The study of substances is a rather complex and interesting matter. Indeed, in their pure form, they are almost never found in nature. Most often, these are mixtures of complex composition, in which the separation of components requires certain efforts, skills and equipment.

After separation, it is equally important to correctly determine the belonging of a substance to a particular class, that is, to identify it. Determine the boiling and melting points, calculate the molecular weight, check for radioactivity, and so on, in general, investigate. For this, they are used different ways, including physical and chemical methods analysis. They are quite diverse and require the use, as a rule, of special equipment. About them and will be discussed further.

Physical and chemical methods of analysis: a general concept

What are these methods of identifying compounds? These are methods based on the direct dependence of all physical properties substances from its structural chemical composition. Since these indicators are strictly individual for each compound, physicochemical research methods are extremely effective and give a 100% result in determining the composition and other indicators.

So, such properties of a substance can be taken as a basis, such as:

• the ability to absorb light;
• thermal conductivity;
• electrical conductivity;
• boiling temperature;
• melting and other parameters.

Physicochemical research methods have a significant difference from purely chemical methods for identifying substances. As a result of their work, there is no reaction, that is, the transformation of a substance, both reversible and irreversible. As a rule, the compounds remain intact both in terms of mass and composition.

Features of these research methods

There are several main features characteristic of such methods for determining substances.

1. The research sample does not need to be cleaned of impurities before the procedure, since the equipment does not require this.
2. Physical and chemical methods of analysis have a high degree sensitivity and increased selectivity. Therefore, a very small amount of the test sample is needed for analysis, which makes these methods very convenient and efficient. Even if it is required to determine an element that is contained in the total wet weight in negligible amounts, this is not an obstacle for the indicated methods.
3. The analysis takes only a few minutes, so another feature is the short duration, or rapidity.
4. The research methods under consideration do not require the use of expensive indicators.

It is obvious that the advantages and features are sufficient to make physicochemical research methods universal and in demand in almost all studies, regardless of the field of activity.

Classification

There are several features on the basis of which the considered methods are classified. However, we will give the most general system, which unites and embraces all the main methods of research related directly to physical and chemical ones.

1. Electrochemical research methods. They are subdivided on the basis of the measured parameter into:

• potentiometry;
• voltammetry;
• polarography;
• oscillometry;
• conductometry;
• electrogravimetry;
• coulometry;
• amperometry;
• dielkometry;
• high frequency conductometry.

2. Spectral. Include:

• optical;
• X-ray photoelectron spectroscopy;
• electromagnetic and nuclear magnetic resonance.

3. Thermal. Subdivided into:

• thermal;
• thermogravimetry;
• calorimetry;
• enthalpymetry;
• delatometry.

4. Chromatographic methods, which are:

• gas;
• sedimentary;
• gel-penetrating;
• exchange;
• liquid.

It is also possible to divide physicochemical methods of analysis into two large groups. The first are those that result in destruction, that is, the complete or partial destruction of a substance or element. The second is non-destructive, preserving the integrity of the test sample.

Practical application of such methods

The areas of use of the considered methods of work are quite diverse, but all of them, of course, in one way or another, relate to science or technology. In general, several basic examples can be given, from which it will become clear why such methods are needed.

1. Control over the flow of complex technological processes in production. In these cases, the equipment is necessary for contactless control and tracking of all structural links of the working chain. The same devices will fix malfunctions and malfunctions and give an accurate quantitative and qualitative report on corrective and preventive measures.
2. Carrying out chemical practical work for the purpose of qualitative and quantitative determination of the yield of the reaction product.
3. The study of a sample of a substance in order to establish its exact elemental composition.
4. Determination of the quantity and quality of impurities in the total mass of the sample.
5. Accurate analysis of intermediate, main and side participants of the reaction.
6. A detailed account of the structure of matter and the properties it exhibits.
7. Discovery of new elements and obtaining data characterizing their properties.
8. Practical confirmation of theoretical data obtained empirically.
9. Analytical work with high purity substances used in various industries technology.
10. Titration of solutions without the use of indicators, which gives a more accurate result and has a completely simple control, thanks to the operation of the apparatus. That is, the influence human factor reduces to zero.
11. The main physicochemical methods of analysis make it possible to study the composition of:

• minerals;
• mineral;
• silicates;
• meteorites and foreign bodies;
• metals and non-metals;
• alloys;
• organic and inorganic substances;
• single crystals;
• rare and trace elements.

Areas of use of methods

• nuclear power;
• physics;
• chemistry;
• radio electronics;
• laser technology;
• space research and others.

The classification of physicochemical methods of analysis only confirms how comprehensive, accurate and versatile they are for use in research.

Electrochemical methods

The basis of these methods is reactions in aqueous solutions and on the electrodes under the action of an electric current, that is, in other words, electrolysis. Accordingly, the type of energy that is used in these methods of analysis is the flow of electrons.

These methods have their own classification of physico-chemical methods of analysis. This group includes the following species.

1. Electrical weight analysis. According to the results of electrolysis, a mass of substances is removed from the electrodes, which is then weighed and analyzed. So get data on the mass of compounds. One of the varieties of such works is the method of internal electrolysis.
2. Polarography. The basis is the measurement of current strength. It is this indicator that will be directly proportional to the concentration of the desired ions in the solution. Amperometric titration of solutions is a variation of the considered polarographic method.
3. Coulometry is based on Faraday's law. The amount of electricity spent on the process is measured, from which they then proceed to the calculation of ions in solution.
4. Potentiometry - based on the measurement of the electrode potentials of the participants in the process.

All the processes considered are physicochemical methods for the quantitative analysis of substances. Using electrochemical research methods, mixtures are separated into constituent components, the amount of copper, lead, nickel and other metals is determined.

Spectral

It is based on the processes of electromagnetic radiation. There is also a classification of the methods used.

1. Flame photometry. To do this, the test substance is sprayed into an open flame. Many metal cations give color certain color, so it is possible to identify them in this way. These are mainly substances such as: alkaline and alkaline earth metals, copper, gallium, thallium, indium, manganese, lead and even phosphorus.
2. Absorption spectroscopy. Includes two types: spectrophotometry and colorimetry. The basis is the determination of the spectrum absorbed by the substance. It operates both in the visible and in the hot (infrared) part of the radiation.
3. Turbidimetry.
4. Nephelometry.
5. Luminescent analysis.
6. Refractometry and polarometry.

Obviously, all the considered methods in this group are methods of qualitative analysis of a substance.

Emission analysis

This causes the emission or absorption of electromagnetic waves. According to this indicator, one can judge the qualitative composition of the substance, that is, what specific elements are included in the composition of the research sample.

Chromatographic

Physicochemical studies are often carried out in different environments. In this case, very convenient and effective methods become chromatographic. They are divided into the following types.

1. Adsorption liquid. At the heart of the different ability of the components to adsorption.
2. Gas chromatography. Also based on adsorption capacity, only for gases and substances in the vapor state. Used on mass production compounds in similar states of aggregation, when the product comes out in a mixture that should be separated.
3. Partition chromatography.
4. Redox.
5. Ion exchange.
6. Paper.
7. Thin layer.
8. Sedimentary.
9. Adsorption-complexing.

Thermal

Physical and chemical studies also involve the use of methods based on the heat of formation or decay of substances. Such methods also have their own classification.

1. Thermal analysis.
2. Thermogravimetry.
3. Calorimetry.
4. Enthalpometry.
5. Dilatometry.

All these methods allow you to determine the amount of heat, mechanical properties, enthalpies of substances. Based on these indicators, the composition of the compounds is quantified.

Methods of analytical chemistry

This section of chemistry has its own characteristics, because the main task facing analysts is the qualitative determination of the composition of a substance, their identification and quantitative accounting. In this regard, analytical methods of analysis are divided into:

• chemical;
• biological;
• physical and chemical.

Since we are interested in the latter, we will consider which of them are used to determine substances.

The main varieties of physicochemical methods in analytical chemistry

1. Spectroscopic - all the same as those discussed above.
2. Mass spectral - based on the action of an electric and magnetic field on free radicals, particles or ions. The physicochemical analysis laboratory assistant provides the combined effect of the indicated force fields, and the particles are separated into separate ionic flows according to the ratio of charge and mass.
3. radioactive methods.
4. Electrochemical.
5. Biochemical.
6. Thermal.

What do such processing methods allow us to learn about substances and molecules? First, the isotopic composition. And also: reaction products, the content of certain particles in especially pure substances, the masses of the desired compounds and other things useful for scientists.

Thus, the methods of analytical chemistry are important ways of obtaining information about ions, particles, compounds, substances and their analysis.

PHYSICO-CHEMICAL ANALYSIS, studies the relationship between the composition and St. you macroscopic. systems made up of several initial in-in(components). Physical and chemical analysis is characterized by the representation of these dependencies graphically, in the form of a composition-property diagram; apply also tables of numerical data and analyte. records. Since the properties of a system depend not only on its composition, but also on other factors that determine the state of the system - pressure, t-ry, degree of dispersion, gravitational strengths. and electromagnet. fields, as well as the time of observation, then in a general form they talk about the diagrams of the equilibrium factor - St., or about the physical-chemical. (chemical) diagrams. In these diagrams, all chem. processes that occur in systems when the c.-l. balance factor, such as the formation and decay of chemical. Comm., the appearance and disappearance of solid and (or) liquid solutions, etc., are expressed as geom. changes in the complex of lines, surfaces and points, which forms a diagram. Therefore, the analysis of the geometry of the diagrams makes it possible to draw conclusions about the corresponding processes in the system.

Two basic principles of physicochemical analysis were formulated by N.S. Kurnakov. According to the correspondence principle, each set of phases that are in equilibrium in a given system in accordance with the phase rule corresponds to a certain geom on the diagram. image. Based on this principle, N.S. Kurnakov determined the physico- chemical analysis like geom. chemical research method. transformations.

The second main the principle of physico-chemical analysis, called. principle of continuity, the following is formulated. way: with a continuous change in the parameters that determine the state of the system, the properties of its individual phases change continuously. St.-va systems as a whole also change continuously, but on condition that new phases do not arise and old ones do not disappear; if the number of phases changes, then the properties of the system also change, and, as a rule, abruptly.

The third principle of physicochemical analysis was proposed by Ya.G. Goroshchenko. He claims that any set of components, regardless of their number and physical. sv-in, can make up a system (the principle of compatibility). It follows from it that the diagram of any system contains all the elements of particular systems (subsystems) of which it is composed. IN common system translation elements of private systems are combined with geom. images for chem. diagram, arising as a display of processes occurring with the participation of all components of the overall system.

One of the main directions of the theory of physico-chemical analysis is the study of the topology of chemical. diagrams. The advantage of physicochemical analysis as a research method is that it does not require isolation of the chemical product. interaction of components from the reaction mixture, as a result of which the method allows you to explore the chemical. transformations in solutions, alloys (especially metallic ones), glasses, etc. objects, which are practically impossible to study using the classical. preparative-synthetic. methods. Physical and chemical analysis was widely used in the study of complex formation in solutions in order to determine the composition and determine the stability of chemical. connections. Schedule composition - sv-in usually has one extremum, as a rule, a maximum. In simple cases, the maximum corresponds to the molar ratio of the components of the system, representing the stoichiometry of the complex compound. In the general case, the extremum points on the curves (or surfaces) of St.-in, as well as the inflection points, do not correspond to the composition of the chemical compounds formed in the system. Comm., but in the limit, when the degree of dissociation of chemical. conn. is equal to zero, the continuous curve of the dependence of St-va on the composition breaks up into two branches intersecting at a singular point, the abscissa of which corresponds to the composition of the chemical. connections.

Diagrams composition - sv-in are the basis of the analyte. methods (colorimetry, potentiometry, etc.). For use to. - l. Holy Island in analyt. purposes, it is desirable that there be an additive dependence of the values ​​of this property on the composition. Therefore, great importance is given to the rational choice of properties (in particular, direct or reverse, for example, electrical conductivity or electrical resistance), as well as the choice of a method for expressing the concentration of system components (massmolar, volume, equivalent fractions or percentages). In modern In physico-chemical analysis, the number of used St. in the system is many tens. In principle, you can use any sv-in, to-swarm m. b. measured or calculated. For example, when solving the theoretical issues, in particular in the derivation of decomp. types of diagrams, use k.-l. thermodynamic potential, to-ry not m. b. measured directly. When choosing St. Islands, it is necessary to take into account both the possible accuracy of determining its values, and its sensitivity to what is happening in the chemical system. transformations. For example, the density of the v-va m. b. determined with great accuracy, but it is insensitive to the formation of chemical. Comm., while the hardness is sensitive to chemical. interaction in the system, but the accuracy of its determination is low. Physical and chemical analysis is characterized by a parallel study and comparison of the results of determining several. St., for example. electrical conductivity, hardness.

Among the chem. diagrams, a special place is occupied by melting (fusibility) diagrams, p-diagrams, vapor pressure diagrams, to-rye are variants of the state diagram. On such diagrams, any point, regardless of whether it is located on the c.-l. lines or lines of the diagram or not, describes the state of the system. The state diagram is the basis of the diagram of any property, since the value of each of the properties in the system generally depends on the composition, and on the t-ry, and on the pressure, i.e. from all equilibrium factors , the ratio between which gives the state diagram . Increasingly, diagrams are being explored and used in practice, showing the dependence of the state of the system simultaneously on the two most important equilibrium factors - pressure and t-ry. These diagrams are referred to as p-T-x diagrams (x is the molar fraction of the component). Even for a binary system, the construction of a p-T-x-diagram requires the use of spaces, a coordinate system, so the composition diagram is a property for double and more complex systems built and investigated, as a rule, at constant pressure, t-re, etc. ext. factors. The complexity of building a chem. diagrams required the development of appropriate methods graphic. Images.

F physical-chemical analysis contributed to the solution of many. theoretical problems of chemistry, in particular, the creation of a theory of the structure of chemical. conn. variable composition (see Nonstoichiometry). Physico-chemical analysis is the basis for the creation of new and modification of known materials - alloys, semiconductors, glasses, ceramics, etc. by, for example, doping. On physico-chemical analysis and fiz.-chem. many technologies are based on diagrams. processes associated, in particular, with crystallization, rectification, extraction, etc., i.e., with phase separation. Such diagrams indicate, in particular, the conditions for isolating the compound, growing single crystals. T. called the method of residual concentrations allows you to explore the district of deposition of chemical. conn. as a result of interaction in r-ra. According to this method, the composition of the solid phases -products of the district - is determined by the difference between the content of the reacting components in a series of initial mixtures and in the corresponding equilibrium p-pas at the end of the interaction. At the same time, a diagram is constructed of the dependence of the equilibrium concentrations of the reacting components in the solution on the ratio between them in the initial mixtures. In parallel, they usually change the pH, the electrical conductivity of solutions, the absorption of light by a suspension, etc. St. Islands.

In the classic The physicochemical analysis of the system was studied only in the equilibrium state. Approaching equilibrium often takes a long time or is generally difficult to achieve, therefore, for practical purposes. using the method, it is necessary to study systems in a non-equilibrium state, in particular, in the process of approaching equilibrium. Strictly speaking, systems are considered nonequilibrium, in which metastaoils participate. modifications in-in, capable of existing indefinitely long time. Tech. the use of materials in a non-equilibrium state, e.g. glassy metal. alloys, composite materials, glassy semiconductors, has led to the need to study composition-composition diagrams for obviously non-equilibrium systems.

Physico-chemical analysis proved to be fruitful for the study and synthesis of new Comm. as a result of irreversible p-tions in non-equilibrium systems. The study of systems in the process of transition to an equilibrium state makes it possible to establish the existence of not only the final products of the p-tion, but also the intermediate ones. in-in, as well as the resulting unstable in-in. Kinetic factor, i.e., the rate of transformation (the rate of approach to equilibrium), is now considered on an equal footing with other criteria and other saints. On the Holy Islands of the system is significantly influenced by its dispersion - mol.-dispersed distribution of components (submicroscopic. state), the state of colloidal dissolution, etc., up to single-crystal. states. Diagrams composition - structure - degree of dispersion - sv-in determine the features of modern. studies in physico-chemical analysis.

The development of computers has led to the fact that the role of the analyte in physicochemical analysis has significantly increased. forms of expression of the dependencies of St. in the system on its composition. This facilitates the storage of information (modern computer systems allow the collection and storage of reference material on chemical diagrams and in graphical form) and, in particular, mat. processing of results, which was previously used in the main. only in the study of complex formation in solutions. To a certain extent, the use of modern calculates, the technique allows you to overcome the limitations of physico-chemical analysis, which lies in the fact that it establishes which chem. transformations take place in the system, but does not answer questions related to the cause and mechanism of these transformations. Calculation methods allow you to extract additional. information from chem. diagrams, eg. determine the degree of dissociation of chemical. conn. in the melt based on the analysis of the curvature of the liquidus line for binary systems or the change in the free energy of the system during the exchange of salts, based on the shape of the liquidus isotherms for ternary reciprocal systems. Attracting diff. theories of solids, models of liquids and states of gas mixtures, along with a generalization of experiments. data, allows you to get physical. diagrams (or their elements) by calculation.

Historical essay. Main the idea of ​​physicochemical analysis was put forward by M.V. Lomonosov (1752), the first attempts to establish education in the chemical system. Comm., based on the dependence of its St. on the composition, belong to the beginning. 19th century All R . 19th century works by P.P. Anosov (1831), G.K. Sorby (1864), D.K. Chernov (1869) laid the foundations for metallurgy; DI. Mendeleev was the first to carry out geom. analysis of diagrams composition - St. in the example of the study of hydrates of sulfuric acid. The works of V.F. Alekseev on the mutual solubility of liquids, D.P. Konovalova - on the elasticity of a pair of solutions (see Konovalov's laws), I.F. Schroeder - on the temperature dependence of solubility (see Pasmicity). At the turn of the 19th-20th centuries. In connection with the needs of technology, the rapid development of physicochemical analysis began (A. Le Chatelier, J. van't Hoff, F. Osmond, W. Roberts-Austen, J. Van Laar, and others). The fundamental theoretical and experiment. works of modern physical and chemical analysis belong to N.S. Kurnakov. They combined the study of alloys and homogeneous solutions into one direction and proposed the term "physico-chemical analysis" (1913). Studies of complex formation in solutions with the works of I.I. Ostromyslensky (1911), P. Job (1928) and the development of methods for determining the composition of chemical. conn. and constants r about shchenko Ya.G., Physico-chemical analysis of homogeneous and heterogeneous systems, K., 1978; Chernogorenko V.B., Pryadko L.F., "Journal of inorg. chemistry", 1982, vol. 27, no. 6, p. 1527-30; Glazov V.M., "Izv. AN SSSR. Ser. inorganic materials", 1984, v. 20, no. 6, p. 925-36; Fedorov P.I., Fedorov P.P., Dr about D.V., Physical and chemical analysis of anhydrous salt systems, M., 1987. P.I. Fedorov.

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Topic14. Physical methods of analysis

These methods are based on measuring the effect caused by the interaction of radiation with matter - the flux of quanta or particles. Radiation plays roughly the same role that a reagent plays in chemical methods of analysis. The measured physical effect is a signal. As a result of several or multiple measurements of the signal magnitude and their static processing, an analytical signal is obtained. It is related to the concentration or mass of the components being determined.

Physical Methods analysis has a number of advantages:

– ease of sample preparation (in most cases) and qualitative analysis of samples;

– greater versatility compared to chemical and physicochemical methods (including the possibility of analyzing multicomponent mixtures);

– the possibility of determining the main impurity and trace components;

– often low detection limits both in concentration (up to 10-8% without the use of concentration), and by weight (10-10 -10-20 g), which allows you to spend extremely small amounts of the sample, and

sometimes conduct non-destructive analysis.

In addition, many physical methods of analysis make it possible to perform both gross and local and layer-by-layer analysis with spatial resolution up to the monatomic level. These methods are convenient for automation.

Let us consider in more detail some of the physical methods of analysis.

14.1. Spectral analysis

Spectral analysis is a physical method for determining the chemical composition and structure of a substance from its spectrum. The spectrum is electromagnetic radiation ordered by wavelength. When a substance is excited by a certain energy, changes occur in it (excitation of valence or internal electrons, rotation or vibration of molecules), which are accompanied by the appearance of lines or bands in its spectrum. Depending on the nature of the excitation and the processes of internal interaction in a substance, methods (principles) of spectral analysis are also distinguished: atomic emission, absorption, luminescence, Raman scattering, radio and X-ray spectroscopy, etc.

Each spectral line is characterized by a wavelength or frequency. In spectral analysis, the wavelength of a line is usually expressed in nanometers (1 nm = 10-9 m) or micrometers (1 μm = 10-6 m). However, a non-systemic unit is also used - the angstrom (1 Å \u003d 0.1 nm \u003d 10-10 m). For example, the wavelength of one of the yellow lines of sodium can be written as: Na 5893 Å,

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Topic 14. Physical methods of analysis

or Na 589.3 nm, or Na 0.5893 µm. Line spectra emit atoms or ions that are at such distances from each other that their emission can be considered independent. Gases and vapors of metals have line spectra. Striped Spectra arise during the radiation of ionized and non-ionized molecules consisting of two or more atoms, if these molecules are so far apart from each other that they do not interact with neighboring molecules. Continuous or continuous spectra emit incandescent liquid or solid bodies. Under certain conditions, they can also be emitted by individual atoms or molecules.

Striped spectra consist of closely spaced lines, which are well observed in the spectra obtained on instruments with large dispersion. For analytical purposes, the ultraviolet, visible and near infrared parts of the spectrum are more often used. The ultraviolet region of the spectrum is conditionally divided into vacuum (10–185 nm), far (185–230 nm) and near (230–400 nm). Visible part spectrum (400–750 nm), unlike other regions of the spectrum, is perceived by the human eye in the form of seven primary colors: violet (390–420 nm), blue (424–455 nm), cyan (455–494 nm), green (494– 565 nm), yellow (565–595 nm), orange (595–640 nm), red (640–723 nm) and their shades. Behind the visible red part of the spectrum is the infrared region of the spectrum, which is divided into near (0.75–25 µm) and far (> 25 µm).

Spectral analysis makes it possible to establish the elemental, isotopic, molecular composition of a substance and its structure.

Atomic emission spectral analysis is a method of analyzing the emission spectra that occur when the sample is evaporated and excited in an arc, spark or flame. Excited atoms and ions spontaneously, spontaneously transfer from excited E k to lower energy states Ei . This process leads to the emission of light with a frequency

v k i = (E k – E i )/h

and the appearance of a spectral line.

Modern photoelectric spectral devices such as quantometers are equipped with a minicomputer, which makes it possible to carry out mass multi-element express analysis of materials of standard composition with an accuracy often not inferior to the accuracy of most chemical methods.

Flame photometry- one of the methods of atomic emission spectral analysis. This method consists in the fact that the analyzed sample is transferred into a solution, which is then converted into an aerosol with the help of a sprayer and fed into the burner flame. The solvent evaporates, and the elements, being excited, emit a spectrum. The analyzed spectral line is isolated using a device - a monochromator or a light filter, and the intensity of its glow is measured by a photocell. The flame compares favorably with electrical sources light by the fact that the gas fuel and oxidizing gas coming from the cylinder give a very stable, evenly burning flame. Due to the low temperature in the flame, elements with low

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Topic 14. Physical methods of analysis

excitation potentials: first of all, alkaline elements, for the determination of which there are practically no express chemical methods, as well as alkaline earth and other elements. In total, more than 70 elements are determined by this method. The use of an induction high-frequency discharge and an arc plasma torch of a plasma torch makes it possible to determine elements with a high ionization potential, as well as elements that form heat-resistant oxides, for the excitation of which the flame is of little use.

Atomic absorption analysis (AAA) is one of the most

common methods of analytical chemistry. Preliminary preparation of the analyzed sample is similar to this operation in flame photometry: transfer of the sample into a solution, spraying, and supply of aerosols to the flame. The solvent evaporates, the salts decompose, and the metals pass into a vapor state, in which they are able to absorb radiation of the wavelength that they themselves could emit at higher temperatures. A beam of light from a hollow cathode lamp emitting the arc spectrum of the element to be determined is directed through the flame to the slit of the spectrometer, which is used to single out an analytical spectral line and measure the degree of absorption of its intensity by the vapors of the element to be determined.

Modern atomic absorption spectrometers are equipped with minicomputers and digital printing devices. Multichannel instruments such as quantometers allow up to 600 determinations per hour.

The use of electrothermal atomizers instead of a flame in combination with chemical concentration methods makes it possible to reduce the detection limit of elements by several orders of magnitude.

Atomic fluorescent the analysis is close to atomic absorption analysis. With the help of this method, not only the tasks performed by atomic absorption analysis are solved, it allows you to determine individual atoms in a gaseous medium. For example, by exciting atomic fluorescence with a laser beam, sodium can be determined in the upper atmosphere at a distance

100 km from Earth.

14.2. Methods based on the interaction of a substance

with magnetic field

Brief information about magnetism. In a magnetic system (macroscopic or microscopic) there are always two magnetic charges of different sign, but equal in absolute value, separated by some distance. Such a magnetic system is a magnetic dipole and, when placed in an external magnetic field with strength H, tends to be parallel lines of force applied field. The force orienting a free dipole in a magnetic field can either pull it into the region of a stronger field or push it out, depending on whether the directions of the vector characterizing the dipole moment and the field gradient dН/dx coincide or not. Unlike electric, individual magnetic charges have not been detected. Elementary

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Topic 14. Physical methods of analysis

the carriers of magnetic properties are magnetic dipoles, which can be modeled by a current-carrying loop. In this case, the resulting magnetic moment μ is directly proportional to the current strength and the area of ​​the loop.

Consider a body consisting of atoms and molecules with magnetic moments μi. If the dimensions of the body are small enough and we can assume that within its limits the field gradient dH/dx does not change, then the total force F acting on it will be equal to

F = ∑ i μi dH = M dH , 1 dx dx

i.e., it can be expressed in terms of the magnetic moment or the magnetization of the entire body M. In real conditions, due to the thermal motions of molecules and the anisotropy of the crystal structure, the vectors μi are not necessarily oriented along the field H. Therefore, the value of the vector M can be many times less arithmetic sumμi and depends on the temperature T, and its direction may not coincide with the direction of H.

To characterize a specific substance, the concept of specific magnetization σ = M/m (m is the body mass) is introduced, which fully reflects the specifics of its interaction with an external field. However, in many cases it is convenient to use the concept of specific magnetic susceptibility χ, which is a proportionality factor in the ratio σ = χН, which does not depend on either the size of the body or the field strength, but is determined only by the fundamental properties of the substance and, in some cases, temperature. Specific susceptibility is sometimes referred to as χ g. For magnetic susceptibility per atom, mole and unit volume, the designations χА, χМ and χV are used. If a body is placed in a medium with magnetic susceptibility χ0, then a force acts on it

F = (χ − χ 0 )mH dH dx .

The magnetic dipoles that make up the sample create their own magnetic fields. Therefore, the effective field inside the sample is the sum of the external field H and the field of dipoles, and such a change in the field compared to vacuum can be described by the equation:

B = H + 4πI ,

where B is the magnetic field induction vector inside the sample; I is the magnetization of a unit volume of a substance.

In an isotropic medium, all three vectors are collinear, so one can introduce a scalar

μ \u003d H B \u003d 1 + 4 πχ,

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Topic 14. Physical methods of analysis

called relative magnetic permeability. As can be seen, μ and χ are dimensionless. For most substances μ ≈ 1, |χ|<< 1 и приближение В ≈ Н выполняется с высокой точностью.

It is known that any system can be characterized by its response to an external influence. If we consider a substance in a condensed state as a system of charges and currents, then it can also be characterized by a response function. In this case, we are mainly interested in the response of such a system to a magnetic field. Here the output is the magnetization and the response function is the magnetic susceptibility. Usually, the change in magnetic susceptibility is used to judge the most important processes occurring in the system, and then the system is analyzed taking into account the identified processes. To implement such a program, it is necessary to know what processes are possible in the system, how they affect the susceptibility, and what is the probability of a particular state of the system under study. Such information is contained in the distribution function of the system, which is determined by the total energy or Hamiltonian, which takes into account all types of interactions in a quantum system.

First of all, attention should be paid to the interactions that are essential in the manifestation of magnetism. In addition, it is necessary to take into account the features of the behavior of the systems under consideration in magnetic fields, the strength of which is constant or varies with time. In this case, the magnetic susceptibility of substances is determined by the expression

χ = χ" + χ"",

where χ" - susceptibility - response to the action of a field that is constant in time; χ"" - dynamic magnetic susceptibility - response to the action of an alternating field.

It can be assumed that in a constant field the system is in thermal equilibrium, and then finding the distribution function is reduced to solving the Bloch equations. In the case of the dependence of the field strength on time, in order to calculate the distribution function, it is necessary to introduce the corresponding Boltzmann equations. The processes considered are the basis of methods used in chemistry to obtain information about the structure and reactivity of substances: methods of static magnetic susceptibility, electron paramagnetic resonance, nuclear magnetic resonance, etc.

Method of static magnetic susceptibility. The expediency of using the experimental method of research with the participation of a magnetic field depends significantly on the behavior of a substance in a magnetic field. According to their magnetic properties, all bodies are divided into diamagnets, paramagnets, ferromagnets, antiferromagnets and ferrimagnets. The diamagnetic susceptibility of an atom is proportional to the number of electrons and the sum of the squares of the radii of the electron orbitals, taken with the opposite sign, in accordance with the Lenz law, according to which, when the magnetic flux changes in

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the system of charges, currents arise, the direction of which is determined by the need to compensate for the change in flow.

The molecular susceptibility of a chemical compound can be expressed as

χM = ∑ N i χi + λ,

where N i is the number of atoms of the i -th element in the compound molecule; χi is the atomic susceptibility of a given element; λ is a corrective factor depending on the nature of the chemical bond between atoms.

For salts take

χ mol = χ cat + χ an.

For mixtures and solutions, the specific magnetic susceptibility is the sum of the magnetic susceptibilities of all components, taking into account their proportion in the sample.

Consider a substance characterized by a set of non-interacting magnetic moments. In the absence of an external magnetic field, under the action of thermal motion, the magnetic moments are completely disordered and the magnetization is zero. In an external magnetic field, the magnetic moments are ordered, which leads to magnetization in the direction of the field and the body is drawn into the region of a strong field due to interaction. This phenomenon is called paramagnetism. Due to the competing influence of thermal motion at T ≠ 0, the ordering is never complete, and the degree of ordering is proportional to H. Usually, for paramagnets, the magnetic susceptibility is the sum of the dia- and paramagnetic contributions:

χ = χpair + χdia .

To estimate the typical values ​​of the susceptibility, we use the fact that the effective magnetic moment, defined as

μ eff \u003d 8χ M T , for an ordinary paramagnet does not depend on T and is equal to 1÷6

units of the Bohr magneton; hence χm ≈ (0.2 ÷ 1.0) 10-2 cm3/mol at T ≈ 300 K. Interpretation of the obtained results requires taking into account a number of effects (for example, the contribution of the orbital momentum, etc.).

Only a complete analysis of the interactions in each specific case can reveal them. In addition to electron shells, their own magnetic

moments are also possessed by most of the nuclei, which have an odd number of protons (1 H, 15 N, 19 F, 3I P, 11 B, 79 Br) or neutrons (13 C, 127 I), but the effect

their interaction with the external field is too small - the magnetic susceptibility of the nuclei has a value of the order of 10-10 cm3 / mol.

There are many ways to measure magnetic susceptibility,

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based on the fact that a sample with a mass m with a specific susceptibility χg placed in an inhomogeneous field, the gradient of which has a direction perpendicular to the direction of the field (we denote the directions Z and X, respectively), is affected by the force

Fz = Hx dH dZ x χ g m ,

which can be measured with a balance.

The most commonly used method is the Faraday method, using a magnet whose poles are carefully machined to create a large area of ​​constant H x (dHx/dZ ). Samples of small size compared to this area are placed in the zone of known values ​​of H x (dHx/dZ ) (determined by calibrating the system against a standard sample, usually Pt ) and the force acting on it is measured. The working sensitivity of the balance is 5 µg.

The range of application areas for various modifications of the described method is very wide: complex formation, kinetics, catalysis, structural studies, analysis of the composition of multicomponent systems, etc. This is determined by the ease of installation, measurement precision, and rapid results, and makes the method easily implemented in process control automation systems. Despite the wide distribution and simplicity of the described modifications of the method, a number of limitations of its information capabilities should be pointed out. First of all, the concentration of the analyte must be sufficiently reliable for registration. The accuracy in studying the behavior of diamagnetic substances must be<< 1 % и может быть достигнута только путем их глубокой очистки от парамагнитных примесей (О2 и др.). Менее жесткие требования предъявляются к процессам с участием парамагнетиков, однако и в этом случае можно различить образование только >2% new component. In addition, the rate of the studied transformations should be low, since the measurement time, even with automatic recording, is at least a few seconds. Often, due to small differences in the magnetic susceptibility of individual reaction products, the method does not allow their identification and determination.

Method of electron paramagnetic resonance (EPR). When entering

When a paramagnetic substance is placed in an alternating magnetic field with a frequency υ, a dispersion of the magnetic permeability (i.e., the dependence of the magnetic permeability on the frequency υ) and absorption of the energy of the external field are observed. In this case, the absorption has a resonant character. Typical conditions for such an experiment are as follows: a sample of a paramagnetic substance is placed in a constant magnetic field H, at a right angle to which an alternating magnetic field with a frequency v is turned on, and the complex magnetic susceptibility χ \u003d χ "+ iχ" is measured. The real part χ "is called high-frequency or dynamic susceptibility, and the imaginary part iχ"" characterizes

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absorption coefficient.

It is possible to find resonance conditions and obtain EPR spectra by changing the radiation frequency or the magnetic field strength. In most cases, experimenters have at their disposal installations with a constant frequency, in which, by changing the field, they adjust to the frequency of the emitter. Paramagnetic resonance is a set of phenomena associated with quantum transitions occurring between the energy levels of macroscopic systems under the influence of an alternating magnetic field of resonant frequency.

The EPR method is used to obtain information about the processes of redox and complex formation, as well as to determine the electronic and geometric structure of compounds when the observed paramagnetic particles are the direct objects of study. To obtain information, the width, line shape, number of lines in the spectrum, g-factor value, number of components and constants of STS and DSTS, signal intensity or area can be used.

The types of particles that determine the signals in the EPR spectrum are as follows: electron (solvated, trapped, in metals); radicals (inorganic, organic); ions; radical ions; complexes.

Important for the analytical aspects of the chemistry of coordination compounds is the manifestation of EPR in the complexes of the following paramagnetic ions: in the group of 3d elements - TiIII, VII, CrIII, CrV, CuII, MnII, FeIII; in Group

4d elements − ZrIII , PdI , PdIII , RhII , NbIV , MoV ; in the group of 5d elements - ReVI , WV , AuIII , RuIII ; in the group of REE and transuranium - GdIII , CeIII , EuIII .

14.3. Vibrational spectroscopy

The energy of vibrational transitions in molecules is comparable to the energy of radiation quanta in the infrared region. The infrared (IR) spectrum and the Raman spectrum (RS) of the molecules of chemical compounds are among the important characteristics of substances. However, since the spectra are of different nature, the intensity of manifestation of the same oscillations in them is different.

IR spectroscopy. Consider a molecule containing N atoms; the position of each atom can be determined by specifying three coordinates (for example, x, y, and z in a rectangular coordinate system). The total number of such coordinate values ​​will be 3N, and since each coordinate can be set independently of the others, we can assume that the molecule has 3N degrees of freedom. Having set all 3N coordinates, we will completely describe the molecule - the lengths of the bonds, the angles between them, as well as its location and orientation in space.

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Fig.14.1. Symmetry and three main types of vibrations of the water molecule.

The motion of the oxygen atom can be neglected, since it is located near the center of gravity of the molecule:

a – stretching symmetric vibration υ1 (parallel); b - deformation symmetric vibration υ2. (parallel); c – stretching antisymmetric vibration υ3 (perpendicular)

To describe the free motion of a molecule in three-dimensional space without changing its configuration, it is necessary to know three coordinates of the position of its center of gravity. Any rotation of a nonlinear molecule can be represented as the sum of rotations about three mutually perpendicular axes. With this in mind, the only remaining independent form of motion of the molecule is its internal oscillations. The number of basic vibrations of a linear molecule will be 3N–5 (taking into account the rotation around the bond axis), non-linear - 3N – 6. In both cases, the molecule (non-cyclic) has N–1 bonds between atoms and N– 1 vibrations are directed along the bonds - they are valence, and the remaining 2N–5 (or 2N–4) change the angles between the bonds - they are deformation vibrations. On fig. 14.1 shows all possible types of vibrations of the water molecule.

In order for the oscillation to manifest itself in the infrared region, it is necessary to change the dipole moment during oscillation along the axis of symmetry or perpendicular to it, i.e., any change in the value or direction of the dipole leads to the appearance of an oscillating dipole, which can absorb energy; interacting with the electrical component of infrared radiation. Since most molecules at room temperature are at the vibrational level υ0 (Fig. 14.2), most of the transitions should occur from the state υ0 to υ1. Symmetrical vibrations of the H2O molecule are designated υ1 for the highest frequency (3651.7 cm-1) and υ2 for the next (1595.0 cm-1), antisymmetric vibration with a frequency of 3755.8 cm-1 is designated υ3.

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Internuclear distance

Rice. 14.2. Vibrational states of a harmonic oscillator

When dividing vibrations into symmetric and antisymmetric, it should be emphasized that the symmetric stretching vibration does not change the dipole moment and therefore does not appear in the infrared region of the spectrum. Therefore, stretching of a homonuclear molecule should not lead to absorption in the IR region. The described simplified picture of oscillations can be realized only if two assumptions are true: 1) each oscillation is purely harmonic; 2) all vibrations are completely independent and do not affect each other.

For really oscillating molecules, the picture of movement is very complex, each atom does not move exactly along one of the paths shown in Fig. 14.1, their movement is a superposition of all possible vibrations in Fig. 14.2. However, such a superposition can be decomposed into components, if, for example, the molecule is observed stroboscopically, illuminating it with impulse frequencies coinciding with the frequencies of each of the fundamental vibrations in turn. This is the essence of infrared spectroscopy, only the frequency of the absorbed radiation plays the role of illumination, and the observation is carried out for changes in the dipole moment.

A complex molecule has a large number of vibrations, many of which can be seen in the IR spectrum. Each such vibration involves the movement of most of the atoms of the molecule, but in some cases the atoms are displaced by approximately the same distances, and in others, some small groups of atoms are displaced more than others. On this basis, vibrations can be divided into two classes: skeletal vibrations and vibrations of characteristic groups.

The frequencies of skeletal vibrations of organic molecules usually fall in the region of 1400–700 cm-1, and it is often difficult to attribute individual frequencies to any of the vibrations possible for a molecule, although the totality of bands quite unambiguously indicates belonging to a certain molecular structure. In such cases, the bands are called the fingerprints of the molecule in the spectrum.

The vibrational frequencies of the characteristic groups depend little on the structure of the molecule as a whole, they are in regions that usually do not overlap.

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with the region of skeletal vibrations, and can be used for analytical purposes.

IR spectroscopy can be used to solve the following problems.

1. Determination of the material composition of synthesis products in various phase states.

2. The study of phase-structural changes in products while maintaining certain technological indicators in a given range.

3. Evaluation of the state of equilibrium, the speed of the process.

4. Evaluation of indicators of the technological scheme as a whole with varying process conditions.

5. Investigation of functional affiliation and consumption of active components.

Quantitative measurements, as in other types of absorption spectroscopy, are based on Bouguer's law.

The analytical capabilities of IR spectroscopy can be demonstrated

rovat, pointing to some: practical results.

Using the characteristic absorption bands at 780 and 800 cm-1, which fall within the transparency region of the filter material and coal dust, and the corresponding calibration curves, it is possible to determine the content of quartz (less than 10 μg) in coal dust deposited on control filters over a certain time. Similar results can be obtained in the determination of asbestos in the air.

14.4. X-ray fluorescence method of analysis

The X-ray spectral method is based on the analysis of the nature and intensity of X-ray radiation. There are two types of method.

1. Proper x-ray analysis. In this method, the sample is placed in an X-ray tube as an anti-cathode. The heated cathode emits a stream of electrons bombarding the anticathode. The energy of these electrons depends on the temperature of the cathode, the voltage applied to the electrodes, and other factors. Under the influence of the energy of electrons in the anticathode of the tube, X-ray radiation is excited, the wavelength of which depends on the material of the anticathode, and the intensity of the radiation depends on the amount of this element in the sample.

By means of special devices it is possible to focus the electron beam on a very small surface area of ​​the target - the anticathode. This makes it possible to determine the qualitative and quantitative composition in the local area of ​​the material under study. This microprobe method is used, for example, if necessary, to determine the nature of the smallest inclusions in minerals or on the surface of metal grains, etc.

Another type of method, namely X-ray fluorescence analysis, has become more widely used.

2. X-ray fluorescence analysis. In this method, the sample is exposed to the primary x-ray radiation of the tube. As a result

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emits secondary X-ray radiation of the sample, the nature of which depends on the qualitative and quantitative composition of the sample.

For high-quality X-ray fluorescence analysis, it is important that the energy of polychromatic radiation (radiation of various wavelengths) of the X-ray tube is equal to or exceeds the energy necessary to knock out K-electrons of the elements that make up the analyzed sample. In this case, the secondary X-ray spectrum contains characteristic X-ray lines. The excess energy of the primary radiation of the tube (in excess of that necessary for the removal of electrons) is released in the form of the kinetic energy of the photoelectron.

For quantitative X-ray fluorescence analysis, the measurement of the intensity of characteristic emission lines is important.

The schematic diagram of the installation for X-ray fluorescence analysis is shown in fig. 14.3. The primary radiation of the x-ray tube falls on sample 2, in which the characteristic secondary x-ray radiation of the atoms of the elements that make up the sample is excited. X-rays of various wavelengths reflected from the surface of the sample pass through the collimator 3, a system of parallel molybdenum plates designed to transmit parallel rays traveling in only one direction. Divergent rays from other directions are absorbed by the inner surface of the tubes. The rays coming from the sample are decomposed into a spectrum, i.e., they are distributed over the wavelengths by means of an analyzer crystal 4. The angle of reflection of the rays 0 from the crystal is equal to the angle of incidence; however

Rice. 14.3. Schematic diagram of the installation for X-ray fluorescence analysis

1 - x-ray tube; 2 - sample; 3, 5 - collimators; 4 - crystal; 6 - receiver; 7 - recorder

at this angle, only rays with a wavelength that is related to θ by the Bragg equation are reflected:

where d is the distance between the planes of the atoms of the analyzer crystal lattice.

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By rotating the latter, one can change the angle θ and, consequently, the wavelength of the reflected rays.

Various substances are used as crystals.

According to the Bragg equation, it is easy to calculate that if, for example, we use a crystal of lithium fluoride (2d = 0.4026 nm) and change the angle θ from 10° to 80° by rotating the crystal, then the wavelengths of the reflected rays will be within 0.068 –0.394 nm. In accordance with this, the lines can identify and quantify elements with atomic numbers from 19 to 42, i.e. from potassium to molybdenum (Kα = 0.0709 nm). With a crystal of ethylenediamine ditartrate, elements with lower atomic numbers, such as aluminum (13), can be determined, and magnesium, sodium, etc., can also be determined with potassium hydrophthalate. Elements with atomic numbers from 13 and above are most reliably determined.

The monochromatic beams reflected from the analyzer crystal pass through the collimator and are fixed by the receiver, which rotates synchronously with the analyzer crystal at twice the speed. Geiger, proportional or scintillation counters are used as receivers. The latter consists of crystalline phosphorus - potassium iodide activated by thallium - which converts X-rays into visible. Light, in turn, is converted into electrical impulses, which are then amplified and recorded by a recording instrument. Curves are drawn on the paper tape of the recorder, the height of which characterizes the radiation intensity, and the position in relation to the abscissa axis - wavelengths - makes it possible to identify the qualitative composition of the sample.

Currently, there are fully automated devices for X-ray fluorescence analysis, which, in combination with a computer that produces statistically processed results, make the analysis fast and fairly accurate.

The X-ray fluorescence method makes it possible to analyze samples containing individual elements (starting from an element with an atomic mass of 13) from ten thousandths of a percent to tens of percent. Like other physical methods, this method is relative, i.e., the analysis is performed using standards of known chemical composition. It is possible to analyze samples of various aggregate states - solid, liquid and gaseous. In the analysis of solid materials, tablets are prepared from them, which are then exposed to radiation from an X-ray tube.

A certain disadvantage of the method is the requirement for complete homogeneity of the surfaces of the reference and analyzed tablets, which is often achieved with great difficulty.

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14.5. Radioactivation method of analysis

Radioactivation analysis is a physical method of analysis that arose and developed after the discovery of atomic energy and the creation of atomic reactors. It is based on the measurement of the radioactive emission of elements. Radioactivity analysis was known before. Thus, by measuring the natural radioactivity of uranium ores, the content of uranium in them was determined. A similar method is known for determining potassium from the radioactive isotope of this element. Activation analysis differs from these methods in that it measures the intensity of radiation of radioisotopes of elements formed as a result of the bombardment of the analyzed sample by a stream of elementary particles. With such a bombardment, nuclear reactions occur and radioactive isotopes of the elements that make up the analyzed sample are formed.

Table 14.1

Detection Limits of Elements by Thermal Neutron Activation Analysis

Elements	Mass - lg g
Mn, Co, Rh, Ag, In, Sm, Ho, Lu, Re, Ir, Au,
Na, Se, V, Cu, Ga, As, Br, Kr, Pd, Sb, I, La
Pr, Tb, Tm, Yb, W, Hg, Th, Zn, Ge, Se, Rb,
Sr, Y, Nb, Cd, Cs, Gd, Er, Hf, Ta, Os, U
Al, Cl, Ar, K, Cr, P, Ni, Mo, Ru,
Sn, Fe, Xe, Ba, Ce, Nd, Pt, Te
Mg, Si, Ca, Ti, Bi

The activation method of analysis is characterized by a low detection limit, Table. 14.1, and this is its main advantage over other methods of analysis.

The table shows that for more than 50 elements the detection limit is below 10-9 g.

The half-lives and radiation energies of the resulting radioactive isotopes are different for individual elements, and therefore it is possible to achieve significant specificity in the determination. In one sample of the analyzed material, a large number of impurity elements can be determined. Finally, the advantage of the method is that there is no need to quantitatively isolate traces of elements - the use of standards allows you to get the correct result even if some part of the element being determined is lost.

The disadvantages of the method include the need to use complex and expensive equipment; in addition, protection of the performers of the analysis from radioactive radiation should be ensured.

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In activation analysis, various elementary particles can be used to irradiate a sample - neutrons, protons, α-particles, as well as γ-radiation. The most commonly used neutron irradiation. This section of activation analysis is called neutron analysis. Usually, a slow thermal neutron flux is used.

Nuclear reactors can serve as sources of neutrons, in which a controlled chain reaction of fission of uranium nuclei occurs. Known neutron generators, in which to obtain neutrons using the reaction of interaction of deuterium with tritium, as well as other devices.

Radioactive isotopes of elements formed as a result of irradiation of a sample with a neutron flux undergo radioactive decay. The main types of such decay are as follows.

1. α-decay is characteristic of the heaviest elements. As a result of such a decay, the charge of the nucleus decreases by two units, and the mass - by four units.

2. β-decay, in which the mass number of the element is preserved, but the charge of the nucleus changes by one - upwards when the nucleus emits electrons and downwards when positrons are emitted. Radiation has a continuous energy spectrum.

After α- or β-decay, the nucleus formed as a result of decay is often in an excited state. The transition of such nuclei from an excited state to the ground state is usually accompanied by γ-radiation. The emission from nuclei is discrete, with very narrow linewidths. Such radiation, in principle, can serve for the unambiguous identification of radioisotopes.

14.6. Choice of scheme and method of analysis

To select the scheme and method of analysis, it is necessary to know the quantitative and semi-quantitative composition of the analyte. The analyst must know what he is dealing with, because depending on the composition of the analyte, the method of analysis is chosen. Before carrying out the analysis, it is necessary to draw up an analysis scheme from which it will be clear which methods can be used to transfer the analyte into solution, which methods must be used to separate the components to be determined and to what extent the components present will interfere with the separation, as far as possible to prevent the interfering effect of the substances present when definition of certain components. In the analysis of silicates, rocks, minerals, and often ores, it is usually necessary to determine almost all components, although in some cases a narrower task can be set. For example, when studying any ore deposit, it is not necessary to conduct a complete analysis of all samples. To do this, it is sufficient to perform a complete analysis of a certain number of samples, but the determination of the main ore component (for example, iron or manganese in the analysis

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Topic 14. Physical methods of analysis

iron or manganese ores) is mandatory for a large number of samples. The course of a complete analysis is usually different from the course of an analysis in the determination of one or more components. When analyzing metals, it is very rare for an analyst to determine the content of the main component, usually it is necessary to determine the content of impurities. The latter greatly affect the quality of the metal. So, when analyzing steels, the iron content is very rarely determined, but always to determine the grade of steel, the content of carbon, sulfur, phosphorus, silicon, manganese, alloying and some other components, which determine the quality of steel, is determined. This often applies to the analysis of high purity substances. However, the approach to the determination of impurities in the analysis of steels and metals of high purity should be different.

Methods for bringing the sample into solution or methods for digesting the sample are entirely dependent on the composition of the analyte. In general, it can be noted that in the analysis of silicates, rocks, minerals, as a rule, alkaline fusion is carried out to decompose samples, less often

– sintering with calcium carbonate, acid decomposition in a mixture of acids. In the analysis of metals and alloys, as a rule, acid decomposition is carried out, sometimes other methods of sample decomposition are used. For example, when analyzing aluminum, the sample is dissolved in an alkali solution. Other methods of transferring the sample into solution may also be proposed. As an example of the choice of an analysis scheme, we present a scheme for the analysis of silicate.

	Scheme of silicate analysis
Silicate (weighed)
Fusion with KNaCO3
Leaching with water and evaporation with HCl
SiO2	Precipitation of NH4OH
		precipitation
	Ca2 C2 O4	(NH4) 2 SECTION 7. REVIEW OF MODERN METHODS OF SUBSTANCE ANALYSIS Topic 14. Physical methods of analysis However, depending on the content of various components in the scheme, the influence of these components and their behavior in the analysis process according to such a scheme should be provided. So, if boron, fluorine and manganese are present in the silicate, then this scheme cannot be accepted without change, because there may be the following deviations: 1) during evaporation with hydrochloric acid, silicon and boron losses will be noticeable; 2) boron will partially precipitate together with silicic acid, and then volatilize when the silicic acid precipitate is treated with hydrofluoric acid; 3) part of the fluorine may remain in solution and will prevent the precipitation of aluminum and iron under the action of an aqueous solution of ammonia; 4) some part of boron will settle together with sesquihydroxides; 5) without the addition of an oxidizing agent, not all manganese precipitates together with sesquihydroxides during precipitation with an aqueous solution of ammonia, then it is partially precipitated in the form of oxalate together with calcium oxalate; 6) when magnesium is precipitated by phosphate, manganese phosphate will also precipitate. Thus, the presented analysis scheme cannot always be applied, and only knowing the qualitative and approximate quantitative composition, it is possible to draw up an analysis scheme taking into account the influence of all components present in the analyzed sample. The choice of determination method also depends on the content of the analyte and on the presence of other substances. So, when determining tenths of carbon in metals in the presence of thousandths and even several hundredths of a percent of sulfur, it is possible to carry out the determination without taking sulfur into account. If the sulfur content exceeds 0.04%, then the influence of sulfur must be taken into account and eliminated. Quizzes and exercises 1. What are the physical methods of analysis based on? 2. What are the advantages of physical methods of analysis over chemical and physical and chemical methods? 3. What is the nature of the analytical signal in spectral analysis? 4. What analytical problems can be solved using spectral analysis methods? 5. How are bodies classified according to their magnetic properties? 6. What is specific magnetization? 7. What is the basis of the method of static magnetic susceptibility? 8. What is paramagnetic resolax? 9. For what purposes can the EPR method be used? 10. What is the essence of the method IR spectroscopy? 11. What type of fluctuations Can the IR spectrum of complex molecules be used for analytical purposes? 12. What are quantitative measurements based on? IR spectroscopy? 13. What is the microprobe method in X-ray spectral analysis? SECTION 7. REVIEW OF MODERN METHODS OF SUBSTANCE ANALYSIS Topic 14. Physical methods of analysis 14. What is the nature of the analytical signal in X-ray fluorescence analysis? 15. How is a qualitative analysis of a sample carried out in the X-ray fluorescence method of analysis? 16. What is the difference between activation analysis and other radioactivity methods? 17. What is the main advantage of the activation method? 18. What is neutron analysis? 19. How is preliminary information about the composition of the sample used before choosing a method and analysis scheme? 20. Why is it necessary to draw up a sample analysis scheme?

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 recorded - 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.

ANALYTICAL CHEMISTRY AND PHYSICO-CHEMICAL METHODS OF ANALYSIS Publishing House TSTU Ministry of Education and Science of the Russian Federation State Educational Institution of Higher Professional 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 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 their structures. 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. A limited number of textbooks and teaching aids for 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 experience in teaching 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 actions. 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, astronautics, semiconductor and laser technology), the leading place belongs to analytical chemistry. The basis of 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 the development of modern methods for the 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 goals. Due to its significance, analytical chemistry has long been constantly taken care of ... ". 1.1 Brief history of the 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 Rus' 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 first years of Soviet power took place in three main directions: – assistance to enterprises in performing analyses; – 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 that are lower than 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 of fundamental importance 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 in this 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. Method of analysis - a detailed description of the performance of analytical reactions, indicating 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 аналитических методов – получение и измерение аналитического сигнала, т.е. любое проявление химических и физических свойств вещества в результате протекания химической реакции. Аналитические реакции можно проводить «сухим» и «мокрым» путем. Примеры реакций, проводимых «сухим» путем: реакции окрашивания пламени (Na + – желтый; Sr 2+ – красный; Ba 2+ – зеленый; K + – фиолетовый; Tl 3+ – зеленый, In + – синий и др.); при сплавлении Na 2 B 4 O 7 и Co 2+ , Na 2 B 4 O 7 и Ni 2+ , Na 2 B 4 O 7 и Cr 3+ образуются «перлы» буры различной окраски. Чаще всего аналитические реакции проводят в растворах. Анализируемый объект (индивидуальное вещество или смесь веществ) может находиться в любом state of aggregation(solid, liquid, gaseous). The object for analysis is called a sample, or sample. The same element in the 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 to the 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 poorly 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 K D KD = [A]II [A]I If both phases are solutions saturated with respect to the solid phase, and the substance to be extracted exists in a single form, then at equilibrium the distribution constant is equal to S II KD = , (1.1) SI where S I , 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 original sample; y and y0 are the final and initial impurity contents. The smaller Rc and the larger Rn, the more perfect the separation.