end.
For potentiometric titration, a circuit is assembled from an indicator electrode in the analyzed solution and a reference electrode. Calomel or silver chloride electrodes are most often used as reference electrodes.
The type of indicator electrode used in potentiometric titration depends on the properties titrimetric mixture and its interaction with the electrode. In acid-base titration, a glass electrode is used, in redox titration, an inert (platinum) electrode or an electrode that is reversible with respect to one of the ions contained in titrimetric mixtures; in the precipitation - a silver electrode; in complexometric- a metal electrode reversible to the titratable metal ion.
To find the equivalence point, a differential curve is often constructed in coordinates D E/ D V-V . The equivalence point is indicated by the maximum of the obtained curve, and the reading along the abscissa corresponding to this maximum gives the volume of titrant, spent for titration to the equivalence point. Determining the equivalence point to a differential curve is much more accurate than using a simple relationship E - V.
The main advantages of the potentiometric titration method are high accuracy and the ability to carry out determinations in dilute solutions, in turbid and colored media, and also to determine several substances in one solution without preliminary separation. The field of practical application of potentiometric titration with the use of non-aqueous solvents is significantly expanding. They make it possible to analyze multicomponent systems that cannot be determined in an aqueous solution, to analyze substances that are insoluble or decomposing in water, etc. Potentiometric titration can easily be automated. The industry produces several types of autotitrators that use potentiometric sensors.
The disadvantages of potentiometric titration include the not always fast establishment of the potential after the addition of the titrant and the need in many cases to carry out a large number of readings during titration.
In potentiometric analysis, various types of potentiometers are the main measuring instruments. They are designed to measure the EMF of the electrode system. Since the EMF depends on the activity of the corresponding ions in the solution, many potentiometers also allow you to directly measure the pX value - the negative logarithm of the activity of the X ion. Such potentiometers, complete with the corresponding ion-selective electrode, are called ionomers. If the potentiometer and electrode system are designed to measure the activity of only hydrogen ions, the instrument is called a pH meter.A.A. Vikharev, S.A. Zuykova, N.A. Chemeris, N.G. Domina
Physicochemical methods of analysis (PCMA) are based on the use of the relationship between the measured physical properties of substances and their qualitative and quantitative composition. Because the physical properties substances are measured using various instruments - "tools", then these methods of analysis are also called instrumental methods.
The greatest practical application among FKhMA have:
- electrochemical methods- based on the measurement of potential, current strength, amount of electricity and other electrical parameters;
- spectral and others optical methods – are based on the phenomena of absorption or emission of electromagnetic radiation (EMR) by atoms or molecules of a substance;
- chromatographic methods– are based on sorption processes occurring under dynamic conditions with directional movement of the mobile phase relative to the stationary one.
The advantages of PCMA include high sensitivity and low detection limit - mass up to 10-9 µg and concentration up to 10-12 g / ml, high selectivity (selectivity), which allows to determine the components of mixtures without their preliminary separation, as well as rapid analysis, the possibility their automation and computerization.
Electrochemical methods are widely used in analytical chemistry. The choice of the analysis method for a particular object of analysis is determined by many factors, including, first of all, the lower limit of element definition.
Data on the lower limit of detection of various elements by some methods are presented in the table.
Limits of detection (µg/ml) of elements by various methods
| Element | MAC | AAS | PTP | WILLOW | Ionometry | Ampere. captions. |
| Ag | 0.1– dithizone | 0,07 | 0,2 | 0.00001 | 0.02 | 0.05 |
| As | 0.05 - molybd.blue | 0,2 | 0,04 | 0,02 | - | 0,05 |
| Au | 0.04-methyl fiol. | 0,3 | 0,005 | 0,001 | - | 0,05 |
| Bi | 0.07-dithizone | 0,005 | 0,00001 | - | 0,5 | |
| CD | 0.04-dithizone | 0,05 | 0,002 | 0,00001 | 0,03 | 0,5 |
| Cr | 0.04-diphenylcarbazide | 0,2 | 0,02 | - | - | |
| Cu | 0.03-dithizone | 0,2 | 0,002 | 0,00002 | 0,01 | 0,05 |
| hg | 0.08-dithizone | - | 0,00005 | |||
| Pb | 0.08-dithizone | 0,6 | 0,003 | 0,00002 | 0,03 | |
| Sb | 0.08-rhodamine | 0,004 | 0,00004 | - | 0,5 | |
| Fe | 0,1-thiocyanate | 0,2 | 0,003 | 0,0002 | 0,3 | 0,5 |
| Se | 0.08-diami-nophthalene | 0,3 | 0,2 | 0,00002 | - | 0,5 |
| sn | 0,07-phenyl-fluriom | 0,4 | 0,003 | 0,00004 | - | 0,5 |
| Te | 0.1-bismuthol | 0,7 | 0,02 | - | - | |
| Tl | 0.06-rhodamine | 0,6 | 0,01 | 0,00002 | - | 0,5 |
| Zn | 0.02-dithizone | 0,02 | 0,003 | 0,0003 | - | 0,5 |
| F- | - | - | - | - | 0,02 | 5-10 |
| NH 4 +, NO 3 - | - | - | - | - | 0,1 | 1-5 |
MAC - molecular absorption spectrometry (photometry);
AAS - atomic absorption spectrometry (flame photometry);
PTP - alternating current polarography;
IVA - stripping voltammetry.
The determination errors in FHMA are about 2–5%; analysis requires the use of complex and expensive equipment.
Distinguish direct and indirect methods of physico-chemical analysis. Direct methods use the dependence of the measured analytical signal on the concentration of the analyte. In indirect methods, the analytical signal is measured in order to find the end point of titration of the analyte component with a suitable titrant, that is, the dependence of the measured parameter on the volume of the titrant is used.
Electrochemical methods of analysis are based on the study and use of processes occurring on the surface of the electrode or in the near-electrode space. Any electrical parameter (potential, electric current, amount of electricity, etc.) functionally related to the concentration of the determined component and amenable to correct measurement can serve as an analytical signal.
According to the nature of the measured analytical signal, electrochemical methods of analysis are divided into potentiometry, voltammetry, coulometry and a number of other methods:
Characteristic dependence of the electrochemical signal on the independent variable
| Method | Measured signal | Dependence of the signal on the independent variable |
| Potentiometry, ionometry | potential E = f(C) C-concentration of analyte | |
| Potentiometric titration | potential E = f(V), V is the volume of titrant reagent |
 |
| polarography, voltammetry | current I = f(E), E is the polarization potential of the electrode |  |
| stripping voltammetry | current I n = f(E) |  |
| chronopotentiometry | potential E =f(t), t – electrode polarization time at I=const. |
 |
| amperometric titration with one indicator electrode | current I = f(V), V is the volume of titrant reagent |  |
| amperometric titration with two indicator electrodes | current I = f(V) V – volume of titrant reagent | |
| coulometry | Q \u003d f (C), C - amount of substance |  |
| conductometry | G = f(C), C is the concentration of ions in the solution |  |
| conductometric titration | electrical conductivity G = f(V), V is the volume of the titrant reagent |  |
Potentiometry
Potentiometric measurements are based on the dependence of the electrode equilibrium potential on the activity (concentration) of the ion being determined. For measurements, it is necessary to make a galvanic cell from a suitable indicator electrode and reference electrode, and also have a device for measuring the potential of the indicator electrode (EMF of a galvanic cell), under conditions close to thermodynamic, when the indicator electrode has an equilibrium (or close to it) potential, that is, without significant current being removed from the galvanic cell when the circuit is closed. In this case, you cannot use a conventional voltmeter, but you should use potentiometer- an electronic device with a high input resistance (1011 - 1012 Ohm), which excludes the occurrence of electrode electrochemical reactions and the occurrence of current in the circuit.
An indicator electrode is an electrode whose potential depends on the activity (concentration) of the ion being determined in the analyzed solution.
A reference electrode is an electrode whose potential remains constant under the conditions of analysis. In relation to the reference electrode, measure the potential of the indicator electrode E(EMF of a galvanic cell).
In potentiometry, two main classes of indicator electrodes are used - electron exchange and ion exchange.
Electron exchange electrodes- these are electrodes on the surface of which electrode reactions occur with the participation of electrons. These electrodes include electrodes of the first and second kind, redox electrodes.
Electrodes of the first kind- these are electrodes that are reversible in a cation common to the electrode material, for example, metal M immersed in a solution of a salt of the same metal. On the surface of such an electrode flows reversible reaction M n+ + ne↔ M and its real potential depends on the activity (concentration) of metal cations in solution in accordance with the Nernst equation:
For a temperature of 250C (298 K) and for conditions where the ion activity is approximately equal to the concentration (γ → 1):
Electrodes of the first kind can be made of various metals, for example, Ag (silver), Cu (copper), Zn (zinc), Pb (lead), etc.
Schematically, the electrodes of the first kind are written as M | M n+ , where the vertical line shows the boundary of the solid (electrode) and liquid (solution) phases. For example, a silver electrode immersed in a solution of silver nitrate is depicted as follows - Ag | Ag+; if necessary, indicate the concentration of the electrolyte - Ag | AgNO 3 (0.1 M).
The electrodes of the first kind include gas hydrogen electrode Pt(H2) | H+ (2Н + + 2nd↔ H 2, E 0 = 0):
Electrodes of the second kind are anion-reversible electrodes, for example, a metal coated with a sparingly soluble salt of this metal, immersed in a solution containing an anion of this sparingly soluble salt M, MA | BUT n-. On the surface of such an electrode, the reversible reaction MA + ne↔ M + A n- and its real potential depends on the activity (concentration) of the anion in solution in accordance with the Nernst equation (at T= 298 K and γ → 1):
Examples of electrodes of the second kind are silver chloride (AgCl + e↔ Ag + Cl -) and calomel (Hg 2 Cl 2 + 2e↔ 2Hg + 2Cl -) electrodes:
Redox electrodes- these are electrodes that consist of an inert material (platinum, gold, graphite, glassy carbon, etc.) immersed in a solution containing oxidized (Ok) and reduced (Boc) forms of the analyte. On the surface of such an electrode, the reversible reaction Ok + ne↔ Vos and its real potential depend on the activity (concentration) of the oxidized and reduced forms of the substance in solution in accordance with the Nernst equation (at T= 298 K and γ → 1):
If hydrogen ions participate in the electrode reaction, then their activity (concentration) is taken into account in the corresponding Nernst equations for each specific case.
Ion exchange electrodes- These are electrodes on the surface of which ion-exchange reactions occur. These electrodes are also called ion-selective or membrane. The most important component of such electrodes is semipermeable membrane a thin solid or liquid film that separates inner part electrode (internal solution) from the analyzed and having the ability to pass only ions of one type X (cations or anions). Structurally, the membrane electrode consists of an internal reference electrode (usually silver chloride) and an internal electrolyte solution with a constant concentration of a potential-determining ion, separated from the external (investigated) solution by a sensitive membrane.
The real potential of ion-selective electrodes, measured relative to any reference electrode, depends on the activity of those ions in the solution that are sorbed by the membrane:
where const- constant depending on the nature of the membrane ( asymmetry potential) and the potential difference between the external and internal reference electrodes, n and a(X n±) are the charge and activity of the potential-determining ion. If the potential of the ion-selective electrode is measured relative to the standard hydrogen electrode, then the constant is the standard electrode potential E 0.
For membrane electrodes, the value slope of the electrode function may differ from theoretical Nernst values (0.059 V); in this case, the real value of the electrode function θ is defined as the tangent of the slope of the calibration curve. Then:
The potential of the membrane electrode in a solution containing, in addition to the determined ion X, a foreign ion B, which affects the potential of the electrode, is described by Nikolsky equation(modified Nernst equation):
where z is the charge of the foreign (interfering) ion, KХ/В is the selectivity coefficient of the membrane electrode.
Selectivity factor K X / B characterizes the sensitivity of the electrode membrane to the determined X ions in the presence of interfering B ions. If K X/V<1, то электрод селективен относительно ионов Х и, чем меньше числовое значение коэффициента селективности, тем выше селективность электрода по отношению к определяемым ионам и меньше мешающее действие посторонних ионов. Если коэффициент селективности равен 0,01, то это означает, что мешающий ион В оказывает на величину электродного потенциала в 100 раз меньшее влияние, чем определяемый ион той же молярной концентрации.
The selectivity coefficient is calculated as the ratio of the activities (concentrations) of the determined and interfering ions, at which the electrode acquires the same potential in solutions of these substances, taking into account their charges:
Knowing the value of the selectivity coefficient, it is possible to calculate the concentration of the interfering ion, which affects the potential of the ion-selective electrode (example).
Example. What concentration of nitrate ions must be created in a 1∙10-3 M solution of sodium fluoride in order for an ion-selective fluoride electrode to be equally sensitive to both ions, if its electrode selectivity coefficient?
Solution.
Since then
This means that the concentration of nitrate ions in the analyzed solution above 0.5 mol/l has a significant effect on the determination of the fluoride ion in its millimolar solutions.
A classic example of a solid membrane ion-selective electrode is a glass electrode with a hydrogen function used to measure the concentration of hydrogen ions in a solution (glass pH electrode). For such electrodes, the membrane is a special glass of a certain composition, and the internal electrolyte is a 0.1 M solution of hydrochloric acid:
Ag, AgCl | 0.1 M HCl | glass membrane | test solution
An ion exchange process takes place on the surface of the glass membrane:
SiO-Na+ (glass) + H+ (solution) → -SiO-H+ (glass) + Na+ (solution)
as a result, a dynamic equilibrium is established between hydrogen ions in glass and a solution of H+ (glass) ↔ H+ (solution), which leads to the emergence of a potential:
E = const + θ lg a(H+) = const – θ pH
A glass electrode with a high content of Al2O3 in the membrane measures the activity of sodium ions in solution (a glass Na electrode, a sodium selective electrode). In this case, the internal solution is 0.1 M sodium chloride solution:
Ag, AgCl | 0.1M NaCl | glass membrane | test solution
On the surface of the glass membrane of the sodium-selective electrode, an equilibrium is established between sodium ions in glass and a solution of Na + (glass) ↔ Na + (solution), which leads to the emergence of a potential:
E = const + θ lg a(Na+) = const – θ pNa
The most perfect electrode with a crystalline membrane is a fluoride-selective electrode, the membrane of which is made of a plate of a single crystal of lanthanum fluoride (LaF3), activated to increase conductivity with europium fluoride (EuF 2):
Ag, AgCl | 0.1 M NaCl, 0.1 M NaF | LaF 3 (EuF 2) | test solution
The potential of the fluoride electrode is determined by the ion exchange process on its surface F- (membrane) ↔ F- (solution):
E = const – θ lg a(F-)= const + θ pF
The values of the constant and slope of the electrode function θ for ion-selective electrodes is determined from the calibration curve E ÷ pX as a segment on the y-axis and the tangent of the slope of the straight line, respectively. For a glass pH electrode, this operation is replaced by the adjustment of instruments (pH meters) using standard buffer solutions with precisely known pH values.
A schematic view of the glass and fluoride-selective electrodes is shown in the figures:
Paired with an indicator electrode to measure its potential (emf of a galvanic cell), a reference electrode with a known and stable potential is used, which does not depend on the composition of the test solution. The most commonly used reference electrodes are silver chloride and calomel electrodes. Both electrodes belong to the electrodes of the second kind and are characterized by high stability in operation.
The potentials of silver chloride and calomel electrodes depend on the activity (concentration) of chloride ions (at T= 298 K and γ → 1):
As reference electrodes, electrodes with a saturated solution of potassium chloride are most often used - at 250C, the potential of a saturated silver chloride reference electrode is +0.201 V, and a saturated calomel electrode is +0.247 V (relative to a standard hydrogen electrode). Potentials for silver chloride and calomel reference electrodes containing 1 M and 0.1 M potassium chloride solutions can be found in the reference tables.
A schematic view of saturated silver chloride and calomel reference electrodes is shown in the figure:
Reference electrodes silver chloride (a) and calomel (b)
1 - asbestos fiber providing contact with the analyzed solution
2 - KCl solution (saturated)
3 - contact hole
4 - KCl solution (saturated), AgCl (solid)
5 - hole for injecting KCl solution
6 - paste from a mixture of Hg2Cl2, Hg and KC1 (saturated)
Potentiometric analysis is widely used to directly determine the activity (concentration) of ions in a solution by measuring the equilibrium potential of the indicator electrode (emf of a galvanic cell) - direct potentiometry (ionometry), as well as to indicate the end point of the titration ( ktt) by changing the potential of the indicator electrode during titration ( potentiometric titration).
In all tricks direct potentiometry the dependence of the indicator electrode on the activity (concentration) of the ion being determined is used, which is described by the Nernst equation. The results of the analysis imply the determination of the concentration of a substance, and not its activity, which is possible when the value of the activity coefficients of ions is equal to unity (γ → 1) or their constant value (constant ionic strength of the solution), therefore, in further reasoning, only concentration dependences are used.
The concentration of the ion being determined can be calculated from the experimentally found potential of the indicator electrode, if the constant component is known for the electrode (standard potential E 0) and the steepness of the electrode function θ . In this case, a galvanic cell is made up, consisting of an indicator electrode and a reference electrode, its EMF is measured, the potential of the indicator electrode (relative to SHE) and the concentration of the ion being determined are calculated.
AT method calibration curve prepare a series of standard solutions with a known concentration of the ion to be determined and a constant ionic strength, measure the potential of the indicator electrode relative to the reference electrode (emf of a galvanic cell) in these solutions, and build a dependence based on the data obtained E÷ p FROM(A) (calibration plot). Then measure the potential of the indicator electrode in the analyzed solution E x (under the same conditions) and determine p according to the schedule FROM x(A) and calculate the concentration of the analyte in the analyzed solution.
AT standard (comparison) method measure the potential of the indicator electrode in the analyzed solution ( E x) and in the standard solution of the analyte ( E st). Calculation of the concentration of the determined ion is carried out on the basis of the Nernst equations for the analyzed sample and the standard sample. The slope of the electrode function for the indicator electrode θ
Using additive method first measure the potential of the indicator electrode in the analyzed solution ( E x), then add to it a certain volume of the standard solution of the analyte and measure the electrode potential in the resulting solution with the addition ( E x+d). Calculation of the concentration of the determined ion is carried out on the basis of the Nernst equations for the analyzed sample and the sample with the additive. The slope of the electrode function for the indicator electrode θ must be known or determined in advance from the calibration curve.
At potentiometric titration measure and record the EMF of the electrochemical cell (the potential of the indicator electrode) after adding each portion of the titrant. Then, according to the results obtained, titration curves are built - integral in coordinates E ÷ V(а) and differential in coordinates ∆ E/∆V ÷ V (b), and determine the end point of the titration ( ktt) in a graphical way:
In potentiometric titration, all the main types of chemical reactions are used - acid-base, redox, precipitation and complexation. They are subject to the same requirements as in visual titrimetry, supplemented by the presence of a suitable indicator electrode to record the change in the concentration of potential-determining ions during the titration.
The determination error during potentiometric titration is 0.5-1%, which is significantly lower than in direct potentiometric measurements (2-10%), however, higher detection limits are observed - more than 10 -4 mol/l.
Coulometry
Coulometry combines methods of analysis based on measuring the amount of electricity spent on an electrochemical reaction. An electrochemical reaction leads to a quantitative electroconversion (oxidation or reduction) of the analyte at the working electrode (direct coulometry) or to the production of an intermediate reagent (titrant) that reacts stoichiometrically with the analyte (indirect coulometry, coulometric titration).
Coulometric methods are based on Faraday's law, which establishes a relationship between the amount of electroconverted (oxidized or reduced) substance and the amount of electricity consumed in this case:
where m is the mass of the electroconverted substance, g; Q is the amount of electricity spent on the electroconversion of a substance, C; F- Faraday number, equal to the amount of electricity required for the electroconversion of one mole equivalent of a substance, 96500 C/mol; M is the molar mass of the substance, g/mol; n is the number of electrons involved in the electrochemical reaction.
A necessary condition for carrying out coulometric analysis is the almost complete consumption of electricity for the transformation of the analyte, that is, the electrochemical reaction must proceed without side processes with 100% current efficiency.
In practice, coulometric analysis is implemented in two versions - at a constant potential ( potentiostatic coulometry) and at a constant current ( amperostatic coulometry).
Potentiostatic coulometry used for direct coulometric measurements, when the directly determined substance is subjected to electrolysis. In this case, the potential of the working electrode using potentiostats is maintained constant and its value is chosen on the basis of polarization curves in the region of the limiting current of the analyte. In the process of electrolysis at a constant potential, the current strength decreases in accordance with a decrease in the concentration of an electroactive substance according to an exponential law:
where Ι - current strength at a time t, BUT; Ι 0 – current strength at the initial moment of electrolysis, A; k is a constant depending on the electrolysis conditions.
Electrolysis is carried out until the residual current is reached Ι , the value of which is determined by the required accuracy - for an acceptable error of 0.1%, electrolysis can be considered complete when Ι = 0,001Ι 0 . To reduce the electrolysis time, a working electrode of a large surface should be used with intensive stirring of the analyzed solution.
Total amount of electricity Q, necessary for the electroconversion of the analyte, is determined by the equation:
The amount of electricity can be determined by measuring the area under the current-time curve using mechanical or electronic integrators, or using chemical coulometers. coulometer is an electrolytic cell in which an electrochemical reaction of known stoichiometry proceeds with 100% current efficiency. The coulometer is connected in series with the coulometric cell under study, therefore, during the electrolysis, the same amount of electricity flows through both cells. If, at the end of electrolysis, the amount (mass) of the substance formed in the coulometer is measured, then according to Faraday's law, the amount of electricity can be calculated. The most commonly used are silver, copper and gas coulometers.
Amperostatic coulometry used for coulometric titration at direct current, during which the analyte reacts with the titrant formed as a result of an electrochemical reaction on the working electrode, and therefore, called electrogenerated titrant.
To ensure a 100% current efficiency, a significant excess of the auxiliary substance is required, from which the titrant is generated, which eliminates the occurrence of competing reactions on the working electrode. In this case, the titrant is generated in an amount equivalent to the analyte, and the amount of electricity spent on the generation of the titrant can be used to calculate the content of the analyte.
The amount of electricity Q in coulometry at constant current Ι calculated by the formula:
where t– electrolysis time, for the determination of which almost all methods of establishing the end point in titrimetry are suitable (visual - indicators, instrumental - potentiometry, amperometry, photometry). With the current strength in amperes and the electrolysis time in seconds, we get the amount of electricity in coulombs (example).
Example. The coulometric titration of an ascorbic acid solution with iodine generated from potassium iodide by a current of 5.00 mA took 8 min 40 s. Calculate the mass of ascorbic acid in the analyzed solution. Suggest a way to fix the end point of the titration.
Solution. The amount of electricity spent on the oxidation of iodide and, accordingly, ascorbic acid is:
Q = Ι t= 5.00∙10 -3 ∙520 = 2.60 C.
Ascorbic acid is oxidized by iodine to dehydroascorbic acid with the release of two electrons (C 6 H 8 O 6 - 2 e→ C 6 H 6 O 6 + 2H +), then according to Faraday's law:
The end point of the titration is determined by the appearance of an excess of iodine in the solution. Therefore, it can be fixed visually with the help of starch added to the analyzed solution (the appearance of a blue color), amperometrically with a dropping mercury or platinum microelectrode by the appearance of the limiting current of iodine, potentiometrically by a sharp increase in the potential of the platinum electrode.
Voltammetry
Voltammetric method of analysis is based on the use of the microelectrode polarization phenomenon, obtaining and interpreting current-voltage (polarization) curves that reflect the dependence of the current on the applied voltage. The current-voltage curve (voltammogram) allows you to simultaneously obtain qualitative and quantitative information about substances that are reduced or oxidized on the microelectrode (depolarizers), as well as about the nature of the electrode process. Modern voltammetry is a highly sensitive and express method for the determination of substances, suitable for the analysis of various objects of inorganic and organic nature, including pharmaceuticals. The minimum detectable concentration in voltammetry reaches values of 10 -8 mol/l with a method error of less than 5%. Voltammetry under optimal experimental conditions makes it possible to determine several components simultaneously in the analyzed solution.
Voltammetry uses two electrodes - worker a polarizable electrode with a small surface (indicator microelectrode) and auxiliary non-polarizable electrode with a large surface (reference electrode). The working electrodes are microelectrodes made of mercury (mercury dripping electrode, RCE), platinum (PE) and conductive carbon materials (graphite, glassy carbon).
When a direct current passes through an electrolytic cell, the process is characterized by the relationship (Ohm's law for an electrolyte solution):
E \u003d Ea - Ek + IR
Where E is the applied external voltage; Ea is the anode potential; Ek is the cathode potential; I- current in the circuit; R is the internal resistance of the electrolytic cell.
During voltammetric measurements, the analyzed solution contains an indifferent (background) electrolyte of high concentration (100 times or more higher than the concentration of the analyte - the resistance of the solution is low), and the current in voltammetry does not exceed 10 -5 A, therefore, the voltage drop in the cell IR can be neglected.
Since in voltammetry one of the electrodes (auxiliary) is not polarized and the potential for it remains constant (it can be taken equal to zero), the voltage applied to the cell manifests itself in a change in the potential of only the working electrode, and then E = Ea for working microanode ( anodic polarization) and E =-Ek for the working microcathode ( cathodic polarization). Thus, the recorded current-voltage curve reflects the electrochemical process that occurs only at the working electrode. If there are substances in the solution that can be electrochemically reduced or oxidized, then when a linearly changing voltage is applied to the cell, the voltammogram has waveform 1 (in the absence of an electrochemical reaction, the dependence of current on voltage is linear 2 in accordance with Ohm's law):
The section of voltammetry in which the RCE serves as a working microelectrode is called polarography, in honor of the Czech electrochemist J. Gejrovsky, who proposed this method in 1922. Voltammograms obtained in a cell with a dropping mercury electrode are called polarograms.
To register classic polarograms, a cell with an RCE (working electrode) and a saturated calomel electrode (auxiliary electrode, reference electrode) is connected to a constant voltage source and the potential is changed at a rate of 2–5 mV/s.
The dropping mercury electrode is almost perfectly polarizable in a wide range of potentials, limited in the anodic region by electrode reactions of mercury oxidation (+0.4 V), and in the cathodic region by hydrogen ion reduction reactions (from -1 to -1.5 V, depending on the acidity of the medium) or background cations (from -2 V for alkali metal cations to -2.5 V for R 4 N +). This makes it possible to study and determine on the RCE substances that are reduced at very high negative potentials, which is impossible on electrodes made of other materials. It should be noted that here and below the potential values are given relative to a saturated calomel electrode and, if necessary, can be recalculated with respect to another reference electrode, for example, saturated silver chloride.
Before registering the polarogram on the RCE, it is necessary to remove dissolved oxygen, since it is electroactive in the negative potential region, giving two recovery waves at -0.2 and -0.9 V. This can be done by saturating the solution with an inert gas (nitrogen, argon, helium). Oxygen is removed from alkaline solutions using sodium sulfite (O 2 + 2Na 2 SO 3 → 2Na 2 SO 4).
The classic polarogram (polarographic wave) in an idealized form is presented below:
The main characteristics of a polarographic wave are the magnitude of the diffusion current ( I e, μA), half-wave potential ( E 1/2, V) - the potential at which the current is equal to half the diffusion, and the slope of the ascending section (0.059 / n is the steepness of the electrode function). These parameters make it possible to use polarography as a method of analysis (current strength is proportional to concentration) and research (half-wave potential and electrode function depend on the nature of the substance).
In the initial section of the polarographic wave (A-B), the current increases very slowly with a change in potential - this is the so-called residual current (I ost) . The main contribution to the residual current is made by the formation of a double electric layer ( charging current), which cannot be excluded and whose value increases with increasing potential. The second term of the residual current is the current due to electroactive impurities, which can be reduced by using pure reagents and water.
Upon reaching point B ( release potential– during reduction at the cathode, the release potential is called recovery potential E vos, during oxidation at the anode - oxidation potential E ok), an electrochemical reaction begins on the electrode, into which an electroactive substance (depolarizer) enters, as a result of which the current increases sharply (section B-C) to a certain limit value, then remaining practically constant (section C-D). The current corresponding to this section is called current limit(I pr), and the difference between the limiting and residual current is diffusion current (I d = I etc - I ost). In the section C-D, with an increase in potential, the limiting and residual currents increase slightly, and the value of the diffusion current remains constant. The rise in current at point G is due to a new electrochemical reaction (for example, the reduction of cations of the supporting electrolyte).
The diffusion current got its name due to the fact that in the given range of potentials, as a result of an electrochemical reaction, an almost complete absence of a depolarizer is observed in the near-electrode layer and its enrichment with a substance occurs due to diffusion of the depolarizer from the depth of the solution, where its concentration remains constant. Since the diffusion rate under these specific conditions remains constant, the diffusion current also retains its constant value.
Dependence of the diffusion current on the concentration of the depolarizer for r.c.e. is expressed by the Ilkovich equation:
I d = 605nD 1/2 m 2/3 t 1/6 s
where D is the diffusion coefficient of an electroactive ion; n is the number of electrons involved in the reaction; m 2/3 t 1/6 - characteristic of the capillary from which mercury flows; c is the concentration of the analyte (depolarizer).
When working with the same capillary and depolarizer, the value 605nD 1/2 m 2/3 t 1/6 = const, therefore, there is a linear relationship between the wave height and the substance concentration
Quantitative polarographic analysis is based on this linear relationship. The relationship between the electrode potential and the emerging current is described by the polarographic wave equation (the Ilkovich-Heyrovskiy equation):
where E and I are, respectively, the potential and the magnitude of the current for a given point of the polarographic curve; I d - the magnitude of the diffusion current; E 1/2 - half-wave potential.
E 1/2 is the potential at which a current equal to half I d is reached. It does not depend on the concentration of the depolarizer. E 1/2 is very close to the normal redox potential of the system (Eo), that is, it is a qualitative characteristic determined only by the nature of the reducing ions and by which it is possible to establish the qualitative composition of the analyzed solution.
Polarogram (voltammogram) contains valuable analytical information - half-wave potential E 1/2 is a qualitative characteristic of the depolarizer (qualitative analytical signal), while the diffusion current I e is linearly related to the concentration of the analyte in the volume of the analyzed solution (quantitative analytical signal) – I d = KC.
Value E 1/2 can be calculated from the polarographic wave equation or defined graphically:
Found value E 1/2, taking into account the background electrolyte used, makes it possible to identify the depolarizer on the basis of tabular data. If there are several substances in the analyzed solution, the half-wave potentials of which differ by more than 0.2 V, then the polarogram will have not one wave, but several - according to the number of electroactive particles. In this case, it should be borne in mind that the reduction (oxidation) of multiply charged particles can occur in steps, giving rise to several waves.
To exclude the movement of the substance to the electrode due to thermal and mechanical convection (mixing), the measurement is carried out in a thermostated solution and in the absence of mixing. The elimination of the electrostatic attraction of the depolarizer by the electrode field (migration) is facilitated by a large excess of the electrically inactive background electrolyte, whose ions shield the electrode charge, reducing the driving force of migration to almost zero.
When using a mercury dropping electrode, the polarogram shows current oscillation(its periodic slight increase and decrease). Each such oscillation corresponds to the emergence, growth, and detachment of a mercury drop from the microelectrode capillary. Polarographs have devices for eliminating oscillations.
Polarograms can be distorted by polarographic maxima- a sharp increase in current above its limit value with a subsequent decline:
The appearance of maxima is due to the mixing of the solution as a result of the movement of the surface of the mercury drop due to the uneven distribution of the charge, and, accordingly, the surface tension (maxima of the first kind), as well as the appearance of eddies when mercury flows out of the capillary (maxima of the second kind). The maxima distort the polarogram and make it difficult to decipher. To remove peaks of the first kind, a surfactant is introduced (for example, agar-agar, gelatin, camphor, fuchsin, synthetic surfactants), which, being adsorbed on the surface of a mercury drop, equalizes surface tension and eliminates the movement of surface layers of mercury. To remove type II maxima, it is sufficient to reduce the mercury pressure in the capillary by lowering the height of the mercury column.
Voltammetry with solid working electrodes differs from polarography with the use of RCE by a different range of polarization of the microelectrode. As shown above, due to the high hydrogen overvoltage on it, the dropping mercury electrode can be used in the region of high negative potentials, but due to the anodic dissolution of mercury at +0.4 V, it cannot be used for research in the field of positive potentials. On graphite and platinum, the discharge of hydrogen ions proceeds much more easily, therefore, their polarization region is limited by much lower negative potentials (-0.4 and -0.1 V, respectively). At the same time, in the region of anodic potentials, platinum and graphite electrodes are suitable up to a potential of +1.4 V (then the electrochemical reaction of oxygen oxidation of water 2H 2 O - 4 e→ О 2 + 4Н +), which makes them suitable for research in the range of positive potentials.
In contrast to RCE, during the recording of a voltammogram, the surface of a solid microelectrode is not renewed and is easily contaminated by the products of the electrode reaction, which leads to a decrease in the reproducibility and accuracy of the results; therefore, before recording each voltammogram, the surface of the microelectrode should be cleaned.
Stationary solid electrodes have not found wide application in voltammetry due to the slow establishment of the limiting current, which leads to distortion of the shape of the voltammogram, however, on rotating microelectrodes conditions for stationary diffusion arise in the near-electrode layer; therefore, the current strength is established quickly and the voltammogram has the same shape as in the case of the RCSE.
The value of the limiting diffusion current on a rotating disk electrode (regardless of the material) is described by the equation of convective diffusion (Levich):
I d = 0.62nFSD 2/3 w 1/2 n -1/6 s
where n is the number of electrons involved in the electrode process;
F is the Faraday number (96500 coulombs);
S is the area of the electrode;
D is the diffusion coefficient of the depolarizer;
w is the angular velocity of the electrode;
n is the kinematic viscosity of the test solution;
c is the concentration of the depolarizer, mol/l.
If it is difficult to decipher the polarograms, the “witness” method is used - after registering the polarogram of the analyzed solution, standard solutions of the proposed compounds are added to it in turn in the electrolytic cell. If the assumption was correct, then the height of the wave of the corresponding substance increases, if the assumption is incorrect, an additional wave will appear at a different potential.
The concentration of the depolarizer in the analyzed solution can be determined by the methods of the calibration graph, the standard (comparison) method, and the additive method. In this case, in all cases, standard solutions should be used, the composition of which is as close as possible to the composition of the analyzed solution, and the conditions for recording polarograms should be the same. The methods are applicable in the concentration range where the directly proportional dependence of the diffusion current on the concentration of the depolarizer is strictly observed. In practice, in quantitative determinations, as a rule, the magnitude of the diffusion current is not fixed in μA, but the height of the polarographic wave is measured. h, as indicated in the previous figure, which is also a linear function of concentration h = KC.
By calibration curve method register polarograms of a series of standard solutions and build a calibration graph in the coordinates h÷C(or I d ÷ FROM), by which for the found value h x in the analyzed solution find the concentration of the analyte in it FROM X.
AT standard (comparison) method under the same conditions, polarograms of the analyzed and standard solutions of the analyte are recorded with concentrations FROM x and FROM st, then:
Using additive method first, a polarogram of the analyzed solution is recorded with a volume V x with concentration FROM x and measure the wave height h x. Then a certain volume of the standard solution of the analyte is added to the electrolytic cell to the analyzed solution. V d with concentration FROM d (preferably V x>> V d and FROM X<FROM e) record the polarogram of the solution with concentration FROM x + d and measure the height of the received wave h x+d. Simple transformations allow using these data to calculate the concentration of the analyte in the analyzed solution (example).
Example. Polarography of 10.0 ml of nicotinamide solution resulted in a wave with a height of 38 mm. After adding 1.50 ml of a standard solution containing 2.00 mg/ml of nicotinamide to this solution, the wave increased to 80.5 mm. Calculate the content of the drug (mg/ml) in the analyzed solution.
Solution. Wave height of nicotinamide in the analyzed solution h x in accordance with the Ilkovich equation is equal to:
and after adding the standard solution ( h x+d):
If we divide the first equation term by term by the second, we get:
Solving the equation for FROM x and substituting the values of the quantities from the condition of the problem.
COURSE WORK
By discipline: ______ ___________
EXPLANATORY NOTE
_______ Potentiometry and potentiometric titration ________
(full name) (signature)
GRADE: _____________
The date: ___________________
CHECKED
Project Manager: Tsybizov A.V. /________________/
(full name) (signature)
St. Petersburg
Department of Metallurgy of Non-Ferrous Metals
COURSE WORK
By discipline _________ Physical and chemical methods of substance analysis __________
(name of the academic discipline according to the curriculum)
EXERCISE
Group student: ONG-10-1 Fandofan A.A. . (group code) (full name)
1. Project theme: Potentiometry and potentiometric titration.
3. List of graphic material: Presentation of results in the form of graphs, tables, figures.
4. Deadline for the completed project 10.12.12
Project Manager: Tsybizov A.V. /________________/
(full name) (signature)
Job issue date: 24.10.12
annotation
This explanatory note is a report on the implementation of the course project. The purpose of the work is to learn how to navigate the main flow of information on analytical chemistry, work with classical and periodic literature in the field of analytical chemistry of non-ferrous metals, technically competently understand and evaluate the proposed methods and methods of analysis.
Pages 17, drawings 0.
The Summary
This explanatory note is a report on the implementation of a course project. The aim is to learn to navigate the mainstream media in analytical chemistry, to work with classical literature and periodicals in the field of analytical chemistry of non-ferrous metals, technically competent to understand and evaluate the proposed methods and analysis techniques.
Pages 17, figures 0.
Abstract.. 3
Introduction. 5
Brief description of electrochemical methods of analysis.. 6
Potentiometry.. 7.
Direct potentiometry.. 10
Potentiometric titration. 13
Conclusion. 16
References.. 17
Introduction
The purpose of the work is to learn how to navigate the main flow of information on analytical chemistry, work with classical and periodic literature in the field of analytical chemistry of non-ferrous metals, technically competently understand and evaluate the proposed methods and methods of analysis.
Taking into account the peculiarities of analytical control in non-ferrous metallurgy (many determined elements, including gangue elements, satellite elements; complex combinations of elements in minerals; a very wide range of element concentrations, etc.), among the methods of physicochemical analysis that have received the most distribution in factory and research laboratories, one should include such classical methods as titrimetry (including complexometry), gravimetry (for high concentrations of elements and arbitration analysis) and optical methods of analysis that have been developing especially intensively recently (spectrophotometry, extraction-photometric method, atomic - absorption analysis, X-ray spectral analysis) and electrochemical (potentiometry, voltammetry).
The variety of raw materials presents us with a wide range of metals and elements that need to be quantified: basic metals of non-ferrous and ferrous metallurgy (copper, nickel, lead, zinc, tin, aluminum, magnesium, titanium, antimony, arsenic, iron, cadmium, silver, chromium etc.), rock-forming elements (silicon, calcium, sodium, chlorine, fluorine, sulfur, phosphorus, etc.) and rare metals (lithium, rubidium, cesium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, tungsten, rhenium , gallium, indium, thallium, germanium, selenium, tellurium, etc.).
Brief description of electrochemical methods of analysis
Electrochemical methods of analysis and research are based on the study and use of processes occurring on the electrode surface or in the near-electrode space. Any electrical parameter (potential, current strength, resistance, etc.) that is functionally related to the concentration of the analyzed solution and can be correctly measured can serve as an analytical signal.
A great convenience is that electrochemical methods use electrical effects, and that the result of this effect (response) is also obtained in the form of an electrical signal. This provides high speed and accuracy of counting, opens up wide possibilities for automation. Electrochemical methods of analysis are distinguished by good sensitivity and selectivity; in some cases, they can be attributed to microanalysis, since sometimes less than 1 ml of solution is sufficient for analysis.
For any kind of electrochemical measurements, an electrochemical circuit or an electrochemical cell is required, the component of which is the analyzed solution. The substance to be determined can be included both in the composition of the electrolyte filling the cell and in the composition of one of the electrodes. If the analytical redox reaction proceeds spontaneously on the electrodes of the cell, that is, without the application of voltage from an external source, but only due to the potential difference (EMF) of its electrodes, then such a cell is called a galvanic cell.
Distinguish direct and indirect electrochemical methods . In direct methods, the dependence of the current strength (potential, etc.) on the concentration of the analyte is used. In indirect methods, the current strength (potential, etc.) is measured in order to find the end point of the titration of the component to be determined with a suitable titrant, i.e. use the dependence of the measured parameter on the volume of the titrant.
According to the types of analytical signal, EMA is divided into: 1) conductometry - measurement of the electrical conductivity of the test solution; 2) potentiometry- measurement of the currentless equilibrium potential of the indicator electrode, for which the substance under study is a potency-determining one; 3) coulometry - measurement of the amount of electricity required for the complete transformation (oxidation or reduction) of the substance under study; 4) voltammetry - measurement of stationary or non-stationary polarization characteristics of electrodes in reactions involving the test substance;
5) electrogravimetry - measurement of the mass of a substance released from a solution during electrolysis.
Potentiometry
Potentiometry (from Latin potentia - strength, power and Greek metreo - measure) is an electrochemical method for determining various physical and chemical quantities, based on measuring the equilibrium electrode potential of an indicator electrode immersed in the test solution. The potential of the indicator electrode, which is determined by the activity of the components of the electrochemical reaction, is measured relative to the reference electrode. Potentiometry is widely used in analytical chemistry to determine the concentration of substances in solutions (potentiometric titration), to measure the concentration of hydrogen ions (pH-metry), as well as other ions (ionometry).
Potentiometry is based on the dependence of the equilibrium electrode potential E from thermodynamic activity a components of the electrochemical reaction:
aA + bB + ... + n e m M+ R P + ...
This dependence is described by the Nernst equation:
E = E° + R T/(n F) log ( a oxide / a restore)
E = E° + R T /(n F) ln ([oxide] ү oxide /([restore] ү restore)), where
R- universal gas constant, equal to 8.31 J / (mol. K); T- absolute temperature; F- Faraday's constant (96500 C/mol); n- the number of electrons involved in the electrode reaction; a oxide, a res - activities of the oxidized and reduced forms of the redox system, respectively; [oxide] and [restore] - their molar concentrations; ү oxide, ү restore - activity coefficients; E° - standard potential of the redox system.
Substituting T= 298.15 K and the numerical values of the constants in the equation, we get:
E = E° + (0.059 / n) lg ( a oxide / a restore)
E = E° + (0.059 / n) lg ([oxide] ү oxide /([restore] ү restore))
For potentiometric measurements, a galvanic cell with an indicator electrode is made up , the potential of which depends on the activity of at least one of the components of the electrochemical reaction, and the reference electrode and measure the electromotive force (emf) of this element.
In potentiometry, galvanic cells are used without transfer, when both electrodes are placed in the same test solution, and with transfer, when the electrodes are in different solutions that have electrolytic contact with each other. The latter is carried out in such a way that the solutions can only mix with each other by diffusion. Usually they are separated by a porous ceramic or plastic partition or a tightly ground glass sleeve. Elements without transfer are mainly used to measure chemical equilibrium constants. reactions, dissociation constants of electrolytes. stability constants of complex compounds, solubility products, standard electrode potentials, as well as activities and activity coefficients of ions. Transfer elements are used to determine "apparent" equilibrium constants (because they do not take into account the liquid potential), activities and activity coefficients of ions, as well as in potentiometric methods of analysis.
Direct potentiometry
Direct potentiometry methods are based on the application of the Nernst equation to find the activity or concentration of an electrode reaction participant from the experimentally measured EMF of the circuit or the electrode potential. Direct potentiometry is used to directly determine a ions (for example, Ag + in a solution of AgNO 3) according to the value of the EMF of the corresponding indicator electrode (for example, silver); in this case, the electrode process should be reversible. Historically, the first methods of direct potentiometry were methods for determining the pH value. . A glass electrode is most often used to determine pH. The main advantages of the glass electrode are ease of operation, rapid equilibrium and the ability to determine pH in redox systems. The disadvantages include the fragility of the electrode material and the complexity of work in the transition to strongly alkaline and strongly acidic solutions.
The appearance of membrane ion-selective electrodes led to the emergence of ionometry (pX-metry), where pX \u003d - lg ahah - the activity of component X of the electrochemical reaction. Sometimes pH-metry is considered as a special case of ionometry. Calibrating the scales of potentiometer instruments by pX values is difficult due to the lack of appropriate standards. Therefore, when using ion-selective electrodes, the activity (concentration) of ions is determined, as a rule, using a calibration graph or the method of additions. The use of such electrodes in non-aqueous solutions is limited due to the instability of their body and membrane to the action of organic solvents.
Direct potentiometry also includes redoxmetry - the measurement of standard and real redox potentials and equilibrium constants of redox reactions. The redox potential depends on the activities of the oxidized (O) and reduced ( a vos) forms of matter. Redoxmetry is also used to determine the concentration of ions in solutions. Direct potentiometry using metal electrodes is used to study the mechanism and kinetics of precipitation and complexation reactions.
The calibration curve method is also used. . To do this, a calibration graph is built in advance in the coordinates of the EMF - lg FROM en using standard solutions of the analyzed ion having the same ionic strength of the solution.
In this case f en(activity ratio) and E differential(diffusion potential) remain constant and the graph becomes linear. Then, using the same ionic strength, the EMF of the circuit with the analyzed solution is measured and the concentration of the solution is determined from the graph. An example of a definition is shown in fig. one.
Fig.1. Calibration curve for determining the concentration by direct potentiometry
Direct potentiometry has important advantages. During measurements, the composition of the analyzed solution does not change. In this case, as a rule, no preliminary separation of the analyte is required. The method can be easily automated, which makes it possible to use it for continuous control of technological processes.
