Environmental engineers must know the chemical composition of raw materials, products and production wastes and environment- air, water and soil; it is important to identify harmful substances and determine their concentration. This problem is solved analytical chemistry - the science of determining the chemical composition of substances.

The problems of analytical chemistry are solved mainly by physicochemical methods of analysis, which are also called instrumental. They use the measurement of some physical or physico-chemical property of a substance to determine its composition. It also includes sections on methods of separation and purification of substances.

The purpose of this course of lectures is to familiarize with the principles of instrumental methods of analysis in order to navigate their capabilities and, on this basis, set specific tasks for specialists - chemists and understand the meaning of the results of analysis.

Literature

Aleskovsky V.B. etc. Physico-chemical methods of analysis. L-d, "Chemistry", 1988

Yu.S. Lyalikov. Physical and chemical methods of analysis. M., publishing house "Chemistry", 1974

Vasiliev V.P. Theoretical basis physico-chemical methods of analysis. M., high school, 1979

A.D. Zimon, N.F. Leshchenko. colloidal chemistry. M., "Agar", 2001

A.I. Mishustin, K.F. Belousova. Colloid chemistry (Methodological guide). Publishing house MIHM, 1990

The first two books are textbooks for students of chemistry and are therefore difficult enough for you. This makes these lectures very useful. However, you can read individual chapters.

Unfortunately, the administration has not yet allocated a separate credit for this course, so the material is included in the general exam, along with the course of physical chemistry.

2. Classification of analysis methods

Distinguish between qualitative and quantitative analysis. The first determines the presence of certain components, the second - their quantitative content. Analysis methods are divided into chemical and physico-chemical. In this lecture, we will consider only chemical methods that are based on the transformation of the analyte into compounds with certain properties.

In the qualitative analysis of inorganic compounds, the test sample is transferred to a liquid state by dissolving in water or an acid or alkali solution, which makes it possible to detect elements in the form of cations and anions. For example, Cu 2+ ions can be identified by the formation of a bright blue 2+ complex ion.

Qualitative analysis is divided into fractional and systematic. Fractional analysis - detection of several ions in a mixture with an approximately known composition.

Systematic analysis is a complete analysis according to a certain method of sequential detection of individual ions. Separate groups of ions with similar properties are isolated by means of group reagents, then groups of ions are divided into subgroups, and those, in turn, into separate ions, which are detected using the so-called. analytical reactions. These are reactions with an external effect - precipitation, gas evolution, change in the color of the solution.

Properties of analytical reactions - specificity, selectivity and sensitivity.

Specificity allows you to detect a given ion in the presence of other ions by a characteristic feature (color, smell, etc.). There are relatively few such reactions (for example, the reaction of detecting the NH 4 + ion by the action of an alkali on a substance when heated). Quantitatively, the specificity of the reaction is estimated by the value of the limiting ratio, which is equal to the ratio of the concentrations of the ion to be determined and the interfering ions. For example, a drop reaction on the Ni 2+ ion by the action of dimethylglyoxime in the presence of Co 2+ ions succeeds at a limiting ratio of Ni 2+ to Co 2+ equal to 1:5000.

Selectivity(or selectivity) of the reaction is determined by the fact that only a few ions give a similar external effect. The selectivity is the greater, the smaller the number of ions that give a similar effect.

Sensitivity reactions are characterized by a detection limit or a dilution limit. For example, the limit of detection in a microcrystalloscopic reaction to the Ca 2+ ion by the action of sulfuric acid is 0.04 μg of Ca 2+ in a drop of solution.

A more difficult task is the analysis of organic compounds. Carbon and hydrogen are determined after the combustion of the sample, recording the released carbon dioxide and water. There are a number of techniques for detecting other elements.

Classification of methods of analysis by quantity.

Components are divided into basic (1 - 100% by weight), minor (0.01 - 1% by weight) and impurity or trace (less than 0.01% by weight).

Depending on the mass and volume of the analyzed sample, macroanalysis is distinguished (0.5 - 1 g or 20 - 50 ml),

semi-microanalysis (0.1 - 0.01 g or 1.0 - 0.1 ml),

microanalysis (10 -3 - 10 -6 g or 10 -1 - 10 -4 ml),

ultramicroanalysis (10 -6 - 10 -9 g, or 10 -4 - 10 -6 ml),

submicroanalysis (10 -9 - 10 -12 g or 10 -7 - 10 -10 ml).

Classification according to the nature of the determined particles:

1.isotopic (physical) - isotopes are determined

2. elemental or atomic - a set of chemical elements is determined

3. molecular - the set of molecules that make up the sample is determined

4. structural group (intermediate between atomic and molecular) - functional groups are determined in the molecules of organic compounds.

5. phase - the components of heterogeneous objects (for example, minerals) are analyzed.

Other types of analysis classification:

Gross and local.

Destructive and non-destructive.

Contact and remote.

discrete and continuous.

Important characteristics of the analytical procedure are the rapidity of the method (speed of analysis), the cost of analysis, and the possibility of its automation.

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, and 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 physical and 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 of 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 condition material absorbing light 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 are taken. the same shape, size and from the same glass. 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 flow of electromagnetic radiation is determined by the minimum interval of wavelengths, which is separated by the used monochromator (light filter, diffraction 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) holding 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 oscillations of 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 method of labeled atoms is used to study the effectiveness of various methods of applying fertilizers to the soil, the 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 KCl 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

ANALYTICAL CHEMISTRY AND PHYSICO-CHEMICAL METHODS OF ANALYSIS TSTU publishing house Ministry of Education and Science of the Russian Federation State educational institution higher vocational education"Tambov State Technical University" M.I. LEBEDEV ANALYTICAL CHEMISTRY AND PHYSICO-CHEMICAL METHODS OF ANALYSIS Lectures for the course Tambov Publishing House TSTU 2005 UDC 543(075) BBK G4ya73-4 L33 Reviewers: Doctor chemical sciences, Professor A.B. Kilimnik Candidate of Chemical Sciences, Associate Professor of the Department of Inorganic and Physical Chemistry, TSU. G.R. Derzhavina A.I. Ryaguzov Lebedeva, M.I. L33 Analytical chemistry and physico-chemical methods of analysis: textbook. allowance / M.I. Lebedev. Tambov: Tamb Publishing House. state tech. un-ta, 2005. 216 p. The main questions of the course "Analytical chemistry and physicochemical methods of analysis" are considered. After the presentation of the theoretical material in each chapter, meaningful blocks are given for testing knowledge using test tasks and a rating of knowledge assessment is given. The third section of each chapter contains solutions to the most difficult problems and their evaluation in points. Designed for students of non-chemical specialties (200401, 200402, 240202, 240802, 240902) and compiled in accordance with standards and curricula. UDC 543(075) BBK G4ya73-4 ISBN 5-8265-0372-6 © Lebedeva M.I., 2005 © Tambov State Technical University (TSTU), 2005 Educational publication Maria Ivanovna LEBEDEVA ANALYTICAL CHEMISTRY AND PHYSICO-CHEMICAL METHODS OF ANALYSIS Lectures to course Editor V.N. Mitrofanova Computer prototyping D.A. Lopukhova Signed for publication May 21, 2005 Format 60 × 84 / 16. Offset paper. Offset printing Typeface Times New Roman. Volume: 12.55 arb. oven l.; 12.50 ed. l. Circulation 200 copies. P. 571M Publishing and Printing Center of the Tambov State technical university, 392000, Tambov, Sovetskaya, 106, k. 14 FOREWORD There is no synthesis without analysis F. Engels Analytical chemistry is the science of methods for identifying chemical compounds, principles and methods for determining the chemical composition of substances and their structure. Analytical chemistry has acquired particular relevance at the present time, since chemical pollution is the main factor in the adverse anthropogenic impact on nature. Determination of their concentration in various natural objects becomes a major task. Knowledge of the fundamentals of analytical chemistry is equally necessary for a modern student, engineer, teacher, and entrepreneur. Limited number of textbooks and teaching aids in the course “Analytical Chemistry and Physical and Chemical Methods of Analysis” for students of the chemical profile and their complete absence for the specialties “Standardization and Certification”, “Food Biotechnology”, “Engineering Environmental Protection”, as well as my many years of teaching experience of this discipline at TSTU led to the need to compile and publish the proposed course of lectures. The proposed edition consists of eleven chapters, in each of which the most important theoretical issues are highlighted, reflecting the sequence of presentation of the material in the lecture course. Chapters I–V are devoted to chemical (classical) methods of analysis, chapters VIII–X are devoted to the main physicochemical methods of analysis, and chapter XI is devoted to organic analytical reagents. It is recommended to complete the study of each section by solving the corresponding substantive block located at the end of the chapter. Blocks of tasks are formulated in three special forms. Theoretical tasks with a choice of answers (type A). For each theoretical question of this type, four attractive answers are offered, only one of which is correct. For any correctly solved task of type A, the student receives one point. Multiple choice tasks (type B)1 are worth two points. They are simple and can be solved practically in one or several steps. The correct answer is selected from four options. Tasks with a detailed answer (type C)2 offer the student to write down the answer in a detailed form and, depending on the completeness of the solution and its correctness, they can be assessed from one to five points. The maximum number of points is given for a completely solved task and is indicated in the last line of the rating table. The total number of points scored on a particular topic is an indicator of the student's knowledge, the level of which can be assessed in the proposed rating system. Score 32 - 40 Excellent 25 - 31 Good 16 - 24 Satisfactory Less than 16 Unsatisfactory . PB-21), Popova S. (gr. Z-31), who took an active part in the design of the work. 1 Some chapters may be missing 2 Some chapters may be missing “Analytical chemistry is responsive to industry demands and draws strength and impetus for further growth from this.” N.S. Kurnakov 1 ANALYTICAL CHEMISTRY AS A SCIENCE. BASIC CONCEPTS In solving major human problems (the problem of raw materials, foodstuffs, nuclear energy, cosmonautics, semiconductor and laser technology), the leading place belongs to analytical chemistry. basis environmental monitoring is a combination of various chemical sciences, each of which needs the results of chemical analysis, since chemical pollution is the main factor in the adverse anthropogenic impact on nature. The goal of analytical chemistry is to determine the concentration of pollutants in various natural objects. They are natural and waste waters of various composition, bottom sediments, atmospheric precipitation, air, soils, biological objects, etc. The widespread introduction of highly effective measures to control the state of the natural environment, without eliminating the disease at the root, is very important for diagnosis. The effect in this case can be obtained much faster and at the lowest cost. The control system makes it possible to detect harmful impurities in time and localize the source of pollution. That is why the role of analytical chemistry in environmental protection is becoming increasingly important. Analytical chemistry is the science of methods for identifying chemical compounds, the principles and methods for determining the chemical composition of substances and their structure. It is the scientific basis for chemical analysis. Chemical analysis is the empirical acquisition of data on the composition and properties of objects. For the first time this concept was scientifically substantiated by R. Boyle in the book "Skeptic Chemist" (1661) and introduced the term "analysis". Analytical chemistry is based on the knowledge gained while studying the courses of inorganic, organic, physical chemistry, physics and mathematics. The purpose of studying analytical chemistry is to master modern methods analysis of substances and their application to solve national economic problems. Careful and constant control of production and environmental objects is based on the achievements of analytical chemistry. W. Ostwald wrote: “Analytical chemistry, or the art of recognizing substances or their constituents, occupies a special place among the applications of scientific chemistry, since the questions that it makes it possible to answer always arise when trying to reproduce chemical processes for scientific or technical purposes. Due to its significance, analytical chemistry has long been constantly taken care of ... ". 1.1 Short story development of analytical chemistry The history of the development of analytical chemistry is inseparable from the history of the development of chemistry and the chemical industry. Separate techniques and methods of chemical analysis have been known since ancient times (recognition of substances by color, smell, taste, hardness). In the IX - X centuries. in Russia they used the so-called “assay analysis” (determination of the purity of gold, silver and ores). Thus, there are records of Peter the Great about his “assay analysis” of ores. At the same time, qualitative analysis (determination of the qualitative composition) always preceded quantitative analysis (determination of the quantitative ratio of components). The founder of qualitative analysis is the English scientist Robert Boyle, who first described methods for detecting SO 2 - - and Cl - - ions using Ba 2 + - and Ag + - ions, and also 4 used organic dyes as indicators (litmus). However, analytical chemistry began to form into a science after the discovery of M.V. Lomonosov of the law of conservation of the weight of substances in chemical reactions and the use of balances in chemical practice. Thus, M.V. Lomonosov is the founder of quantitative analysis. A contemporary of Lomonosov, Academician T.E. Lovitz established the relationship between the shape of crystals and their chemical composition: "microcrystalloscopic analysis". The first classical works on chemical analysis belong to Academician V.M. Severgin, who published the "Guidelines for the testing of mineral waters". In 1844 professor of Kazan University K.K. Klaus, analyzing "raw platinum", discovered a new element - ruthenium. A turning point in the development of analytical chemistry, in its formation as a science, was the discovery of the periodic law by D.I. Mendeleev (1869). Proceedings of D.I. Mendeleev formed the theoretical foundation of the methods of analytical chemistry and determined the main direction of its development. In 1871, the first manual on qualitative and quantitative analysis was published by N.A. Menshutkin "Analytical Chemistry". Analytical chemistry was created by the works of scientists from many countries. An invaluable contribution to the development of analytical chemistry was made by Russian scientists: A.P. Vinogradov, N.A. Tananaev, I.P. Alimarin, Yu.A. Zolotov, A.P. Kreshkov, L.A. Chugaev, M.S. Color, E.A. Bozhevolnov, V.I. Kuznetsov, S.B. Savvin et al. The development of analytical chemistry in the early years Soviet power took place in three main directions: – assistance to enterprises in performing analyzes; – development of new methods for the analysis of natural and industrial objects; – obtaining chemical reagents and preparations. During the Second World War, analytical chemistry performed defense tasks. For a long time, the so-called "classical" methods of analysis dominated in analytical chemistry. Analysis was regarded as an "art" and depended sharply on the "hands" of the experimenter. Technological progress required faster, simpler methods of analysis. Currently, most bulk chemical analyzes are performed using semi-automatic and automatic instruments. At the same time, the price of the equipment pays off with its high efficiency. At present, it is necessary to apply powerful, informative and sensitive methods of analysis in order to control the concentrations of pollutants below the MPC. Indeed, what does the normative "absence of a component" mean? Perhaps its concentration is so low that it cannot be determined by the traditional method, but it still needs to be done. Indeed, protecting the environment is a challenge for analytical chemistry. It is fundamentally important that the limit of detection of pollutants by analytical methods is not lower than 0.5 MPC. 1.2 TECHNICAL ANALYSIS At all stages of any production, technical control is carried out - i.e. work is carried out to control the quality of products during the technological process in order to prevent defects and ensure the release of products that comply with technical specifications and state standards. Technical analysis is divided into general - analysis of substances found in all enterprises (H2O, fuel, lubricants) and special - analysis of substances found only at a given enterprise (raw materials, semi-products, production waste, final product). To this end, thousands of analytical chemists perform millions of analyzes every day, in accordance with the relevant International State Standards. Analysis Method - detailed description performing analytical reactions with indication of the conditions for their implementation. Its task is to master the skills of experiment and the essence of analytical reactions. The methods of analytical chemistry are based on different principles. 1.3 CLASSIFICATION OF METHODS OF ANALYSIS 1 According to the objects of analysis: inorganic and organic. 2 By purpose: qualitative and quantitative. Quantitative analysis allows you to establish the quantitative ratio of the constituent 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 object under study. Methods of qualitative and quantitative analysis, which make it possible to determine the content of individual elements in the analyzed substance, are called elemental 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 are limited from each other by interfaces is called phase analysis. 3 According to the method of execution: chemical, physical and physico-chemical (instrumental) methods. 4 By sample weight: macro– (>> 0.10g), semimicro– (0.10–0.01g), micro– (0.01–10 −6 g), ultramicroanalysis (< 10 −6 г). 1.4 АНАЛИТИЧЕСКИЕ РЕАКЦИИ 1.4.1 Способы выполнения аналитических реакций В основе analytical methods – acquisition and measurement of an analytical signal, i.e. any manifestation of the chemical and physical properties of a substance as a result of a chemical reaction. Analytical reactions can be carried out "dry" and "wet" way. Examples of reactions carried out in a “dry” way: flame coloring reactions (Na + - yellow; Sr 2+ - red; Ba 2+ - green; K + - violet; Tl 3+ - green, In + - blue, etc.); when Na 2 B 4 O 7 and Co 2+, Na 2 B 4 O 7 and Ni 2+, Na 2 B 4 O 7 and Cr 3+ are fused, “pearls” of borax of various colors are formed. Most often, analytical reactions are carried out in solutions. The analyzed object (individual substance or mixture of substances) can be in any state of aggregation (solid, liquid, gaseous). The object for analysis is called a sample, or sample. The same element in a sample can be in different chemical forms. For example: S 0 , S 2− , SO 2 − , SO 3 - etc. Depending on the goals and objectives of the analysis, after transferring the sample into a solution, elemental analysis (determination of the total sulfur content) or phase analysis (determination of the sulfur content in each phase or in its individual chemical forms) is carried out. When performing this or that analytical reaction, it is necessary to strictly observe certain conditions for its course (temperature, pH of the solution, concentration) so that it proceeds quickly and has a sufficiently low detection limit. 1.4.2 Classification of analytical reactions 1 Group reactions: the same reagent reacts with a group of ions, giving the same signal. So, to separate a group of ions (Ag +, Pb 2+, Hg 2+), their reaction with Cl - - ions is used, while 2 white precipitates are formed (AgCl, PbCl 2, Hg 2 Cl 2). 2 Selective (selective) reactions. Example: starch iodine reaction. It was first described in 1815 by the German chemist F. Stromeyer. For these purposes, organic reagents are used. Example: dimethylglyoxime + Ni 2+ → formation of an alo - red precipitate of nickel dimethylglyoximate. By changing the conditions for the course of an analytical reaction, it is possible to make nonselective reactions selective. Example: if the reactions Ag +, Pb 2 +, Hg 2 + + Cl - are carried out when heated, then PbCl 2 does not precipitate, since it 2 is highly soluble in hot water. 3 Complexation reactions are used for the purpose of masking interfering ions. Example: to detect Co 2+ in the presence of Fe 3+ - ions using KSCN , the reaction is carried out in the presence of F - - ions. In this case, Fe 3+ + 4F − → − , K n = 10 −16, therefore, Fe 3+ - ions are complexed and do not interfere with the determination of Co 2+ - ions. 1.4.3 Reactions used in analytical chemistry 1 Hydrolysis (cation, anion, cation and anion) Al 3+ + HOH ↔ Al(OH) 2+ + H + ; CO 3 - + HOH ↔ HCO 3 + OH - ; 2 − Fe 3+ + (NH 4) 2 S + HOH → Fe(OH) 3 + ... 4 + 2H 2 SO 4  3 Complex formation reactions СuSO 4 + 4 NH 4 OH → SO 4 + 4H 2 O 4 Precipitation reactions Ba 2+ + SO 2− →↓ BaSO 4 4 1.4.4 Signals of qualitative analysis methods 1 Formation or dissolution of the precipitate Hg 2+ + 2I − →↓ HgI 2 ; red HgI 2 + 2KI - → K 2 colorless 2 Appearance, change, disappearance of the color of the solution (color reactions) Mn 2 + → - MnO 4 → MnO 2 - 4 colorless violet green 3 Gas evolution SO 3 - + 2H + → SO 2 + H 2 O. 2 4 Reactions of the formation of crystals of a strictly defined shape (microcrystalloscopic reactions). 5 Flame color reactions. 1.5 Analytical classification of cations and anions There are two classifications for cations: acid-base and hydrogen sulfide. Hydrogen sulfide classification of cations is presented in Table. 1.1. 1.1 Hydrogen sulfide classification of cations Analytical Analytical Cations Group reagent group form І K + , Na + , NH + , Mg 2 + 4   (NH 4) 2 CO 3 + NH 4 OH + NH 4 Cl II Ba 2 + , Sr 2 + , Ca 2 + MeCO3 ↓ pH ~ 9 Al3 + , Cr 3 + (NH 4) 2 S + NH 4 OH + NH 4 Cl Me(OH)m ↓ III Zn 2 + , Mn 2 + , Ni 2 + , Co 2 + , Fe 2 + , Fe3 + pH ~ 9 MeS ↓ Cu 2 + , Cd 2 + , Bi 3 + , Sn 2 + , Sn 4 + H 2S → HCl, IV MeS ↓ Hg 2 + , As3 + , As5 + , Sb 3 + , Sb 5 + pH ~ 0.5 V Ag + , Pb 2 + , 2 + HCl MeCl m ↓ All anions are divided into two groups: 1 Group reagent - BaCl 2 ; in this case, soluble barium salts are formed: - - - Cl, Br, I, NO 3, CH 3 COO - , SCN - , - , 4- 3- 2 - ClO - , ClO - , ClO 3 , ClO - . − , BrO3 4 2 Anions form sparingly soluble barium salts, which are soluble in acetic, hydrochloric and nitric acids (with the exception of BaSO 4): F − , CO 3 − , SO 2− , SO 3 − , S 2 O 3 − , SiO 3 − , CrO 2− , PO 3− . 2 4 2 2 2 4 4 1.5.1 Scheme of analysis for the identification of an unknown substance 1 Color of dry matter: black: FeS, PbS, Ag 2 S, HgS, NiS, CoS, CuO, MnO 2, etc.; orange: Cr2 O 7− and others; 2 yellow: CrO 2−, HgO, CdS; 4 red: Fe(SCN) 3 , Co 2+ ; blue: Cu 2+ . 2 Flame coloring. 3 Check for the presence of water of crystallization. 4 Action of acids on dry salt (gas). 5 Solvent selection (at room temperature, with heating): H 2 O, CH 3 COOH, HCl, H 2 SO 4, aqua regia, fusion with Na 2CO3 and subsequent leaching. It should be remembered that practically all nitrates, all salts of potassium, sodium and ammonium are soluble in water. 6 Solution pH control (only for water-soluble objects). 7 Preliminary tests (Fe 2+ , Fe 3+ , NH +). 4 8 Detection of a group of cations, anions. 9 Detection of the cation. 10 Anion detection. 1.6 Methods of separation and concentration Separation is an operation (process), as a result of which the components that make up the initial mixture are separated from one another. Concentration is an operation (process), as a result of which the ratio of the concentration or amount of microcomponents to the concentration or amount of macrocomponents increases. The need for separation and concentration may be due to the following factors: - the sample contains components that interfere with the determination; – the concentration of the analyte is below the detection limit of the method; – determined components are unevenly distributed in the sample; – there are no standard samples for calibrating instruments; – the sample is highly toxic, radioactive or expensive. Most separation methods are based on the distribution of a substance between two phases: I - aqueous and II - organic. For example, for substance A, the equilibrium A I ↔ A II takes place. Then the ratio of the concentration of substance A in the organic phase to the concentration of the substance in the aqueous phase is called the distribution constant KD KD = [A]II [A]I If both phases are solutions saturated with respect to the solid phase, and the extractable substance exists in a single form, then at equilibrium the distribution constant is equal to S II KD = , (1.1) SI where SI , S II are the solubility of the substance in the aqueous and organic phases. Absolutely complete extraction, and, consequently, separation is theoretically impracticable. The efficiency of extracting substance A from one phase to another can be characterized by two factors: the completeness of extraction Rn and the degree of separation of impurities Rc. x y Rn = ; Rc = , (1.2) x0 y0 where x and x0 are the content of the extracted substance and its content in the initial sample; y and y0 are the final and initial impurity contents. The smaller Rc and the larger Rn, the more perfect the separation.

All existing methods of analytical chemistry can be divided into methods of sampling, decomposition of samples, separation of components, detection (identification) and determination.

Almost all methods are based on the relationship between the composition of a substance and its properties. To detect a component or its amount, measure analytical signal.

Analytical signal is the average of the measurements of the physical quantity at the final stage of the analysis. The analytical signal is functionally related to the content of the determined component. This may be the current strength, EMF of the system, optical density, radiation intensity, etc.

If it is necessary to detect any component, the appearance of an analytical signal is usually fixed - the appearance of a precipitate, a color, a line in the spectrum, etc. The appearance of an analytical signal must be reliably recorded. At a certain amount of the component, the magnitude of the analytical signal is measured: the mass of the deposit, the current strength, the intensity of the lines of the spectrum, etc. Then the content of the component is calculated using the functional dependence analytical signal - content: y=f(c), which is established by calculation or experience and can be presented in the form of a formula, table or graph.

In analytical chemistry, there are chemical, physical and physico-chemical methods of analysis.

In chemical methods of analysis, the element or ion being determined is converted into a compound that has one or another characteristic property, on the basis of which it can be established that this particular compound was formed.

Chemical Methods analysis have a specific scope. Also, the speed of performing analyzes using chemical methods does not always satisfy the needs of production, where it is very important to get analyzes in a timely manner, while it is still possible to regulate the technological process. Therefore, along with chemical methods, physical and physico-chemical methods of analysis are becoming more widespread.

Physical Methods analyzes are based on the measurement of some

a system parameter that is a function of composition, such as emission absorption spectra, electrical or thermal conductivity, potential of an electrode immersed in a solution, permittivity, refractive index, nuclear magnetic resonance, etc.

Physical analysis methods make it possible to solve problems that cannot be resolved by chemical analysis methods.

For the analysis of substances, physicochemical methods of analysis are widely used, based on chemical reactions, the course of which is accompanied by a change in the physical properties of the analyzed system, for example, its color, color intensity, transparency, thermal and electrical conductivity, etc.

Physical and chemical methods of analysis are characterized by high sensitivity and rapid execution, make it possible to automate chemical-analytical determinations and are indispensable in the analysis of small amounts of substances.

It should be noted that it is not always possible to draw a strict boundary between physical and physicochemical methods of analysis. Sometimes they are combined under the general name "instrumental" methods, because. to perform certain measurements, instruments are required that allow one to measure with great accuracy the values ​​of certain parameters that characterize certain properties of a substance.

These methods of analysis are used in the presence of a relationship between the measured physical properties in-in and their qualitative and quantitative composition. Because to measure physical st-in-in various devices (tools) are used, then these methods are called instrumental. Classification of physical and physico-chemical methods of analysis. Based on taking into account the measured physical and physico-chemical sv-v-va or the system under study. Optical methods are based on the measurement of optical St-in-in. Chromatographic on the use of the ability of various substances to selective sorption. Electrochemical methods are based on the measurement of electrochemical properties in the system. Radiometric based on the measurement of radioactive sv-in in-in. Thermal on the measurement of the thermal effects of the relevant processes. Mass spectrometry in the study of ionized fragments ("fragments") in-in. Ultrasonic, magnetochemical, pycnometric, etc. Advantages of instrumental methods of analysis: low detection limit 1 -10 -9 µg; low limiting concentration, up to 10 -12 g / ml of the determined in-va; high sensitivity, formally determined by the value of the tangent of the slope of the corresponding calibration curve, which graphically reflects the dependence of the measured physical parameter, which is usually plotted along the ordinate axis, on the quantity or concentration of the determined substance (abscissa axis). The greater the tangent of the slope of the curve to the x-axis, the more sensitive the method, which means the following: to obtain the same “response” - a change in physical property - a smaller change in the concentration or amount of the measured substance is required. The advantages include the high selectivity (selectivity) of the methods, i.e., the constituent components of mixtures can be determined without separating and isolating these components; short duration of analysis, the possibility of their automation and computerization. Disadvantages: hardware complexity and high cost; greater error (5 -20%) than in classical chemical analysis (0.1 -0.5%); worse reproducibility. Optical methods of analysis are based on the measurement of optical properties in the islands (emission, absorption, scattering, reflection, refraction, polarization of light), manifested by the interaction of electromagnetic radiation with the island.

Classification according to the objects under study: atomic and molecular spectral analysis. By the nature of the interaction of electromagnetic radiation with in-ohm. In this case, the following methods are distinguished. Atomic absorption analysis, which is based on the measurement of the absorption of monochromatic radiation by atoms of the substance being determined in the gas phase after the atomization of the substance. Emission spectral analysis is a measurement of the intensity of light emitted by an object (most often atoms or ions) during its energy excitation, for example, in an electric discharge plasma. Flame photometry - the use of a gas flame as a source of energy excitation of radiation. Nephelometry - measurement of light scattering by light particles of a dispersed system (environment). Turbidimetric analysis - measurement of the attenuation of the intensity of radiation during its passage through a dispersed medium. Refractometric analysis measurement of light refraction indices in-in. Polarimetric analysis is the measurement of the magnitude of optical rotation - the angle of rotation of the plane of polarization of light by optically active objects. The following methods are classified according to the area of ​​​​the electromagnetic spectrum used: spectroscopy (spectrophotometry) in the UVI region of the spectrum, i.e., in the nearest ultraviolet region of the spectrum - in the wavelength range of 200 - 400 nm and in the visible region - in the wavelength range of 400 - 700 nm. Infrared spectroscopy, which studies a portion of the electromagnetic spectrum in the range of 0.76 - 1000 μm (1 μm = 10 -6 m), less often X-ray and microwave spectroscopy. By the nature of energy transitions in various spectra - electronic (change in the energy of the electronic states of atoms, ions, radicals, molecules, crystals in the UVI region); vibrational (when changing the energy of vibrational states of 2- and polyatomic ions, radicals, molecules, as well as liquid and solid phases in the IR region); rotational also in the IR and microwave region. That. The interaction between molecules and electromagnetic radiation lies in the fact that by absorbing electromagnetic radiation, the molecules pass into an excited state. In this case, an important role is played by energy, i.e., the wavelength of the absorbed radiation.

So, in x-rays, the wavelength of which is 0.05 - 5 nm, the process of excitation of internal electrons in atoms and molecules occurs; in ultraviolet rays (5 - 400 nm) the process of excitation of external electrons in atoms and molecules occurs; visible light (400 - 700 nm) is the excitation of external electrons in conjugated p-electron systems; infrared radiation (700 nm - 500 microns) is the process of excitation of vibrations of molecules; microwaves (500 microns - 30 cm) the process of excitation of the rotation of molecules; radio waves (more than 30 cm) the process of excitation of spin transitions in atomic nuclei (nuclear magnetic resonance). The absorption of radiations makes it possible to measure and record them in spectrometry. In this case, the incident radiation is divided into reference and measured at the same intensity. The measured radiation passes through the sample; when absorption occurs, the intensity changes. When absorbing the energy of electromagnetic radiation, particles in the islands (atoms, molecules, ions) increase their energy, i.e., they pass into a higher energy state. Electronic, vibrational, rotational energy states of the particles in the islands can only change discretely, by a strictly defined amount. For each particle there is an individual set of energy states - energy levels (terms), for example, electronic energy levels. Electronic energy levels of molecules and polyatomic ions have a fine structure - vibrational sublevels; therefore, vibrational transitions also take place simultaneously with purely electronic transitions.

Each electronic (electronic-vibrational) transition from a lower energy level to a higher lying electronic level corresponds to a band in the electronic absorption spectrum. Since the difference between the electronic levels for each particle (atom, ion, molecule) is strictly defined, the position of the band in the electronic absorption spectrum corresponding to one or another electronic transition is also strictly defined, i.e. the wavelength (frequency, wave number) absorption band maximum. Differences in intensity are measured by a detector and recorded on a recorder in the form of a signal (peak), page 318, chemistry, schoolchildren's and student's handbook, spectrometer scheme. Ultraviolet spectroscopy and absorption spectroscopy in the visible region. Absorption of electromagnetic radiation from the ultraviolet and visible parts of the spectrum; excites transitions of electrons in molecules from occupied to unoccupied energy levels. The greater the difference in energy between energy levels, the greater the energy, i.e. shorter wavelength, must have radiation. The part of the molecule that largely determines the absorption of light is called the chromophore (literally, color carriers) - these are atomic groups that affect the absorption of light by the molecule, especially conjugated and aromatic p-electron systems.

Structural elements of chromophores are mainly involved in the absorption of a quantum of light energy, which leads to the appearance of bands in a relatively narrow region of the absorption spectrum of compounds. The region from 200 to 700 nm is of practical importance for determining the structure of organic molecules. Quantitative measurement: along with the position of the absorption maximum, the value of extinction (attenuation) of radiation, i.e., the intensity of its absorption, is important for analysis. In accordance with the law of Lambert - Beer E \u003d lgI 0 / I \u003d ecd, E - extinction, I 0 - intensity of incident light, I - intensity of transmitted light, e - molar extinction coefficient, cm 2 / mol, c - concentration, mol / l, d - thickness of the sample layer, cm. Extinction depends on the concentration of the absorbing substance. Absorption analysis methods: colorimetry, photoelectrocolorimetry, spectrometry. Colorimetry is the simplest and oldest method of analysis, based on a visual comparison of the color of liquids (determination of soil pH using an Alyamovsky instrument) - the simplest method of comparison with a series of reference p-s. 3 methods of colorimetry are widely used: standard series method (scale method), color equalization method and dilution method. Glass colorimetric test tubes, glass burettes, colorimeters, photometers are used. The scale method is the determination of pH on an Alyamovsky instrument, i.e. a series of test tubes with different concentrations in the islands and different in terms of changing the intensity of the color of the solution or reference solutions. Photocolorimetry - the method is based on measuring the intensity of a non-monochromatic light flux that has passed through the analyzed solution using photocells.

The luminous flux from the radiation source (incandescent lamp) passes through a light filter that transmits radiation only in a certain wavelength range, through a cuvette with the analyzed p-ohm and enters a photocell that converts light energy into photocurrent recorded by an appropriate device. The greater the light absorption of the analyzed solution (i.e., the higher its optical density), the lower the energy of the light flux falling on the photocell. FECs are supplied with n-mi filters that have a maximum light transmission at different wavelengths. In the presence of 2 photocells, 2 light fluxes are measured, one through the analyzed solution, the other through comparison solution. The concentration of the studied substance is found according to the calibration curve.

Electrochemical methods of analysis are based on electrode reactions and on the transfer of electricity through solutions. In quantitative analysis, the dependence of the values ​​of the measured parameters of electrochemical processes (difference in electrical potentials, current, amount of electricity) on the content of the determined substance in the solution involved in this electrochemical process is used. Electrochemical processes are those processes that are accompanied by the simultaneous occurrence of chemical reactions and a change in the electrical properties of the system, which in such cases can be called an electrochemical system. Basic Principles of Potentiometry

As the name of the method implies, the potential is measured in it. To clarify what the potential is and why it arises, consider a system consisting of a metal plate and a solution in contact with it containing ions of the same metal (electrolyte) (Fig. 1). Such a system is called an electrode. Any system tends to a state that corresponds to the minimum of its internal energy. Therefore, at the first moment after the metal is immersed in the solution, processes begin to occur at the phase boundary, leading to a decrease in the internal energy of the system. Let us assume that the ionized state of the metal atom is energetically more "favorable" than the neutral state (the reverse version is also possible). Then, at the first moment of time, the metal atoms will pass from the surface layer of the plate into the solution, leaving their valence electrons in it. In this case, the surface of the plate acquires a negative charge, and this charge grows with the increase in the number of metal atoms that have passed into the solution in the form of ions. The electrostatic forces of attraction of unlike charges (negatively charged electrons in the plate and positive metal ions in solution) do not allow these charges to move away from the phase boundary, and also cause the reverse process of the transition of metal ions from solution to the metal phase and their reduction there. When the rates of the forward and reverse processes become the same, equilibrium occurs. The equilibrium state of the system is characterized by the separation of charges at the phase boundary, i.e., a “jump” of the potential appears. It should be noted that the described mechanism of the occurrence of the electrode potential is not the only one; in real systems, many other processes also occur, leading to the formation of a “jump” of potentials at the interface. In addition, a potential “jump” can occur at the phase boundary not only when the electrolyte comes into contact with the metal, but also when the electrolyte comes into contact with other materials, such as semiconductors, ion exchange resins, glasses, etc.

In this case, ions whose concentration affects the potential of the electrode are called potential-determining. The electrode potential depends on the nature of the material in contact with the electrolyte, the concentration of potential-determining ions in the solution, and the temperature. This potential is measured relative to another electrode whose potential is constant. Thus, having established this relationship, it is possible to use it in analytical practice to determine the concentration of ions in a solution. In this case, the electrode, the potential of which is measured, is called the measuring one, and the electrode, relative to which the measurements are made, is called the auxiliary or reference electrode. The constancy of the potential of the reference electrodes is achieved by the constancy of the concentration of potential-determining ions in its electrolyte (electrolyte No. 1). The composition of electrolyte #2 may vary. To prevent mixing of two different electrolytes, they are separated by an ion-permeable membrane. The potential of the measuring electrode is taken equal to the measured emf of the reduced electrochemical system. Using solutions of a known composition as electrolyte No. 2, it is possible to establish the dependence of the potential of the measuring electrode on the concentration of potential-determining ions. This dependence can later be used in the analysis of a solution of unknown concentration.

To standardize the potential scale, a standard hydrogen electrode was adopted as a reference electrode, the potential of which was assumed to be zero at any temperature. However, in conventional measurements, the hydrogen electrode is rarely used because of its bulkiness. In everyday practice, other simpler reference electrodes are used, the potential of which relative to the hydrogen electrode is determined. Therefore, if necessary, the result of the potential measurement carried out with respect to such electrodes can be recalculated with respect to the hydrogen electrode. The most widely used are silver chloride and calomel reference electrodes. The potential difference between the measuring electrode and the reference electrode is a measure of the concentration of the ions to be determined.

The electrode function can be described using the linear Nernst equation:

E \u003d E 0 + 2.3 RT / nF * lg a,

where E is the potential difference between the measuring electrode and the reference electrode, mV; E 0 - constant, depending mainly on the properties of the reference electrode (standard electrode potential), mV; R - gas constant, J * mol -1 * K -1. ; n is the charge of the ion, taking into account its sign; F - Faraday number, C/mol; T - absolute temperature, 0 K; the term 2.3 RT/nF included in the Nernst equation at 25 0 C is 59.16 mV for singly charged ions. The method without imposing an external (extraneous) potential is classified as a method based on taking into account the nature of the source of electrical energy in the system. In this method, the source of el.en. the electrochemical system itself serves, which is a galvanic cell (galvanic circuit) - potentiometric methods. EMF and electrode potentials in such a system depend on the soda of the determined substance in the solution. The electrochemical cell includes 2 electrodes - indicator and reference electrode. The value of the EMF generated in the cell is equal to the potential difference of these 2 electrodes.

The potential of the reference electrode under the conditions of the potentiometric determination remains constant, then the EMF depends only on the potential of the indicator electrode, that is, on the activities (concentrations) of certain ions in the solution. This is the basis for the potentiometric determination of the concentration of a given substance in the anal-th solution. Both direct potentiometry and potentiometric titration are used. When determining the pH of the solutions as indicator electrodes, the potential of which depends on the concentration of hydrogen ions is used: glass, hydrogen, quinhydrone (redox electrode in the form of a platinum wire immersed in HC1 solution, saturated with quinhydrone - an equimolecular compound quinone with hydroquinone) and some others. Membrane or ion-selective electrodes have a real potential, depending on the activity of those ions in the p-re, which are sorbed by the electrode membrane (solid or liquid), the method is called ionometry.

Spectrophotometers are devices that make it possible to measure the light absorption of samples in beams of light narrow in spectral composition (monochromatic light). Spectrophotometers allow decomposing white light into a continuous spectrum, separating a narrow wavelength range from this spectrum (1-20 nm width of the selected spectrum band), passing an isolated beam of light through the analyzed solution and measuring the intensity of this beam with high accuracy. The absorption of light by the colored solution in the solution is measured by comparing it with the absorption of the zero solution. The spectrophotometer combines two devices: a monochromator for obtaining a monochromatic light flux and a photoelectric photometer for measuring light intensity. The monochromator consists of a light source, a dispersing device (decomposing white light into a spectrum) and a device for regulating the magnitude of the wavelength interval of the light beam incident on the solution.

From a variety of physico-chemical and physical methods of analysis highest value have 2 groups of methods: 1 - methods based on the study of the spectral characteristics of the Islands; 2 - methods based on the study of physico-chemical parameters. Spectral methods are based on phenomena that occur when a substance interacts with various types of energy (electromagnetic radiation, thermal energy, electrical energy, etc.). The main types of interaction in-va with radiant energy include absorption and emission (emission) of radiation. The nature of the phenomena due to absorption or emission is in principle the same. When radiation interacts with matter, its particles (atoms of the molecule) pass into an excited state. After some time (10 -8 s), the particles return to the ground state, emitting excess energy in the form of electromagnetic radiation. These processes are associated with electronic transitions in an atom or molecule.

Electromagnetic radiation can be characterized by wavelength or frequency n, which are interconnected by the ratio n=s/l, where c is the speed of light in vacuum (2.29810 8 m/s). The totality of all wavelengths (frequencies) of electromagnetic radiation makes up the electromagnetic spectrum from g-rays (short-wave region, photons have high energy) to the visible region of the spectrum (400 - 700 nm) and radio waves (long-wave region, photons with low energy).

In practice, one deals with radiation characterized by a certain interval of wavelengths (frequencies), i.e., with a certain section of the spectrum (or, as they say, with a radiation band). Often, for analytical purposes, monochromatic light is also used (a luminous flux in which electromagnetic waves have the same wavelength). Selective absorption by atoms and molecules of radiation with certain wavelengths leads to the fact that each in-in is characterized by individual spectral characteristics.

For analytical purposes, both the absorption of radiation by atoms and molecules (respectively, atomic absorption spectroscopy) and the emission of radiation by atoms and molecules (emission spectroscopy and luminescence) are used.

Spectrophotometry is based on the selective absorption of electromagnetic radiation in-vom. By measuring the absorption in-tion of radiation of various wavelengths, one can obtain an absorption spectrum, i.e., the dependence of absorption on the wavelength of the incident light. The absorption spectrum is a qualitative characteristic of the island. A quantitative characteristic is the amount of absorbed energy or the optical density of the solution, which depends on the concentration of the absorbing substance according to the Bouguer-Lambert-Beer law: D \u003d eIs, where D is the optical density, i is the layer thickness; с - concentration, mol/l; e is the molar absorption coefficient (e = D at I=1 cm and c=1 mol/l). The value of e serves as a sensitivity characteristic: the larger the value of e, the smaller the amount of v-va can be determined. Many substances (especially organic ones) intensively absorb radiation in the UV and visible regions, which makes it possible to directly determine them. Most ions, on the contrary, weakly absorb radiation in the visible region of the spectrum (е? 10…1000), so they are usually transferred to other, more intensely absorbing compounds, and then measurements are taken. To measure absorption (optical density), two types of spectral instruments are used: photoelectrocolorimeters (with coarse monochromatization) and spectrophotometers (with finer monochromatization). The most common is the photometric method of analysis, quantitative determinations in which are based on the Bouguer-Lambert-Beer law. The main methods of photometric measurements are: the method of molar light absorption coefficient, the calibration curve method, the standard method (comparison method), the additive method. In the method of molar light absorption coefficient, the optical density D of the investigated solution is measured and, according to the known value of the molar light absorption coefficient e, the concentration of the absorbing substance in the solution is calculated: c \u003d D / (e I). In the calibration curve method, a series of standard solutions are prepared with known value concentrations from the determined component and determine their value of optical density D.

According to the data obtained, a calibration graph is built - the dependence of the optical density of the solution on the concentration of the in-va: D = f (c). According to the Bucher-Lambert-Beer law, the graph is a straight line. Then the optical density D of the test solution is measured and the concentration of the analyte is determined from the calibration curve. The method of comparison (standards) is based on a comparison of the optical density of the standard and test solutions:

D st \u003d e * I * s st and D x \u003d e * I * s x,

whence D x / D st \u003d e * I * s x / e * I * s st and c x \u003d s st * D x / D st. In the addition method, the values ​​of the optical density of the test solution are compared with the same solution with the addition (with a) of a known amount of the component to be determined. Based on the results of the determinations, the concentration of the substance in the test solution is calculated: D x \u003d e * I * c x and D x + a \u003d e * I * (c x + c a), whence D x / D x + a \u003d e * I * c x / e * I * (c x + c a) and c x \u003d c a * D x / D x + a - D x. .

Atomic absorption spectroscopy is based on the selective absorption of radiation by atoms. To transfer the substance to the atomic state, the sample solution is injected into the flame or heated in a special cuvette. As a result, the solvent volatilizes or burns out, and the solid matter is atomized. Most of the atoms remain in an unexcited state, and only a small part is excited with subsequent emission of radiation. The set of lines corresponding to the wavelengths of the absorbed radiation, i.e., the spectrum, is a qualitative characteristic, and the intensity of these lines is, respectively, a quantitative characteristic of the island.

Atomic emission spectroscopy is based on measuring the intensity of light emitted by excited atoms. Excitation sources can be a flame, a spark discharge, an electric arc, etc. To obtain emission spectra, a sample in the form of a powder or solution is introduced into the excitation source, where the substance passes into a gaseous state or partially decays into atoms and simple (by composition) molecules. A qualitative characteristic of a substance is its spectrum (i.e., a set of lines in the emission spectrum), and a quantitative characteristic is the intensity of these lines.

Luminescence is based on the emission of radiation by excited molecules (atoms, ions) during their transition to the ground state. In this case, sources of excitation can be ultraviolet and visible radiation, cathode rays, the energy of a chemical reaction, etc. The energy of radiation (luminescence) is always less than the absorbed energy, since part of the absorbed energy is converted into heat even before the emission begins. Therefore, luminescent emission always has a shorter wavelength than the wavelength of the light absorbed during excitation. Luminescence can be used both to detect substances (by wavelength) and to quantify them (by radiation intensity). Electrochemical methods of analysis are based on the interaction of the in-va with electric shock. The processes proceeding in this case are localized either on the electrodes or in the near-electrode space. Most methods are of the first of these types. Potentiometry. An electrode process is a heterogeneous reaction in which a charged particle (ion, electron) is transferred through the phase boundary. As a result of such a transfer, a potential difference arises on the surface of the electrode, due to the formation of a double electric layer. Like any process, the electrode reaction eventually comes to equilibrium, and an equilibrium potential is established on the electrode.

Measuring the values ​​of equilibrium electrode potentials is the task of the potentiometric method of analysis. Measurements are carried out in an electrochemical cell consisting of 2 half-cells. One of them contains an indicator electrode (the potential of which depends on the concentration of the ions to be determined in the solution in accordance with the Nernst equation), and the other a reference electrode (the potential of which is constant and does not depend on the composition of the solution). The method can be implemented as direct potentiometry or as potentiometric titration. In the first case, the potential of the indicator electrode in the analyzed solution is measured relative to the reference electrode, and the concentration of the ion to be determined is calculated using the Nernst equation. In the variant of potentiometric titration, the ion to be determined is titrated with a suitable reagent, while simultaneously monitoring the change in the potential of the indicator electrode. Based on the data obtained, a titration curve is built (dependence of the indicator electrode potential on the volume of added titrant). On the curve near the equivalence point, there is a sharp change in the potential value (potential jump) of the indicator electrode, which makes it possible to calculate the content of the ion being determined in the solution. Electrode processes are very diverse. In general, they can be classified into 2 large groups: processes that occur with the transfer of electrons (i.e., the actual electrochemical processes), and processes associated with the transfer of ions (in this case, the electrode has ionic conductivity). In the latter case, we are talking about the so-called ion-selective membrane electrodes, which are widely used at present. The potential of such an electrode in a solution containing ions to be determined depends on their concentration according to the Nernst equation. The glass electrode used in pH-metry also belongs to the same type of electrodes. The possibility of creating a large number of membrane electrodes with high selectivity to certain ions has singled out this area of ​​potentiometric analysis into an independent branch - ionometry.

Polarography. During the passage of current in an electrochemical cell, a deviation of the values ​​of the electrode potentials from their equilibrium values ​​is observed. For a number of reasons, the so-called electrode polarization occurs. The phenomenon of polarization that occurs during electrolysis on an electrode with a small surface underlies this method of analysis. In this method, an increasing potential difference is applied to the electrodes dipped into the test solution. With a small potential difference, there is practically no current through the solution (the so-called residual current). With an increase in the potential difference to a value sufficient for the decomposition of the electrolyte, the current increases sharply. This potential difference is called the expansion potential. By measuring the dependence of the strength of the current passing through the solution on the magnitude of the applied voltage, one can construct the so-called. current-voltage curve, which allows you to determine the qualitative and quantitative composition of the solution with sufficient accuracy. At the same time, a qualitative characteristic of a substance is the magnitude of the potential difference sufficient for its electrochemical decomposition (half-wave potential E S), and a quantitative characteristic is the magnitude of the increase in current strength due to its electrochemical decomposition in solution (wavelength H, or the difference in the values ​​of the limiting diffusion current and residual current). To quantify the concentration of a substance in a solution, the following methods are used: the calibration curve method, the standard method, the additive method. The conductometric method of analysis is based on the dependence of the electrical conductivity of the solution on the concentration of the electrolyte. It is used, as a rule, in the variant of conductometric titration, the equivalence point in which is determined by the inflection of the titration curve (the dependence of electrical conductivity on the amount of added titrant). Amperometric titration is a kind of potentiometric titration, only the indicator electrode is a polarographic device, i.e. applied microelectrode with superimposed voltage.