There are many types of analysis. They can be classified according to different criteria:
- by the nature of the information received. Distinguish qualitative analysis(in this case, they find out what this substance consists of, which components are included in its composition) and quantitative analysis(determine the content of certain components, for example, in% by weight, or the ratio of different components). The line between qualitative and quantitative analysis is very conditional, especially in the study of microimpurities. So, if in the course of a qualitative analysis a certain component was not detected, then it is necessary to indicate what the minimum amount of this component could be detected using this method. Perhaps the negative result of a qualitative analysis is not due to the absence of a component, but to the insufficient sensitivity of the method used! On the other hand, quantitative analysis is always performed taking into account the previously found qualitative composition of the material under study.
- classification by objects of analysis: technical, clinical, forensic and etc.
- classification by objects of definition.
Do not confuse terms - analyze and determine. Objects definitions name the components whose content needs to be established or reliably detected. Taking into account the nature of the component being determined, various types of analysis are distinguished (Table 1.1).
Table 1-1. Classification of types of analysis (by objects of definition or detection)
| Type of analysis | Object of definition (or detection) | Example | Application area |
| Isotopic | Atoms with given values of nuclear charge and mass number (isotopes) | 137 Cs, 90 Sr, 235 U | Nuclear energy, environmental pollution control, medicine, archeology, etc. |
| elemental | Atoms with given nuclear charge values (elements) | Cs, Sr, U, Cr, Fe, Hg | Everywhere |
| Real | Atoms (ions) of an element in a given oxidation state or in compounds of a given composition (element shape) | Cr(III), Fe 2+ , Hg in complex compounds | Chemical Technology, environmental pollution control, geology, metallurgy, etc. |
| Molecular | Molecules with a given composition and structure | Benzene, glucose, ethanol | Medicine, environmental pollution control, agrochemistry, chemical technology, criminalistics. |
| Structural group or functional | The sum of molecules with given structural characteristics and similar properties (the sum of isomers and homologues) | Limit hydrocarbons, monosaccharides, alcohols | Chemical technology, food industry, medicine. |
| phase | Phase or element within a given phase | Graphite in steel, quartz in granite | Metallurgy, geology, technology of building materials. |
The classification "by objects of definition" is very important because it helps to choose suitable way analysis (analytical method). Yes, for elemental analysis often used spectral methods based on the registration of radiation of atoms at different wavelengths. Most spectral methods involve complete destruction (atomization) of the analyte. If it is necessary to establish the nature and quantitative content of various molecules that make up the composition of the organic substance under study ( molecular analysis), then one of the most suitable methods will be chromatographic, which does not involve the destruction of molecules.
During elemental analysis identify or quantify elements, regardless of their degree of oxidation or on the inclusion in the composition of certain molecules. The full elemental composition of the test material is determined in rare cases. It is usually sufficient to determine some elements that significantly affect the properties of the object under study.
Real analysis began to be singled out as an independent form relatively recently, earlier it was considered as part of the elemental one. The purpose of material analysis is to separately determine the content of different forms of the same element. For example, chromium(III) and chromium(VI) in waste water. In petroleum products, “sulphate sulfur”, “free sulfur” and “sulfide sulfur” are separately determined. Investigating the composition of natural waters, they find out what part of mercury exists in the form of strong (non-dissociating) complex and elemental elements. organic compounds, and which - in the form of free ions. These tasks are more difficult than those of elemental analysis.
Molecular analysis especially important in the study organic matter and materials of biogenic origin. An example would be the determination of benzene in gasoline or acetone in exhaled air. In such cases, it is necessary to take into account not only the composition, but also the structure of the molecules. Indeed, in the material under study there may be isomers and homologues of the determined component. Thus, it is often necessary to determine the content of glucose in the presence of many of its isomers and other related compounds, such as sucrose.
When it comes to determining the total content of all molecules that have some common structural features, the same functional groups, and hence similar chemical properties, use the term structural-group(or functional) analysis. For example, the amount of alcohols (organic compounds having an OH group) is determined by carrying out a reaction common to all alcohols with metallic sodium, and then measuring the volume of hydrogen released. Amount unsaturated hydrocarbons(having double or triple bonds) is determined by oxidizing them with iodine. The total content of the same type of components is sometimes also established in inorganic analysis - for example, the total content of rare earth elements.
A specific type of analysis is phase analysis. So, carbon in cast irons and steels can dissolve in iron, can form chemical compounds with iron (carbides), or can form a separate phase (graphite). The physical properties of the product (strength, hardness, etc.) depend not only on the total carbon content, but also on the distribution of carbon between these forms. Therefore, metallurgists are interested not only in the total carbon content in cast iron or steel, but also in the presence of a separate phase of graphite (free carbon) in these materials, as well as the quantitative content of this phase.
The main focus of the basic course in analytical chemistry is elemental and molecular analysis. In other types of analysis, very specific methods are used, and the program basic course isotopic, phase and structural group analyzes are not included.
Classification according to the accuracy of the results, the duration and cost of the analyzes. A simplified, fast and cheap version of the analysis is called express analysis. For their implementation, they often use test methods. For example, anyone (not an analyst) can evaluate the content of nitrates in vegetables (sugar in urine, heavy metals in drinking water, etc.) using a special indicator paper. The result will be visible to the eye, since the content of the component is determined using the color scale attached to the paper. Test methods do not require the delivery of a sample to the laboratory, any processing of the test material; these methods do not use expensive equipment and do not perform calculations. It is only important that the result does not depend on the presence of other components in the material under study, and for this it is necessary that the reagents with which the paper is impregnated during its manufacture would be specific. It is very difficult to ensure the specificity of test methods, and this type of analysis has become widespread only in last years XX century.. Of course, test methods cannot provide high accuracy of analysis, but it is not always required.
The direct opposite of express analysis - arbitration analysis. The main requirement for it is to ensure the greatest possible accuracy of the results. Arbitration analyzes are carried out quite rarely (for example, to resolve a conflict between a manufacturer and a consumer of industrial products). To perform such analyzes, the most qualified performers are involved, the most reliable and repeatedly proven methods are used. The time spent on performing such an analysis, as well as its cost, are of no fundamental importance.
An intermediate place between express and arbitrage analysis - in terms of accuracy, duration, cost and other indicators - is occupied by the so-called routine tests. The main part of the analyzes performed in the factory and other control and analytical laboratories is of this type.
There are other ways of classification, other types of analysis. For example, take into account the mass of the material under study, directly used in the course of the analysis. Within the framework of the corresponding classification, there are macroanalysis(kilograms, liters), semi-microanalysis(fractions of a gram, milliliters) and microanalysis. In the latter case, weighings of the order of a milligram or less are used, the volumes of solutions are measured in microliters, and the result of the reaction sometimes has to be observed under a microscope. Microanalysis is rarely used in analytical laboratories.
1.3. Analysis Methods
The concept of "method of analysis" is the most important for analytical chemistry. This term is used when they want to reveal the essence of this or that analysis, its main principle. The method of analysis is a fairly universal and theoretically justified way of conducting an analysis, regardless of which component is determined and what exactly is analyzed. There are three main groups of methods (Fig. 1-1). Some of them are aimed primarily at separating the components of the mixture under study (subsequent analysis without this operation turns out to be inaccurate or even impossible). In the course of separation, the concentration of the components to be determined usually also occurs (see Chapter 8). An example would be extraction methods or ion exchange methods. Other methods are used in the course of qualitative analysis, they serve for reliable identification (identification) of the components of interest to us. The third, the most numerous, are intended for the quantitative determination of components. The respective groups are called methods of separation and concentration, methods of identification and methods of determination. The methods of the first two groups, as a rule , play a supporting role; they will be discussed later. The most important for practice are determination methods.
In addition to the three main groups, there are hybrid methods. Figure 1.1 does not show these methods. In hybrid methods, separation, identification and determination of components are organically combined in one instrument (or in a single set of instruments). The most important of these methods is chromatographic analysis. In a special device (chromatograph), the components of the test sample (mixture) are separated, since they move at different speeds through a column filled with powder solid(sorbent). By the time of release of the component from the column, its nature is judged and thus all components of the sample are identified. The components leaving the column in turn fall into another part of the device, where a special device - a detector - measures and records the signals of all components. Often, the automatic calculation of the contents of all components is immediately carried out. It is clear that chromatographic analysis cannot be considered only as a method of separation of components, or only as a method of quantitative determination, it is precisely a hybrid method.
Each method of determination combines many specific methods in which the same physical quantity is measured. For example, to carry out a quantitative analysis, one can measure the potential of an electrode immersed in the test solution, and then, using the found potential value, calculate the content of a certain component of the solution. All methods, where the main operation is to measure the potential of the electrode, are considered special cases. potentiometric method. When attributing the methodology to one or another analytical method it doesn’t matter what object is being studied, what substances are determined and with what accuracy, what device is used and how calculations are carried out - it is only important what we are measuring. The physical quantity measured during the analysis, which depends on the concentration of the analyte, is usually called analytical signal.
In a similar way, one can single out the method spectral analysis. In this case, the main operation is the measurement of the intensity of light emitted by the sample at a certain wavelength. Method titrimetric (volumetric) analysis is based on measuring the volume of the solution spent on the chemical reaction with the determined component of the sample. The word "method" is often omitted, they simply say "potentiometry", "spectral analysis", "titrimetry", etc. AT refractometric analysis the signal is the refractive index of the test solution, in spectrophotometry- absorption of light (at a certain wavelength). The list of methods and their corresponding analytical signals can be continued; in total, several dozen independent methods are known.
Each method of determination has its own theoretical basis and is associated with the use of specific equipment. The areas of application of different methods differ significantly. Some methods are mainly used for the analysis of petroleum products, others - for the analysis of drugs, others - for the study of metals and alloys, etc. Similarly, methods for elemental analysis, methods of isotopic analysis, etc. can be distinguished. There are also universal methods used in the analysis of a wide variety of materials and suitable for determining the most diverse components in them. For example, the spectrophotometric method can be used for elemental, molecular, and structural group analysis.
Accuracy, sensitivity, and other characteristics of individual methods related to the same analytical method differ, but not as much as the characteristics of different methods. Any analytical problem can always be solved by several different methods (for example, chromium in alloyed steel can be determined by the spectral method, and titrimetric, and potentiometric). The analyst chooses a method, taking into account the known capabilities of each of them and the specific requirements for this analysis. It is impossible to choose the “best” and “worst” methods once and for all, everything depends on the problem being solved, on the requirements for the analysis results. Thus, gravimetric analysis, as a rule, gives more accurate results than spectral analysis, but it requires a lot of labor and time. Therefore, gravimetric analysis is good for arbitration analysis, but not suitable for express analysis.
The determination methods are divided into three groups: chemical, physical and physico-chemical. Often, physical and physico-chemical methods are combined under the common name “instrumental methods”, since in both cases instruments are used, and the same ones. In general, the boundaries between groups of methods are very arbitrary.
Chemical Methods are based on carrying out a chemical reaction between the determined component and a specially added reagent. The reaction proceeds according to the scheme:
Hereinafter, the symbol X denotes the component being determined (molecule, ion, atom, etc.), R is the added reagent, Y is the totality of reaction products. The group of chemical methods includes classical (long-known and well-studied) methods of determination, primarily gravimetry and titrimetry. The number of chemical methods is relatively small, they all have the same theoretical foundations (theory chemical equilibria, laws of chemical kinetics, etc.). As an analytical signal in chemical methods, the mass or volume of a substance is usually measured. Complex physical instruments, with the exception of analytical balances, and special standards chemical composition are not used in chemical methods. These methods have much in common in terms of their capabilities. They will be discussed in chapter 4.
Physical Methods not associated with chemical reactions and the use of reagents. Their main principle is the comparison of the same type of analytical signals of the X component in the material under study and in a certain reference (sample with a precisely known concentration of X). Having built a calibration graph in advance (the dependence of the signal on the concentration or mass X) and measuring the signal value for a sample of the material under study, the X concentration in this material is calculated. There are other ways to calculate concentrations (see Chapter 6). Physical methods are usually more sensitive than chemical ones; therefore, the determination of microimpurities is carried out mainly by physical methods. These methods are easy to automate and require less time for analysis. However, physical methods require special standards, rather complex, expensive and highly specialized equipment. In addition, they are usually less accurate than chemical ones.
An intermediate place between chemical and physical methods in terms of their principles and capabilities is occupied by physical and chemical analysis methods. In this case, the analyst conducts a chemical reaction, but its course or its result is followed not visually, but with the use of physical instruments. For example, it gradually adds to the test solution another - with a known concentration of the dissolved reagent, and at the same time controls the potential of the electrode dipped into the titrated solution (potentiometric titration), The analyst judges the completion of the reaction by the jump in potential, measures the volume of titrant spent on it, and calculates the result of the analysis. Such methods are generally as accurate as chemical methods and almost as sensitive as physical methods.
Instrumental methods are often divided according to another, more clearly expressed feature - the nature of the measured signal. In this case, subgroups of optical, electrochemical, resonant, activation and other methods are distinguished. There are also few and as yet underdeveloped methods biological and biochemical methods.
Lecture plan:
1. general characteristics physical and chemical methods
2. General information about spectroscopic methods of analysis.
3. Photometric analysis method: photocolorimetry, colorimetry, spectrophotometry.
4. General information about nephelometric, luminescent, polarimetric methods of analysis.
5. Refractometric method of analysis.
6. General information about mass-spectral, radiometric analyses.
7. Electrochemical methods of analysis (potentiometry, conductometry, coulometry, amperometry, polarography).
8. Chromatographic method of analysis.
The essence of physico-chemical methods of analysis. Their classification.
Physico-chemical methods of analysis, like chemical methods, are based on carrying out one or another chemical reaction. In physical methods, chemical reactions are absent or are of secondary importance, although in spectral analysis the line intensity always depends significantly on chemical reactions in a carbon electrode or in a gas flame. Therefore, sometimes physical methods are included in the group of physicochemical methods, since there is no sufficiently strict unambiguous difference between physical and physicochemical methods, and the allocation of physical methods to a separate group is not of fundamental importance.
Chemical methods of analysis were not able to satisfy the diverse demands of practice, which increased as a result of scientific and technological progress, the development of the semiconductor industry, electronics and computers, the widespread use of pure and ultrapure substances in technology.
The use of physical and chemical methods of analysis is reflected in the technochemical control of food production, in research and production laboratories. These methods are characterized by high sensitivity and fast analysis. They are based on the use of physical chemical properties substances.
When performing analyzes by physicochemical methods, the equivalence point (the end of the reaction) is determined not visually, but with the help of instruments that record the change in the physical properties of the test substance at the equivalence point. For this purpose, devices with relatively complex optical or electrical circuits are usually used, so these methods are called methods. instrumental analysis.
In many cases, these methods do not require a chemical reaction to perform the analysis, unlike chemical methods of analysis. It is only necessary to measure the indicators of any physical properties of the analyzed substance: electrical conductivity, light absorption, light refraction, etc. Physicochemical methods allow continuous monitoring of raw materials, semi-finished products and finished products in industry.
Physicochemical methods of analysis began to be used later than chemical methods of analysis, when the relationship between the physical properties of substances and their composition was established and studied.
The accuracy of physicochemical methods varies greatly depending on the method. The highest accuracy (up to 0.001%) has coulometry, based on the measurement of the amount of electricity that is spent on the electrochemical oxidation or reduction of the ions or elements being determined. Most physicochemical methods have an error within 2-5%, which exceeds the error of chemical methods of analysis. However, such a comparison of errors is not entirely correct, since it refers to different concentration regions. With a low content of the determined component (about 10 -3% or less), classical chemical methods of analysis are generally unsuitable; at high concentrations, physicochemical methods successfully compete with chemical ones. Among the significant shortcomings of most physicochemical methods is the mandatory availability of standards and standard solutions.
Among the physicochemical methods, the most practical applications are:
1. spectral and other optical methods (refractometry, polarimetry);
2. electrochemical methods of analysis;
3. chromatographic methods of analysis.
In addition, there are 2 more groups of physico-chemical methods:
1. radiometric methods based on measuring the radioactive emission of a given element;
2. mass spectrometric methods of analysis based on the determination of the masses of individual ionized atoms, molecules and radicals.
The most extensive in terms of the number of methods and important in terms of practical value is the group of spectral and other optical methods. These methods are based on the interaction of substances with electromagnetic radiation. There are many different types of electromagnetic radiation: x-rays, ultraviolet, visible, infrared, microwave and radio frequency. Depending on the type of interaction of electromagnetic radiation with matter optical methods are classified as follows.
On the measurement of the effects of polarization of the molecules of a substance are based refractometry, polarimetry.
Analyzed substances can absorb electromagnetic radiation and, based on the use of this phenomenon, a group is distinguished absorption optical methods.
The absorption of light by atoms of analytes is used in atomic absorption analysis. The ability to absorb light by molecules and ions in the ultraviolet, visible and infrared regions of the spectrum made it possible to create molecular absorption analysis (colorimetry, photocolorimetry, spectrophotometry).
The absorption and scattering of light by suspended particles in a solution (suspension) has led to the emergence of methods turbidimetry and nephelometry.
Methods based on measuring the intensity of radiation resulting from the release of energy by excited molecules and atoms of the analyzed substance are called emission methods. To molecular emission methods include luminescence (fluorescence), to atomic emission- emission spectral analysis and flame photometry.
Electrochemical methods analyzes are based on the measurement of electrical conductivity ( conductometry); potential difference ( potentiometry); the amount of electricity passing through the solution coulometry); the dependence of the current on the applied potential ( voltammetry).
To the group chromatographic methods of analysis includes methods of gas and gas-liquid chromatography, distribution, thin-layer, adsorption, ion-exchange and other types of chromatography.
Spectroscopic methods of analysis: general information
The concept of the spectroscopic method of analysis, its varieties
Spectroscopic methods of analysis- physical methods based on the interaction of electromagnetic radiation with matter. The interaction leads to various energy transitions, which are recorded instrumentally in the form of radiation absorption, reflection and scattering of electromagnetic radiation.
Classification:
Emission spectral analysis is based on the study of emission (radiation) spectra or emission spectra various substances. A variation of this analysis is flame photometry, based on measuring the intensity of atomic radiation excited by heating a substance in a flame.
Absorption spectral analysis is based on the study of the absorption spectra of the analyzed substances. If radiation is absorbed by atoms, then the absorption is called atomic, and if by molecules, then it is called molecular. There are several types of absorption spectral analysis:
1. Spectrophotometry - takes into account the absorption of light with a certain wavelength by the analyzed substance, i.e. absorption of monochromatic radiation.
2. Photometry - based on measuring the absorption of light by the analyzed substance is not strictly monochromatic radiation.
3. Colorimetry is based on measuring the absorption of light by colored solutions in the visible part of the spectrum.
4. Nephelometry is based on the measurement of the intensity of light scattered by solid particles suspended in solution, i.e. light scattered by the suspension.
Luminescence spectroscopy uses the glow of the object under study, which occurs under the action of ultraviolet rays.
Depending on in which part of the spectrum absorption or emission occurs, spectroscopy is distinguished in the ultraviolet, visible and infrared regions of the spectrum.
Spectroscopy is a sensitive method for determining more than 60 elements. It is used to analyze numerous materials, including biological media, plant materials, cements, glasses, and natural waters.
Photometric methods of analysis
Photometric methods of analysis are based on the selective absorption of light by the analyte or its combination with a suitable reagent. The absorption intensity can be measured by any method, regardless of the nature of the colored compound. The accuracy of the method depends on the method of measurement. There are colorimetric, photocolorimetric and spectrophotometric methods.
Photocolorimetric method of analysis.
The photocolorimetric method of analysis makes it possible to quantitatively determine the intensity of light absorption by the analyzed solution using photoelectrocolorimeters (sometimes they are simply called photocolorimeters). To do this, prepare a series of standard solutions and plot the dependence of the light absorption of the analyte on its concentration. This dependence is called a calibration curve. In photocolorimeters, the light fluxes passing through the solution have a wide absorption region - 30-50 nm, so the light here is polychromatic. This leads to loss of reproducibility, accuracy and selectivity of the analysis. The advantages of the photocolorimeter lie in the simplicity of design and high sensitivity due to the large luminosity of the radiation source - an incandescent lamp.
Colorimetric method of analysis.
The colorimetric method of analysis is based on measuring the absorption of light by a substance. In this case, the color intensity is compared, i.e. optical density of the test solution with the color (optical density) of a standard solution, the concentration of which is known. The method is very sensitive and is used to determine micro- and semi-micro quantities.
The analysis by colorimetric method requires much less time than by chemical analysis.
In visual analysis, equality of the intensity of staining of the analyzed and stained solution is achieved. This can be achieved in 2 ways:
1. equalize the color by changing the layer thickness;
2. select standard solutions of different concentrations (method of standard series).
However, it is visually impossible to quantify how many times one solution is colored more intensely than another. In this case, it is possible to establish only the same color of the analyzed solution when comparing it with the standard one.
Basic law of light absorption.
If the light flux, the intensity of which is I 0, is directed to a solution located in a flat glass vessel (cuvette), then one part of its intensity I r is reflected from the surface of the cuvette, the other part with intensity I a is absorbed by the solution and the third part with intensity I t passes through solution. There is a relationship between these values:
I 0 \u003d I r + I a + I t (1)
Because the intensity I r of the reflected part of the light flux when working with identical cuvettes is constant and insignificant, then it can be neglected in the calculations. Then equality (1) takes the form:
I 0 \u003d I a + I t (2)
This equality characterizes the optical properties of the solution, i.e. its ability to absorb or transmit light.
The intensity of the absorbed light depends on the number of colored particles in the solution, which absorb light more than the solvent.
The light flux, passing through the solution, loses part of the intensity - the greater, the greater the concentration and thickness of the solution layer. For colored solutions, there is a relationship called the Bouguer-Lambert-Beer law (between the degree of light absorption, the intensity of the incident light, the concentration of the colored substance and the layer thickness).
According to this law, the absorption of monochromatographic light passing through a layer of colored liquid is proportional to the concentration and thickness of its layer:
I \u003d I 0 10 - kCh,
where I is the intensity of the light flux passing through the solution; I 0 is the intensity of the incident light; FROM- concentration, mol/l; h– layer thickness, cm; k is the molar absorption coefficient.
Molar absorption coefficient k is the optical density of a solution containing 1 mol/l absorbing substance, with a layer thickness of 1 cm. It depends on the chemical nature and physical state of the light-absorbing substance and on the wavelength of monochromatic light.
Standard series method.
The standard series method is based on obtaining the same color intensity of the test and standard solutions at the same layer thickness. The color of the test solution is compared with the color of a number of standard solutions. At the same color intensity, the concentrations of the test and standard solutions are equal.
To prepare a series of standard solutions, 11 test tubes of the same shape, size and glass are taken. Pour the standard solution from the burette in a gradually increasing amount, for example: into 1 test tube 0.5 ml, in the 2nd 1 ml, in the 3rd 1.5 ml, etc. - before 5 ml(in each next test tube 0.5 ml more than in the previous one). Equal volumes of a solution are poured into all test tubes, which gives a color reaction with the ion being determined. The solutions are diluted so that the liquid levels in all tubes are the same. The tubes are stoppered, the contents are thoroughly mixed and placed in a rack in increasing concentrations. In this way a color scale is obtained.
The same amount of reagent is added to the test solution in the same test tube, diluted with water to the same volume as in other test tubes. Close the cork, mix the contents thoroughly. The color of the test solution is compared with the color of standard solutions on a white background. Solutions should be well lit with diffused light. If the color intensity of the test solution coincides with the color intensity of one of the solutions on the color scale, then the concentrations of this and the test solutions are equal. If the color intensity of the test solution is intermediate between the intensity of two adjacent scale solutions, then its concentration is equal to the average concentration of these solutions.
The use of the method of standard solutions is advisable only for the mass determination of a substance. The prepared series of standard solutions has a relatively short time.
Method for equalizing the color intensity of solutions.
The method of equalizing the color intensity of the test and standard solutions is carried out by changing the layer height of one of the solutions. To do this, colored solutions are placed in 2 identical vessels: test and standard. Change the height of the solution layer in one of the vessels until the color intensity in both solutions is the same. In this case, determine the concentration of the test solution With research. , comparing it with the concentration of the standard solution:
From research \u003d C st h st / h research,
where h st and h research are the layer heights of the standard and test solutions, respectively.
Devices used to determine the concentrations of the studied solutions by equalizing the color intensity are called colorimeters.
There are visual and photoelectric colorimeters. In visual colorimetric determinations, the color intensity is measured by direct observation. Photoelectric methods are based on the use of photocells-photocolorimeters. Depending on the intensity of the incident light beam, an electric current is generated in the photocell. The strength of the current caused by exposure to light is measured with a galvanometer. The deflection of the arrow indicates the intensity of the color.
Spectrophotometry.
Photometric method is based on measuring the absorption of light of non-strictly monochromatic radiation by the analyzed substance.
If monochromatic radiation (radiation of one wavelength) is used in the photometric method of analysis, then this method is called spectrophotometry. The degree of monochromaticity of the electromagnetic radiation flux is determined by the minimum wavelength interval, which is distinguished by the used monochromator (light filter, grating or prism) from a continuous flow of electromagnetic radiation.
To spectrophotometry also include the field of measuring technology, which combines spectrometry, photometry and metrology and develops a system of methods and instruments for quantitative measurements of spectral coefficients of absorption, reflection, radiation, spectral brightness as characteristics of media, coatings, surfaces, emitters.
Stages of spectrophotometric research:
1) carrying out a chemical reaction to obtain systems suitable for spectrophotometric analysis;
2) measurements of the absorption of the resulting solutions.
The essence of the method of spectrophotometry
The dependence of the absorption of a solution of a substance on the wavelength on the graph is depicted as an absorption spectrum of a substance, on which it is easy to distinguish the absorption maximum located at the wavelength of light that is maximally absorbed by the substance. Measurement of the optical density of solutions of substances on spectrophotometers is carried out at the wavelength of the absorption maximum. This makes it possible to analyze in one solution substances whose absorption maxima are located at different wavelengths.
In spectrophotometry in the ultraviolet and visible regions, electronic absorption spectra are used.
They characterize the highest energy transitions, which are capable of a limited range of compounds and functional groups. In inorganic compounds, electronic spectra are associated with a high polarization of the atoms that make up the molecule of the substance, and usually appear in complex compounds. In organic compounds, the appearance of electronic spectra is caused by the transition of electrons from the ground to excited levels.
The position and intensity of the absorption bands are strongly affected by ionization. Acid-type ionization results in the appearance of an additional lone pair of electrons in the molecule, which leads to an additional bathochromic shift (a shift to the long-wavelength region of the spectrum) and an increase in the intensity of the absorption band.
The spectrum of many substances has several absorption bands.
For spectrophotometric measurements in the ultraviolet and visible regions, two types of instruments are used - non-registering(the result is observed on the instrument scale visually) and recording spectrophotometers.
Luminescent method of analysis.
Luminescence- the ability to self-luminescence, arising under various influences.
Classification of processes that cause luminescence:
1) photoluminescence (excitation by visible or ultraviolet light);
2) chemiluminescence (excitation due to the energy of chemical reactions);
3) cathodoluminescence (excitation by electron impact);
4) thermoluminescence (excitation by heating);
5) triboluminescence (excitation by mechanical action).
In chemical analysis, the first two types of luminescence matter.
Classification of luminescence by the presence of afterglow. It can stop immediately with the disappearance of excitation - fluorescence or continue for a certain time after the cessation of the exciting effect - phosphorescence. The phenomenon of fluorescence is mainly used, so the method is named fluorimetry.
Application of fluorimetry: analysis of traces of metals, organic (aromatic) compounds, vitamins D, B 6 . Fluorescent indicators are used for titration in cloudy or dark-colored media (titration is carried out in the dark, illuminating the titrated solution, where the indicator is added, with the light of a fluorescent lamp).
Nephelometric analysis.
Nephelometry was proposed by F. Kober in 1912 and is based on measuring the intensity of light scattered by a suspension of particles using photocells.
With the help of nephelometry, the concentration of substances that are insoluble in water, but form stable suspensions, is measured.
For nephelometric measurements, nephelometers, similar in principle to colorimeters, with the only difference being that with nephelometry
When conducting photonephelometric analysis first, based on the results of determining a series of standard solutions, a calibration graph is built, then the test solution is analyzed and the concentration of the analyte is determined from the graph. To stabilize the resulting suspensions, a protective colloid is added - a solution of starch, gelatin, etc.
Polarimetric analysis.
Electromagnetic vibrations natural light occur in all planes perpendicular to the direction of the beam. The crystal lattice has the ability to transmit rays only in a certain direction. Upon exiting the crystal, the beam oscillates only in one plane. A beam whose oscillations are in the same plane is called polarized. The plane in which vibrations occur is called oscillation plane polarized beam, and the plane perpendicular to it - plane of polarization.
The polarimetric method of analysis is based on the study of polarized light.
Refractometric method of analysis.
The basis of the refractometric method of analysis is the determination of the refractive index of the substance under study, since an individual substance is characterized by a certain refractive index.
Technical products always contain impurities that affect the refractive index. Therefore, the refractive index can in some cases serve as a characteristic of the purity of the product. For example, varieties of purified turpentine are distinguished by refractive indices. So, the refractive indices of turpentine at 20 ° for yellow, denoted by n 20 D (the entry means that the refractive index was measured at 20 ° C, the wavelength of the incident light is 598 mmk), are equal to:
First class Second class Third class
1,469 – 1,472 1,472 – 1,476 1,476 – 1,480
The refractometric method of analysis can be used for binary systems, for example, to determine the concentration of a substance in aqueous or organic solutions. In this case, the analysis is based on the dependence of the refractive index of the solution on the concentration of the solute.
For some solutions there are tables of dependence of refractive indices on their concentration. In other cases, they are analyzed using the calibration curve method: a series of solutions of known concentrations are prepared, their refractive indices are measured, and a plot of refractive indices versus concentration is plotted, i.e. build a calibration curve. It determines the concentration of the test solution.
refractive index.
When a beam of light passes from one medium to another, its direction changes. He breaks. The refractive index is equal to the ratio of the sine of the angle of incidence to the sine of the angle of refraction (this value is constant and characteristic of a given medium):
n = sinα / sinβ,
where α and β are the angles between the direction of the rays and the perpendicular to the interface of both media (Fig. 1)
The refractive index is the ratio of the speeds of light in air and in the medium under study (if a beam of light falls from air).
The refractive index depends on:
1. The wavelength of the incident light (as the wavelength increases, the indicator
refraction decreases).
2. temperature (with increasing temperature, the refractive index decreases);
3. pressure (for gases).
The index of refraction indicates the wavelengths of the incident light and the temperature of the measurement. For example, the entry n 20 D means that the refractive index is measured at 20°C, the wavelength of the incident light is 598 microns. In technical handbooks, the refractive indices are given at n 20 D.
Determination of the refractive index of a liquid.
Before starting work, the surface of the prisms of the refractometer is washed with distilled water and alcohol, the correctness of the zero point of the device is checked, and the refractive index of the liquid under study is determined. To do this, the surface of the measuring prism is carefully wiped with a cotton swab moistened with the liquid under study, and a few drops of it are applied to this surface. The prisms are closed and, rotating them, direct the border of light and shade to the cross of the eyepiece threads. The compensator eliminates the spectrum. When reading the refractive index, three decimal places are taken on the refractometer scale, and the fourth is taken by eye. Then they shift the border of chiaroscuro, again combine it with the center of the sighting cross and make a second count. That. 3 or 5 readings are made, after which the working surfaces of the prisms are washed and wiped. The test substance is again applied to the surface of the measuring prism and a second series of measurements is carried out. From the data obtained, the arithmetic mean is taken.
Radiometric analysis.
Radiometric analysis h is based on the measurement of radiation from radioactive elements and is used for the quantitative determination of radioactive isotopes in the test material. In this case, either the natural radioactivity of the element being determined is measured, or the artificial radioactivity obtained using radioactive isotopes.
Radioactive isotopes are identified by their half-life or by the type and energy of the radiation emitted. In the practice of quantitative analysis, the activity of radioactive isotopes is most often measured by their α-, β-, and γ-radiation.
Application of radiometric analysis:
Study of the mechanism of chemical reactions.
The labeled atom method is used to investigate the efficiency various tricks application of fertilizers to the soil, ways of penetration into the body of microelements applied to the leaves of a plant, etc. Radioactive phosphorus 32 P and nitrogen 13 N are especially widely used in agrochemical research.
Analysis of radioactive isotopes used for the treatment of oncological diseases and for the determination of hormones, enzymes.
Mass spectral analysis.
Based on the determination of the masses of individual ionized atoms, molecules and radicals as a result of the combined action of electric and magnetic fields. Registration of separated particles is carried out by electrical (mass spectrometry) or photographic (mass spectrography) methods. The determination is carried out on instruments - mass spectrometers or mass spectrographs.
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. Analytical signal- electrical parameter (potential, current strength, resistance), which depends on the concentration of the analyte.
Distinguish straight and indirect electrochemical methods. In direct methods, the dependence of the current strength on the concentration of the analyte is used. In indirect - the current strength (potential) is measured to find the end point of the titration (equivalence point) of the component being determined by the titrant.
Electrochemical methods of analysis include:
1. potentiometry;
2. conductometry;
3. coulometry;
4. amperometry;
5. polarography.
Electrodes used in electrochemical methods.
1. Reference electrode and indicator electrode.
Reference electrode- This is an electrode with a constant potential, insensitive to the ions of the solution. The reference electrode has a reproducible potential that is stable in time, which does not change when a small current is passed, and the potential of the indicator electrode is reported relative to it. Silver chloride and calomel electrodes are used. The silver chloride electrode is a silver wire coated with a layer of AgCl and placed in a KCI solution. The electrode potential is determined by the concentration of chlorine ion in the solution:
The calomel electrode consists of metallic mercury, calomel and KCI solution. The electrode potential depends on the concentration of chloride ions and temperature.
Indicator electrode- this is an electrode that reacts to the concentration of the ions being determined. The indicator electrode changes its potential with a change in the concentration of "potential-determining ions". Indicator electrodes are divided into irreversible and reversible. Potential jumps of reversible indicator electrodes at interphase boundaries depend on the activity of participants in electrode reactions in accordance with thermodynamic equations; equilibrium is established fairly quickly. Irreversible indicator electrodes do not meet the requirements of reversible ones. In analytical chemistry, reversible electrodes are used, for which the Nernst equation is satisfied.
2. Metal electrodes: electron exchange and ion exchange.
Electron exchange electrode at the interfacial boundary, a reaction occurs with the participation of electrons. The electron exchange electrodes are divided into electrodes first kind and electrodes second kind. Electrodes of the first kind - a metal plate (silver, mercury, cadmium) immersed in a solution of a highly soluble salt of this metal. Electrodes of the second kind - a metal coated with a layer of a sparingly soluble compound of this metal and immersed in a solution of a highly soluble compound with the same anion (silver chloride, calomel electrodes).
Ion exchange electrodes- electrodes, the potential of which depends on the ratio of the concentrations of the oxidized and reduced forms of one or more substances in solution. Such electrodes are made of inert metals such as platinum or gold.
3. Membrane electrodes they are a porous plate impregnated with a liquid immiscible with water and capable of selective adsorption of certain ions (for example, solutions of Ni 2+, Cd 2+, Fe 2+ chelates in an organic solution). The operation of membrane electrodes is based on the occurrence of a potential difference at the phase boundary and the establishment of an exchange equilibrium between the membrane and the solution.
Potentiometric method of analysis.
The potentiometric method of analysis is based on measuring the potential of an electrode immersed in a solution. In potentiometric measurements, a galvanic cell is made up with an indicator electrode and a reference electrode and the electromotive force (EMF) is measured.
Varieties of potentiometry:
Direct potentiometry used to directly determine the concentration by the value of the potential of the indicator electrode, provided that the electrode process is reversible.
Indirect potentiometry is based on the fact that a change in the concentration of an ion is accompanied by a change in the potential at the electrode immersed in the titrated solution.
In potentiometric titration, an end point is found in terms of a potential jump, due to the replacement of an electrochemical reaction with another one in accordance with the values of E ° (standard electrode potential).
The value of the potential depends on the concentration of the corresponding ions in the solution. For example, the potential of a silver electrode immersed in a silver salt solution changes with a change in the concentration of Ag + -ions in the solution. Therefore, by measuring the potential of an electrode immersed in a solution of a given salt of unknown concentration, it is possible to determine the content of the corresponding ions in the solution.
The electrode, by the potential of which the concentration of the ions to be determined in the solution is judged, is called indicator electrode.
The potential of the indicator electrode is determined by comparing it with the potential of another electrode, which is commonly called reference electrode. As a reference electrode, only such an electrode can be used, the potential of which remains unchanged when the concentration of the ions being determined changes. A standard (normal) hydrogen electrode is used as a reference electrode.
In practice, a calomel rather than a hydrogen electrode is often used as a reference electrode with a known value of the electrode potential (Fig. 1). The potential of the calomel electrode with a saturated solution of CO at 20 °C is 0.2490 V.
Conductometric method of analysis.
The conductometric method of analysis is based on measuring the electrical conductivity of solutions, which changes as a result of chemical reactions.
The electrical conductivity of a solution depends on the nature of the electrolyte, its temperature, and the concentration of the solute. The electrical conductivity of dilute solutions is due to the movement of cations and anions, which differ in different mobility.
With an increase in temperature, the electrical conductivity increases, as the mobility of the ions increases. At a given temperature, the electrical conductivity of an electrolyte solution depends on its concentration: as a rule, the higher the concentration, the greater the electrical conductivity! Therefore, the electrical conductivity of a given solution serves as an indicator of the concentration of a solute and is determined by the mobility of the ions.
In the simplest case of conductometric quantification, when the solution contains only one electrolyte, a graph is plotted as a function of the electrical conductivity of the analyte solution versus its concentration. Having determined the electrical conductivity of the test solution, the concentration of the analyte is found from the graph.
Thus, the electrical conductivity of barite water changes in direct proportion to the content of Ba(OH) 2 in the solution. This dependence is graphically expressed by a straight line. To determine the content of Ba(OH) 2 in barite water of unknown concentration, it is necessary to determine its electrical conductivity and, using the calibration graph, find the concentration of Ba(OH) 2 corresponding to this value of electrical conductivity. If a measured volume of gas containing carbon dioxide is passed through a solution of Ba (OH) 2, whose electrical conductivity is known, then CO 2 reacts with Ba (OH) 2:
Ba (OH) 2 + CO 2 → BaCO 3 + H 2 0
 |
As a result of this reaction, the content of Ba(OH) 2 in the solution will decrease and the electrical conductivity of barite water will decrease. By measuring the electrical conductivity of barite water after it has absorbed CO 2 , one can determine how much the concentration of Ba(OH) 2 in the solution has decreased. By the difference in concentrations of Ba (OH) 2 in barite water, it is easy to calculate the amount of absorbed
The vast majority of information about substances, their properties and chemical transformations was obtained using chemical or physicochemical experiments. Therefore, the main method used by chemists should be considered a chemical experiment.
The traditions of experimental chemistry have evolved over the centuries. Back when there was no chemistry exact science, in ancient times and in the Middle Ages, scientists and artisans sometimes accidentally, and sometimes purposefully discovered ways to obtain and purify many substances that were used in economic activity: metals, acids, alkalis, dyes, etc. The accumulation of such information was greatly facilitated alchemists (see Alchemy).
Thanks to this, already early XIX in. chemists were well versed in the basics of experimental art, in particular the methods of purification of various liquids and solids, which allowed them to make many important discoveries. Nevertheless, chemistry began to become a science in the modern sense of the word, an exact science, only in the 19th century, when the law of multiple ratios was discovered and the atomic-molecular theory was developed. Since that time, the chemical experiment began to include not only the study of the transformations of substances and methods of their isolation, but also the measurement of various quantitative characteristics.
A modern chemical experiment includes many different measurements. The equipment for setting up experiments and chemical glassware have also changed. In a modern laboratory, you will not find homemade retorts - they have been replaced by standard glass equipment produced by industry and adapted specifically for performing a particular chemical procedure. Work methods have also become standard, which in our time no longer have to be reinvented by every chemist. Description of the best of them, proven by many years of experience, can be found in textbooks and manuals.
Methods for studying matter have become not only more universal, but also much more diverse. An increasing role in the work of a chemist is played by physical and physicochemical research methods designed to isolate and purify compounds, as well as to establish their composition and structure.
The classical technique for purifying substances was extremely labor intensive. There are cases when chemists spent years of work on the isolation of an individual compound from a mixture. Thus, salts of rare earth elements could be isolated in pure form only after thousands of fractional crystallizations. But even after that, the purity of the substance could not always be guaranteed.
The sophistication of technology has reached such a high level that it has become possible to accurately determine the rate of even "instantaneous", as previously believed, reactions, for example, the formation of water molecules from hydrogen cations H + and anions OH - . With an initial concentration of both ions equal to 1 mol/l, the time of this reaction is several hundred-billionths of a second.
Physicochemical research methods are also specially adapted for the detection of short-lived intermediate particles formed in the course of chemical reactions. To do this, the devices are equipped with either high-speed recording devices or attachments that ensure operation at very low temperatures. Such methods successfully capture the spectra of particles whose lifetime under normal conditions is measured in thousandths of a second, such as free radicals.
In addition to experimental methods, calculations are widely used in modern chemistry. Thus, the thermodynamic calculation of a reacting mixture of substances makes it possible to accurately predict its equilibrium composition (see Fig.
The analysis of a substance can be carried out in order to establish its qualitative or quantitative composition. Accordingly, a distinction is made between qualitative and quantitative analysis.
Qualitative analysis allows you to establish what chemical elements the analyzed substance consists of and what ions, groups of atoms or molecules are included in its composition. When studying the composition of an unknown substance, a qualitative analysis always precedes a quantitative one, since the choice of method for the quantitative determination of the constituent parts of the analyzed substance depends on the data obtained during its qualitative analysis.
Qualitative chemical analysis is mostly based on the transformation of the analyte into some new compound with characteristic properties: a color determined by physical condition, crystalline or amorphous structure, specific odor, etc. The chemical transformation that occurs in this case is called a qualitative analytical reaction, and the substances that cause this transformation are called reagents (reagents).
When analyzing a mixture of several substances with similar chemical properties, they are first separated and only then characteristic reactions are carried out for individual substances (or ions), therefore, qualitative analysis covers not only individual reactions for detecting ions, but also methods for their separation.
Quantitative analysis allows you to establish the quantitative ratio of the parts of a given compound or mixture of substances. Unlike qualitative analysis, quantitative analysis makes it possible to determine the content of individual components of the analyte or the total content of the analyte in the test product.
Methods of qualitative and quantitative analysis, allowing to determine the content of individual elements in the analyzed substance, are called elements of analysis; functional groups - functional analysis; individual chemical compounds, characterized by a certain molecular weight - molecular analysis.
A set of various chemical, physical and physico-chemical methods for separating and determining individual structural (phase) components of heterogeneous systems that differ in properties and physical structure and bounded from each other by interfaces, is called phase analysis.
Qualitative analysis methods
Qualitative analysis uses characteristic chemical or physical properties of the substance to establish the composition of the substance under investigation. There is absolutely no need to isolate the discovered elements in their pure form in order to detect their presence in the analyzed substance. However, the isolation of metals, nonmetals, and their compounds in pure form is sometimes used in qualitative analysis for their identification, although this way of analysis is very difficult. To detect individual elements, simpler and more convenient methods of analysis are used, based on chemical reactions characteristic of the ions of these elements and occurring under strictly defined conditions.
An analytical sign of the presence of the desired element in the analyzed compound is the release of a gas that has a specific odor; in the other - the precipitation, characterized by a certain color.
Reactions between solids and gases. Analytical reactions can take place not only in solutions, but also between solid and gaseous substances.
An example of a reaction between solids is the reaction of the release of metallic mercury when dry salts of it are heated with sodium carbonate. The formation of white smoke from the interaction of gaseous ammonia with hydrogen chloride can serve as an example of an analytical reaction involving gaseous substances.
The reactions used in qualitative analysis can be divided into the following groups.
1. Precipitation reactions accompanied by the formation of precipitates various colors. For example:
CaC2O4 - white
Fe43 - blue,
CuS - brown - yellow
HgI2 - red
MnS - flesh - pink
PbI2 - golden
The resulting precipitates may differ in a certain crystal structure, solubility in acids, alkalis, ammonia, etc.
2. Reactions accompanied by the formation of gases with a known odor, solubility, etc.
3. Reactions accompanied by the formation of weak electrolytes. Among such reactions, which result in the formation of: CH3COOH, H2F2, NH4OH, HgCl2, Hg(CN)2, Fe(SCN)3, etc. Reactions of the same type can be considered reactions of acid-base interaction, accompanied by the formation of neutral water molecules, reactions of the formation of gases and precipitates that are poorly soluble in water, and complex formation reactions.
4. Reactions of acid-base interaction, accompanied by the transition of protons.
5. Complexation reactions accompanied by the addition of various legends - ions and molecules - to the atoms of the complexing agent.
6. Complexation reactions associated with acid-base interaction
7. Oxidation reactions - reductions, accompanied by the transition of electrons.
8. Oxidation reactions - reductions associated with acid - base interaction.
9. Oxidation-reduction reactions associated with complex formation.
10. Oxidation reactions - reductions, accompanied by the formation of precipitation.
11. Ion exchange reactions occurring on cation exchangers or anion exchangers.
12. Catalytic reactions used in kinetic methods of analysis
Wet and dry analysis
The reactions used in qualitative chemical analysis are most often carried out in solutions. The analyte is first dissolved and then the resulting solution is treated with appropriate reagents.
To dissolve the analyte, distilled water, acetic and mineral acids, aqua regia, aqueous ammonia, organic solvents, etc. are used. The purity of the solvents used is an important condition for obtaining correct results.
The substance transferred into solution is subjected to systematic chemical analysis. A systematic analysis consists of a series of preliminary tests and sequentially performed reactions.
The chemical analysis of test substances in solutions is called wet analysis.
In some cases, substances are analyzed dry, without transferring them into solution. Most often, such an analysis comes down to testing the ability of a substance to color a colorless burner flame in a characteristic color or to impart a certain color to a melt (the so-called pearl) obtained by heating a substance with sodium tetraborate (borax) or sodium phosphate ("phosphorus salt") in a platinum wire.
Chemical and physical method of qualitative analysis.
Chemical methods of analysis. Methods for determining the composition of substances based on the use of their chemical properties are called chemical methods of analysis.
Chemical methods of analysis are widely used in practice. However, they have a number of disadvantages. So, to determine the composition of a given substance, it is sometimes necessary to first separate the component to be determined from foreign impurities and isolate it in its pure form. The isolation of substances in pure form is often a very difficult and sometimes impossible task. In addition, in order to determine small amounts of impurities (less than 10-4%) contained in the analyte, it is sometimes necessary to take large samples.
Physical methods of analysis. The presence of one or the other chemical element can be detected in the sample without resorting to chemical reactions, based directly on the study of the physical properties of the substance under study, for example, coloring a colorless burner flame in characteristic colors volatile compounds some chemical elements.
Methods of analysis, by which it is possible to determine the composition of the substance under study, without resorting to the use of chemical reactions, are called physical methods of analysis. Physical methods of analysis include methods based on the study of optical, electrical, magnetic, thermal and other physical properties of the analyzed substances.
Among the most widely used physical methods of analysis are the following.
Spectral qualitative analysis. Spectral analysis is based on the observation of emission spectra (emission spectra, or radiation) of the elements that make up the analyte.
Luminescent (fluorescent) qualitative analysis. Luminescent analysis is based on the observation of luminescence (light emission) of analytes caused by the action of ultraviolet rays. The method is used to analyze natural organic compounds, minerals, medicines, a number of elements, etc.
To excite the luminescence, the test substance or its solution is irradiated with ultraviolet rays. In this case, the atoms of matter, having absorbed a certain amount of energy, pass into an excited state. This state is characterized by a larger supply of energy than the normal state of matter. During the transition of a substance from an excited to a normal state, luminescence occurs due to excess energy.
Luminescence that decays very quickly after cessation of irradiation is called fluorescence.
Observing the nature of the luminescent glow and measuring the intensity or brightness of the luminescence of a compound or its solutions, one can judge the composition of the substance under study.
In some cases, the definitions are based on the study of fluorescence resulting from the interaction of the analyte with certain reagents. Fluorescent indicators are also known, which are used to determine the reaction of the medium by changing the fluorescence of the solution. Luminescent indicators are used in the study of colored media.
X-ray diffraction analysis. With the help of X-rays, it is possible to establish the sizes of atoms (or ions) and their mutual arrangement in the molecules of the sample under study, i.e. it is possible to determine the structure crystal lattice, the composition of the substance and sometimes the presence of impurities in it. The method does not require chemical treatment of the substance and its large quantities.
Mass spectrometric analysis. The method is based on the determination of individual ionized particles deflected by an electromagnetic field to a greater or lesser extent depending on the ratio of their mass to charge (for more details, see Book 2).
Physical methods of analysis, having a number of advantages over chemical ones, in some cases make it possible to solve problems that cannot be resolved by methods of chemical analysis; using physical methods, it is possible to separate elements that are difficult to separate by chemical methods, as well as to conduct continuous and automatic recording of readings. Very often, physical methods of analysis are used along with chemical ones, which makes it possible to use the advantages of both methods. The combination of methods is of particular importance when determining negligible amounts (traces) of impurities in the analyzed objects.
Macro, semi-micro and micro methods
Analysis of large and small quantities of the test substance. In the old days, chemists used large quantities of the substance to be analyzed. In order to determine the composition of a substance, samples of several tens of grams were taken and dissolved in a large volume of liquid. This also required chemical glassware of the appropriate capacity.
At present, chemists manage in analytical practice with small amounts of substances. Depending on the amount of the analyte, the volume of solutions used for analysis, and mainly on the technique used to perform the experiment, analysis methods are divided into macro-, semi-micro- and micro-methods.
When performing a macro analysis, a few milliliters of a solution containing at least 0.1 g of the substance is taken to carry out the reaction, and at least 1 ml of the reagent solution is added to the test solution. The reactions are carried out in test tubes. During precipitation, voluminous precipitates are obtained, which are separated by filtration through funnels with paper filters.
Drop analysis
Technique for carrying out reactions in drop analysis. The so-called drop analysis, introduced into analytical practice by N. A. Tananaev, has acquired great importance in analytical chemistry.
When using this method great importance have the phenomena of capillarity and adsorption, with the help of which it is possible to open and separate various ions in their joint presence. In drop analysis, individual reactions are carried out on porcelain or glass plates or on filter paper. In this case, a drop of the test solution and a drop of a reagent that causes a characteristic coloration or the formation of crystals are applied to the plate or paper.
When performing the reaction on filter paper, the capillary-adsorption properties of the paper are used. The liquid is absorbed by the paper, and the resulting colored compound is adsorbed on a small area of the paper, thereby increasing the sensitivity of the reaction.
Microcrystalloscopic analysis
The microcrystalloscopic method of analysis is based on the detection of cations and anions by means of a reaction, as a result of which a compound is formed that has a characteristic crystal shape.
Previously, this method was used in qualitative microchemical analysis. Currently, it is also used in drip analysis.
To examine the resulting crystals in microcrystalloscopic analysis, a microscope is used.
crystals characteristic form are used when working with pure substances by introducing a drop of a solution or a crystal of a reagent into a drop of the test substance placed on a glass slide. After a while, clearly distinguishable crystals of a certain shape and color appear.
Powder grinding method
To detect some elements, the method of grinding a powdered analyte with a solid reagent in a porcelain plate is sometimes used. The element to be discovered is detected by the formation of characteristic compounds that differ in color or odor.
Methods of analysis based on heating and fusion of a substance
pyrochemical analysis. For the analysis of substances, methods based on heating the test solid or its fusion with appropriate reagents are also used. Some substances, when heated, melt at a certain temperature, others sublime, and precipitation characteristic of each substance appears on the cold walls of the device; some compounds, when heated, decompose with the release of gaseous products, etc.
When the analyte is heated in a mixture with the appropriate reagents, reactions occur, accompanied by a change in color, the release of gaseous products, and the formation of metals.
Spectral qualitative analysis
In addition to the above-described method of observing with the naked eye the coloring of a colorless flame when a platinum wire with an analyte is introduced into it, other methods of studying light emitted by incandescent vapors or gases are currently widely used. These methods are based on the use of special optical devices, the description of which is given in the physics course. In such spectral devices, the decomposition into a spectrum of light with different wavelengths occurs, emitted by a sample of a substance heated in a flame.
Depending on the method of observing the spectrum, spectral instruments are called spectroscopes, which are used to visually observe the spectrum, or spectrographs, in which spectra are photographed.
Chromatographic analysis method
The method is based on the selective absorption (adsorption) of individual components of the analyzed mixture by various adsorbents. Adsorbents are called solid bodies on the surface of which the adsorbed substance is absorbed.
The essence of the chromatographic method of analysis is briefly as follows. A solution of a mixture of substances to be separated is passed through a glass tube (adsorption column) filled with an adsorbent.
Kinetic methods of analysis
Methods of analysis based on measuring the reaction rate and using its magnitude to determine the concentration are combined under the general name of kinetic methods of analysis (K. B. Yatsimirsky).
Qualitative detection of cations and anions by kinetic methods is carried out quite quickly and relatively simply, without the use of complex instruments.
1. Sampling:
A laboratory sample consists of 10-50 g of material, which is taken so that its average composition corresponds to the average composition of the entire lot of the analyte.
2. Decomposition of the sample and its transfer to the solution;
3. Carrying out a chemical reaction:
X is the component to be determined;
P is the reaction product;
R is a reagent.
4. Measurement of any physical parameter of the reaction product, reagent or analyte.
Classification of chemical methods of analysis
I By reaction components
1. Measure the amount of reaction product P formed (gravimetric method). Create conditions under which the analyte is completely converted into a reaction product; further, it is necessary that the reagent R does not give minor reaction products with foreign substances, the physical properties of which would be similar to the physical properties of the product.
2. Based on the measurement of the amount of the reagent consumed in the reaction with the analyte X:
– the action between X and R must be stoichiometric;
- the reaction must proceed quickly;
– the reagent must not react with foreign substances;
– a way to establish the equivalence point is needed, i.e. the moment of titration when the reagent is added in an equivalent amount (indicator, color change, o-in capacity, electrical conductivity).
3. Records the changes that occur with the analyte X itself in the process of interaction with the reagent R (gas analysis).
II Types of chemical reactions
1. Acid-base.
2. Formation of complex compounds.
Acid-base reactions: used mainly for the direct quantitative determination of strong and weak acids and bases and their salts.
Reactions for the formation of complex compounds: determined substances are converted into complex ions and compounds by the action of reagents.
The following separation and determination methods are based on complex formation reactions:
1) Separation by means of precipitation;
2) Extraction method (water-insoluble complex compounds often dissolve well in organic solvents - benzene, chloroform - the process of transferring complex compounds from aqueous phases to dispersed ones is called extraction);
3) Photometric (Co with nitrous salt) - measure the optimal density of solutions of complex compounds;
4) Titrimetric analysis method
5) Gravimetric method of analysis.
1) cementation method - reduction of metal Me ions in solution;
2) electrolysis with a mercury cathode - during the electrolysis of a solution with a mercury cathode, ions of many elements are reduced electric shock to Me, which dissolve in mercury, forming an amalgam. The ions of other Me remain in solution;
3) identification method;
4) titrimetric methods;
5) electrogravimetric - an el is passed through the test solution. a current of a certain voltage, while the Me ions are restored to the Me state, the released is weighed;
6) coulometric method - the amount of a substance is determined by the amount of electricity that must be spent for the electrochemical transformation of the analyzed substance. Analysis reagents are found according to Faraday's law:
M is the amount of the element being determined;
F is the Faraday number (98500 C);
A is the atomic mass of the element;
n is the number of electrons involved in the electrochemical transformation of a given element;
Q is the amount of electricity (Q = I ∙ τ).
7) catalytic method of analysis;
8) polarographic;
III Classification of separation methods based on the use of various types of phase transformations:
The following types of equilibria between phases are known:
Equilibrium L-G or T-G is used in the analysis when substances are released into the gas phase (CO 2 , H 2 O, etc.).
Equilibrium W 1 - W 2 is observed in the extraction method and in electrolysis with a mercury cathode.
Zh-T is typical for the processes of deposition and the processes of precipitation on the surface of the solid phase.
Analysis methods include:
1. gravimetric;
2. titrimetric;
3 optical;
4. electrochemical;
5. catalytic.
Separation methods include:
1. precipitation;
2. extraction;
3. chromatography;
4. ion exchange.
Concentration methods include:
1. precipitation;
2. extraction;
3. grouting;
4. stripping.
Physical methods of analysis
A characteristic feature is that they directly measure any physical parameters of the system associated with the amount of the element being determined without prior chemical reaction.
Physical methods include three main groups of methods:
I Methods based on the interaction of radiation with a substance or on the measurement of the radiation of a substance.
II Methods based on measuring the parameters of el. or magnetic properties of matter.
IIIMethods based on the measurement of density or other parameters of the mechanical or molecular properties of substances.
Methods based on the energy transition of the outer valence electrons of atoms: include atomic emission and atomic absorption methods of analysis.
Atomic emission analysis:
1) Flame photometry - the analyzed solution is sprayed into the flame of a gas burner. Under the influence of high temperature, the atoms go into an excited state. The outer valence electrons move to higher energy levels. The reverse transition of electrons to the main energy level is accompanied by radiation, the wavelength of which depends on the atoms of which element were in the flame. The intensity of the radiation under certain conditions is proportional to the number of atoms of the element in the flame, and the wavelength of the radiation characterizes the qualitative composition of the sample.
2) Emission method of analysis - spectral. The sample is introduced into the flame of an arc or a condensed spark, under high temperature the atoms pass into an excited state, while the electrons pass not only to the closest to the main, but also to more distant energy levels.
Radiation is a complex mixture of light vibrations of different wavelengths. The emission spectrum is decomposed into the main parts of the special. instruments, spectrometers, and photographing. Comparison of the position of the intensity of individual lines of the spectrum with the lines of the corresponding standard, allows you to determine the qualitative and quantitative analysis of the sample.
Atomic absorption methods of analysis:
The method is based on measuring the absorption of light of a certain wavelength by unexcited atoms of the element being determined. A special radiation source produces resonant radiation, i.e. radiation corresponding to the transition of an electron to the lowest orbital with the lowest energy, from the orbital closest to it with more high level energy. The decrease in the intensity of light when it passes through the flame due to the transfer of the electrons of the atoms of the element being determined into an excited state is proportional to the number of unexcited atoms in it. In atomic absorption, combustible mixtures with temperatures up to 3100 ° C are used, which increases the number of elements to be determined, in comparison with flame photometry.
X-ray fluorescent and X-ray emission
X-ray fluorescent: the sample is exposed to x-ray radiation. top electrons. The orbitals closest to the nucleus of the atom are knocked out of the atoms. Their place is taken by electrons from more distant orbitals. The transition of these electrons is accompanied by the appearance of secondary X-ray radiation, the wavelength of which is functionally related to atomic number element. Wavelength - qualitative composition of the sample; intensity - the quantitative composition of the sample.
Methods based on nuclear reactions - radioactive. The material is exposed to neutron radiation, nuclear reactions occur and radioactive isotopes of elements are formed. Next, the sample is transferred into a solution and the elements are separated by chemical methods. After that, the intensity of radioactive radiation of each element of the sample is measured, and the reference sample is analyzed in parallel. The intensity of radioactive radiation of individual fractions of the reference sample and the analyzed material is compared and conclusions are drawn about the quantitative content of elements. Limit of detection 10 -8 - 10 -10%.
1. Conductometric - based on measuring the electrical conductivity of solutions or gases.
2. Potentiometric - there is a method of direct and potentiometric titration.
3. Thermoelectric - based on the occurrence of thermoelectromotive force, which arose when heating the place of contact of steel, etc. Me.
4. Mass spectral - applied with the help of strong elements and magnetic fields, separation occurs gas mixtures into components according to the atoms or molecular weights of the components. It is used in the study of a mixture of isotopes. inert gases, mixtures of organic substances.
Densitometry - based on the measurement of density (determination of the concentration of substances in solutions). To determine the composition, viscosity, surface tension, sound speed, electrical conductivity, etc. are measured.
To determine the purity of substances, the boiling point or melting point is measured.
Prediction and calculation of physical and chemical properties
Theoretical foundations for predicting the physicochemical properties of substances
Approximate prediction calculation
Prediction implies an assessment of physicochemical properties based on a minimum number of readily available initial data, and may also assume the complete absence of experimental information about the properties of the substance under study (“absolute” prediction relies only on information about the stoichiometric formula of the compound).
