Any method of analysis uses a specific analytical signal, which, under given conditions, is given by specific elementary objects (atoms, molecules, ions) that make up the substances under study.

The analytical signal provides information of both qualitative and quantitative nature. For example, if precipitation reactions are used for analysis, qualitative information is obtained from the appearance or absence of precipitation. Quantitative information is obtained from the sediment mass. When a substance emits light under certain conditions, qualitative information is obtained from the appearance of a signal (emission of light) at a wavelength corresponding to a characteristic color, and quantitative information is obtained from the intensity of light radiation.

Based on the origin of the analytical signal, analytical chemistry methods can be classified into chemical, physical and physicochemical.

IN chemical methods carry out a chemical reaction and measure either the mass of the resulting product - gravimetric (weight) methods, or the volume of the reagent spent on interaction with the substance - titrimetric, gas-volumetric (volumetric) methods.

Gas volumetric analysis (gas volumetric analysis) is based on the selective absorption of the components of a gas mixture in vessels filled with one or another absorber, followed by measurement of the decrease in gas volume using a burette. Thus, carbon dioxide is absorbed with a solution of potassium hydroxide, oxygen with a solution of pyrogallol, and carbon monoxide with an ammonia solution of copper chloride. Gas volumemetry refers to rapid methods of analysis. It is widely used for the determination of carbonates in minerals and minerals.

Chemical methods of analysis are widely used for the analysis of ores, rocks, minerals and other materials to determine components in them with contents from tenths to several tens of percent. Chemical methods of analysis are characterized by high accuracy (the analysis error is usually tenths of a percent). However, these methods are gradually being replaced by more rapid physicochemical and physical methods of analysis.

Physical methods analyzes are based on the measurement of any physical property of substances, which is a function of composition. For example, refractometry is based on measuring the relative refractive indices of light. In activation analysis, the activity of isotopes, etc. is measured. Often, the analysis involves a chemical reaction first, and the concentration of the resulting product is determined by physical properties, for example, the intensity of absorption of light radiation by the colored reaction product. Such methods of analysis are called physicochemical.

Physical methods of analysis are characterized by high productivity, low detection limits of elements, objectivity of analysis results, and a high level of automation. Physical methods of analysis are used in the analysis of rocks and minerals. For example, the atomic emission method is used to determine tungsten in granites and shales, antimony, tin and lead in rocks and phosphates; atomic absorption method - magnesium and silicon in silicates; X-ray fluorescence - vanadium in ilmenite, magnesite, alumina; mass spectrometric - manganese in lunar regolith; neutron activation - iron, zinc, antimony, silver, cobalt, selenium and scandium in oil; by isotope dilution method - cobalt in silicate rocks.

Physical and physicochemical methods are sometimes called instrumental, since these methods require the use of instruments (equipment) specially adapted for carrying out the main stages of analysis and recording its results.

Physico-chemical methods analysis may include chemical transformations of the analyte, sample dissolution, concentration of the analyzed component, masking of interfering substances, and others. Unlike “classical” chemical methods of analysis, where the analytical signal is the mass of a substance or its volume, physicochemical methods of analysis use radiation intensity, current strength, electrical conductivity, and potential difference as an analytical signal.

Of great practical importance are methods based on the study of the emission and absorption of electromagnetic radiation in various regions of the spectrum. These include spectroscopy (for example, luminescent analysis, spectral analysis, nephelometry and turbidimetry, and others). Important physicochemical methods of analysis include electrochemical methods that use measurement of the electrical properties of a substance (coulometry, potentiometry, etc.), as well as chromatography (for example, gas chromatography, liquid chromatography, ion exchange chromatography, thin layer chromatography). Methods based on measuring the rates of chemical reactions (kinetic methods of analysis), the thermal effects of reactions (thermometric titration), as well as the separation of ions in a magnetic field (mass spectrometry) are being successfully developed.

Main purpose of analytical chemistry- to ensure, depending on the task at hand, accuracy, high sensitivity, rapidity and (or) selectivity of analysis. Methods are being developed that make it possible to analyze microobjects (see Microchemical analysis), conduct local analysis (at a point, on a surface, etc.), analysis without destroying the sample (see Non-destructive analysis), at a distance from it (remote analysis), continuous analysis (for example, in a flow), and also to establish in the form of what chemical compound and in what phase the component being determined exists in the sample (phase analysis). Important trends in the development of analytical chemistry are automation of analyses, especially in the control of technological processes, and mathematization, in particular the widespread use of computers.

Structure. Three major areas of analytical chemistry can be distinguished: general theoretical foundations; development of analysis methods; analytical chemistry of individual objects. Depending on the purpose of the analysis, a distinction is made between qualitative analysis and quantitative analysis. The task of the first is to detect and identify the components of the analyzed sample, the second is to determine their concentrations or masses. Depending on which components need to be detected or determined, there are isotopic analysis, elemental analysis, structural group analysis (including functional analysis), molecular analysis, and phase analysis. Based on the nature of the analyzed object, the analysis of inorganic and organic substances is distinguished.

In theoretical In the fundamentals of analytical chemistry, metrology of chemical analysis, including statistical processing of results, occupies a significant place. The theory of analytical chemistry also includes the study of the selection and preparation of analytical samples. about drawing up an analysis scheme and choosing methods, principles and ways to automate analysis, the use of computers, as well as the fundamentals of national economies. use of chemical results. analysis. The peculiarity of analytical chemistry is the study of not general, but individual, specific properties and characteristics of objects, which ensures the selectivity of many. analytical methods. Thanks to close connections with the achievements of physics, mathematics, biology and so on. fields of technology (this especially concerns methods of analysis), analytical chemistry has been turned into a discipline at the intersection of sciences.

In analytical chemistry, there are methods of separation, determination (detection) and hybrid ones, combining methods of the first two groups. Determination methods are divided into chemical methods of analysis (gravimetric analysis, titrimetry), physical and chemical methods of analysis (for example, electrochemical, photometric, kinetic), physical methods of analysis (spectral, nuclear physical and others) and biological methods of analysis. Sometimes determination methods are divided into chemical, based on chemical reactions, physical, based on physical phenomena, and biological, using the response of organisms to changes in the environment.

Analytical chemistry defines the general approach to the selection of analytical pathways and methods. Methods for comparing methods, conditions for their interchangeability and combination, principles and ways to automate analysis are being developed. For practical purposes using the analysis, it is necessary to develop ideas about its result as an indicator of product quality, the doctrine of express control of technology. processes, creating cost-effective methods. Of great importance for analysts working in various sectors of the national economy is the unification and standardization of methods. A theory is being developed to optimize the amount of information required to solve an analytical problem.

Analysis methods. Depending on the mass or volume of the analyzed sample, separation and determination methods are sometimes divided into macro-, micro- and ultra-micro methods.

Separation of mixtures is usually resorted to in cases where direct determination or detection methods do not provide the correct result due to the interfering influence of other components of the sample. Particularly important is the so-called relative concentration - the separation of small quantities of analyte components from significantly larger quantities of the main components of the sample. The separation of mixtures can be based on differences in the thermodynamic, or equilibrium, characteristics of the components (ion exchange constants, stability constants of complexes) or kinetic parameters. The main methods used for separation are chromatography, extraction, precipitation, distillation, as well as electrochemical methods such as electrodeposition.

Physico-chemical methods of analysis, are based on the dependence of the physical properties of a substance on its nature, and the analytical signal is a value of a physical property, functionally related to the concentration or mass of the component being determined. Physicochemical methods of analysis may include chemical transformations of the compound being analyzed, sample dissolution, concentration of the analyzed component, masking of interfering substances, and others. Unlike “classical” chemical methods of analysis, where the analytical signal is the mass of a substance or its volume, physicochemical methods of analysis use radiation intensity, current strength, electrical conductivity, potential difference, etc. as an analytical signal.

Of great practical importance are methods based on the study of the emission and absorption of electromagnetic radiation in various regions of the spectrum. These include spectroscopy (for example, luminescent analysis, spectral analysis, nephelometry and turbidimetry, and others). Important physicochemical methods of analysis include electrochemical methods that use the measurement of the electrical properties of a substance.

T.N.ORKINA

CHEMICAL AND PHYSICAL-CHEMICAL ANALYSIS

Tutorial

Orkina T. N. Chemistry. Chemical and physicochemical analysis. Textbook / St. Petersburg: Publishing House of the Polytechnic University, 2012. – 45 p.

The manual presents the goals and objectives of modern analytical chemistry - chemical, physicochemical and physical methods of analysis.

Methods for conducting qualitative and quantitative analysis are described in detail. A description of laboratory work on the qualitative analysis of solutions and metal alloys is given, as well as calculations and methods for conducting titrimetric (volumetric) analysis. The fundamentals of physical

chemical analysis - construction of phase diagrams, thermal analysis of metal alloys and construction of fusibility diagrams.

The manual complies with the educational standard of the disciplines “Chemistry” and “Inorganic Chemistry” and is intended for students of higher educational institutions studying in various areas and specialties in the field of engineering and technology in the field of

“Materials Science”, “Metallurgy” and others. The manual can be useful for students studying in any technical specialty within the discipline "Chemistry".

INTRODUCTION

Analytical chemistry is a branch of chemistry that studies the properties and processes of transformation of substances in order to establish their chemical composition. Establishing the chemical composition of substances (chemical identification) is the answer to the question of which elements or their compounds and in what quantitative ratios are contained in the analyzed sample. Analytical chemistry develops the theoretical foundations of the chemical analysis of substances and materials, develops methods for identifying, detecting, separating and determining chemical elements and their compounds, as well as methods for establishing the structure of a substance. Detection or, as they say, discovery of elements or ions that make up the substance under study constitutes the subject qualitative analysis. Determining the concentrations or quantities of chemical substances that make up the analyzed objects is a task quantitative analysis. Qualitative analysis usually precedes quantitative analysis, since to perform quantitative analysis it is necessary to know the qualitative composition of the sample being analyzed. When the composition of the object being studied is known in advance, qualitative analysis is carried out as necessary.

1. METHODS OF ANALYTICAL CHEMISTRY

To detect a component, a so-called analytical signal is usually used. A lytic signal– these are visible changes in the object of study itself (formation of sediment, change in color, etc.) or changes in the parameters of measuring instruments

(deviation of the instrument needle, change in digital reading, appearance of a line in the spectrum, etc.). To obtain an analytical signal, chemical reactions of different types are used (ion exchange, complexation, redox), various processes (for example,

precipitation, gas evolution), as well as various chemical, physical and biological properties of the substances themselves and the products of their reactions. That's why

Analytical chemistry has various methods for solving its problems.

Chemical methods (chemical analysis) are based on a chemical reaction between the sample being studied and specially selected reagents. In chemical methods, the analytical signal resulting from a chemical reaction is observed mainly visually.

Physico-chemical analysis methods are based on a quantitative study of the dependence composition - physical property object. The analytical signal is the electric potential, current strength,

resistance, etc., or any other parameter (temperature of phase transformations, hardness, density, viscosity, saturated vapor pressure, etc.) associated with a certain functional relationship with the composition and concentration of the object of study. Physicochemical research methods usually require the use of highly sensitive equipment. The advantages of these methods are their objectivity,

possibility of automation and speed of obtaining results. An example of a physicochemical method of analysis is the potentiometric determination of the pH of a solution using measuring instruments - potentiometers. This method allows not only to measure, but also to continuously monitor changes in pH when any processes occur in solutions.

IN physical methods of analysis analytical signal is usually

are received and recorded using special equipment. Physical methods primarily include optical spectroscopic methods of analysis, based on the ability of atoms and molecules to emit, absorb and scatter electromagnetic radiation.

By recording the emission, absorption or scattering of electromagnetic waves by the analyzed sample, a set of signals is obtained,

characterizing its qualitative and quantitative composition.

There is no sharp boundary between all three methods, so this division is somewhat arbitrary. For example, in chemical methods the sample is first exposed to some reagent, i.e. carry out a certain chemical reaction, and only after that the physical property is observed and measured. When analyzing by physical methods, observation and measurement are performed directly on the material being analyzed using special equipment, and chemical reactions, if carried out, play a supporting role. In accordance with this, in

chemical methods of analysis focus on the correct execution of a chemical reaction, while in physicochemical and physical methods the main emphasis is on the appropriate measurement equipment - the determination of a physical property.

2. CLASSIFICATION OF CHEMICAL AND PHYSICAL

CHEMICAL METHODS

Chemical and physicochemical methods of analysis are classified depending on the mass and volume of the analyzed samples. Based on the amount of substance or mixture of substances (sample) used for analysis, macro-, semi-micro-, submicro-, and ultramicroanalysis are distinguished. Table 1 shows the ranges of mass and volume of sample solutions recommended by the IUPAC Division of Analytical Chemistry (an abbreviation from the English abbreviation of the International Union of Pure and Applied Chemistry).

		Table 1
Type of analysis	Sample weight, g
		solution, ml
Macroanalysis		10-103
Semi-microanalysis		10-1 – 10
Microanalysis		10-2 – 1
Submicroanalysis	10-4 – 10-3	less than 10-2
Ultramicroanalysis	less than 10-4	less than 10-3

Depending on the nature of the task at hand, the following types of analysis are distinguished.

1 . Elemental analysis– establishing the presence and content of individual elements in a given substance, i.e. finding its elemental composition.

2. Phase analysis – establishing the presence and content of individual phases of the material being studied. For example, carbon in steel can be in the form of graphite or in the form of iron carbides. The task of phase analysis is to find how much carbon is contained in the form of graphite and how much is in the form of carbides.

3. Molecular analysis (material analysis) - establishing the presence and content of molecules of various substances (compounds) in the material.

For example, the amount of CO, CO2, N2, O2 and other gases in the atmosphere is determined.

4 . Functional analysis – establishing the presence and content of functional groups in molecules of organic compounds, for example amino groups (-NH2), nitro (-NO2), hydroxyl (-OH) and other groups.

Depending on the nature of the analyzed material, there are

analysis of inorganic and organic substances. The separation of the analysis of organic substances into a separate section of analytical chemistry is associated with the characteristics of organic substances. Even the first stage of analysis - transferring the sample into solution - differs significantly for organic and inorganic substances.

The main stages of any chemical analysis of complex

materials are the following steps.

1. Sampling for analysis. The average composition of the sample must correspond to the average composition of the entire batch of the analyzed material.

2. Decomposing the sample and transferring it into solution. The sample is dissolved in water or acids, fused with various substances, or other methods or chemical influences are used.

	Carrying out a chemical reaction:							P, where X –
sample component; R – reagent; P is the reaction product.
	Fixation		measurement	any physical parameter
reaction product, reagent or analyte.
Let's consider			in detail			chemical		analysis –

qualitative and quantitative analysis.

3. QUALITATIVE ANALYSIS

The task of qualitative analysis is to identify components and determine the qualitative composition of a substance or mixture of substances. The detection or, as they say, discovery of elements or ions in the composition of the substance under study is carried out by converting them into a compound that has some characteristic properties, i.e., the appearance of an analytical signal is recorded. The chemical transformations that occur are called analytical reactions. The substance with which the discovery is carried out - a reagent or reagent.

There are different methods of qualitative analysis that require the use of different quantities of the test substance in accordance with Table 1. For example: in macroanalytical method take about 1 g of the substance (0.5 g for metals and alloys) and dissolve it in 20-30 ml of water.

Reactions are carried out in test tubes (tube analysis). In the case of microanalysis, substances are taken approximately 100 times less compared to macroanalysis (milligrams of solid matter and several tenths of milliliters of solution). Highly sensitive reactions are used to open individual parts to detect the presence of small amounts of an element or ion. Reactions are performed either by microcrystalline or drop method. Microcrystalline reactions performed on a glass slide and the presence of the element is judged by the shape of the resulting crystals, which are examined under a microscope. Drip reactions, accompanied by a change in the color of the solution and the formation of colored precipitates, are performed on a strip of filter paper, applying the test solutions and reagents drop by drop onto it. Sometimes drop reactions are carried out on a special “drop plate” - a porcelain plate with indentations, as well as on a watch glass or in a small porcelain crucible. Semi-microanalysis (semi-micromethod)

occupies an intermediate position between macro- and microanalysis.

The amount of substance required to study the composition is approximately 20-25 times less than when conducting macroanalysis - about 50 mg of solid substance and 1 ml of solution. This method retains the system of macroanalysis and discovery of ions, but all reactions are performed with small quantities of the substance, using special techniques and equipment. For example, reactions are carried out in small 1-2 ml test tubes, into which solutions are introduced using pipettes. Sedimentation is carried out only by centrifugation. Submicroanalysis and ultramicroanalysis are carried out using special techniques using microscopes of varying degrees of magnification, electron microscopes and other equipment. Their consideration is beyond the scope of this manual.

In qualitative analysis, chemical reactions are most often carried out in solution, the so-called “wet method”. But sometimes it is possible to carry out solid-phase reactions, i.e. reactions "dry way". The substance and the corresponding reagents are taken in solid form and heated to high temperatures to carry out the reactions. An example of such reactions is the reaction of flame coloring with salts of certain metals. It is known that

sodium salts color the flame bright yellow, potassium salts – purple, copper salts – green. This color can be used to detect the presence of these elements in the substance under study. “Dry” reactions also include reactions of formation colored pearls – glassy alloys of various salts. For example, borax – Na2 B4 O7

· 10H2 O or double salt pearls NaNH4 HPO4 · 4H2 O. These methods are called pyrochemical and are widely used for the determination of minerals and rocks. But basically, in qualitative analysis, reactions are carried out

"wet path" between dissolved substances.

3.1. Methodology for conducting qualitative analysis

The first step in any analysis is to bring the sample into solution using various solvents. When analyzing inorganic substances, water, aqueous solutions of acids, alkalis, and less often other inorganic substances are most often used as solvents. Then the characteristic ion opening reactions are carried out. Qualitative discovery reactions

ions are chemical reactions that are accompanied by an external effect (change in the color of the solution, release of gas, formation of a precipitate), on the basis of which it can be judged that the reaction is taking place.

Most often they deal with aqueous solutions of salts, acids, bases, between which ion exchange reactions occur (less often - oxidation reactions).

restorative).

This or that analytical reaction must be performed under certain conditions, depending on the properties of the resulting compounds. If these conditions are not met, the results of the discovery of ions may be unreliable. For example, acid-soluble precipitates do not fall out of solution when there is an excess of acid. Therefore, the following must be observed

reaction conditions.

1. The proper environment of the test solution, which is created by adding an acid or alkali.

2. A certain temperature of the solution. For example, reactions that form precipitates, the solubility of which increases greatly with temperature, are carried out in the “cold”. On the contrary, if the reaction proceeds extremely slowly,

heating is required.

3. A fairly high concentration of the ion being opened, since at low concentrations the reaction does not proceed, i.e. the reaction is insensitive.

Concept "response sensitivity" is quantitatively characterized by two indicators: opening minimum and maximum dilution. To experimentally determine sensitivity, the reaction is repeated many times with the test solutions, gradually reducing the amount of solute and the volume of solvent. Opening minimum(Υ) is the smallest amount of a substance that can be discovered through a given reaction under certain conditions for its implementation. Expressed in micrograms (1Υ - millionths of a gram, 10-6 g). The opening minimum cannot fully characterize the sensitivity of the reaction, since the concentration of the opened ion in the solution matters. Limit dilution(1:G) characterizes the lowest concentration of a substance (ion) at which it can be opened through this reaction; where G is the mass amount of solvent per unit mass of the substance or ion being discovered. IN

In macroanalysis and semi-micromethod, those reactions are used whose sensitivity exceeds 50Υ, and the maximum dilution is 1: 1000.

When performing analytical reactions, not only sensitivity should be taken into account, but also reaction specificity– the possibility of opening a given ion in the presence of other ions. Discovery of ions through

specific reactions produced in separate portions of the test substance

solution in random order, called fractional analysis . But there are not many specific reactions. More often you have to deal with reagents that give the same or similar reaction effect with many ions. For example, barium chloride precipitates carbonate and

sulfathions in the form of precipitation BaCO3 and BaSO4. Reagents giving

identical analytical signal with a limited number of ions,

called selective or selective . The smaller the number of ions exposed by a given reagent, the higher the degree of selectivity of the reagent.

Sometimes foreign ions do not react with a given reagent, but reduce the sensitivity of the reaction or change the nature of the products formed. In this case, it is necessary to take into account the maximum ratio of the concentrations of the discovered and foreign ions, and also use masking agents (techniques or reagents). The interfering ion is converted into low-dissociation compounds or complex ions, its concentration in the solution decreases, and this ion no longer interferes with the discovery of the analyzed ions. All of the above features and techniques

are used in developing the sequence of chemical reactions during the analysis process. If the reactions used in the analysis

are nonspecific, and the interfering influence of foreign ions cannot be eliminated, then the use of the fractional method becomes impossible and they resort to

systematic course of analysis.

A systematic course of analysis is a specific sequence of reactions designed in such a way that the discovery of each ion is carried out only after the discovery and removal of all ions interfering with this discovery. In a systematic analysis, individual groups of ions are isolated from a complex mixture of ions, using their similar relationship to the action of certain reagents, called a group reagent. For example, one of the group reagents is sodium chloride,

which produces a similar effect on Ag+, Pb2+, Hg2 2+ ions. The action of sodium chloride on soluble salts containing these cations leads to the formation of precipitates insoluble in hydrochloric acid:

Ag+ + Cl- = AgCl↓

Pb2 + Cl- = PbCl2 ↓

Hg2 2+ + 2Cl- = Hg2 Cl2 ↓

All other ions, if exposed to HCl, will go into solution, and the three cations Ag+, Pb2+ and Hg2 2+ will be separated from the others using the group reagent NaCl. The use of group reagents provides great convenience: a complex problem is broken down into a number of simpler ones. Besides,

if any group of ions is completely absent, then its group reagent will not produce any precipitate with the analyzed solution. In this case, it makes no sense to carry out reactions on individual ions of this group. The result is significant savings in labor, time and reagents.

From the above it follows that in qualitative analysis the basis for the classification of ions is the difference in the solubility of some of the compounds they form; The method of separating one group of ions from another is based on this difference. The main classification of cations was introduced by the outstanding Russian chemist N.A. Menshutkin (1871).

IN The classification of anions is based on the solubility of barium salts

And silver in the corresponding acids. This classification is not strictly established, since different authors divide anions into different numbers of groups. One of the most common options is to divide the anions being studied into three groups:

Anions that form barium salts insoluble in water;

Topic 14. Physical methods of analysis

These methods are based on measuring the effect caused by the interaction of radiation with matter - a flow of quanta or particles. Radiation plays approximately the same role as a reagent in chemical methods of analysis. The physical effect being measured 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 of analysis have a number of advantages:

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

– greater versatility compared to chemical and physicochemical methods (including the ability to analyze multicomponent mixtures);

– the ability to determine the main impurity and trace components;

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

sometimes carry out non-destructive analysis.

In addition, many physical analysis methods allow performing both bulk and local and layer-by-layer analysis with spatial resolution down 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. Spectrum is electromagnetic radiation ordered by wavelength. When a substance is excited with 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 excitation and 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 = 0.1 nm = 10-10 m). For example, the wavelength of one of the yellow sodium lines could 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 metal vapors have line spectra. Banded spectra arise from the emission of ionized and non-ionized molecules consisting of two or more atoms, if these molecules are so far away from each other that they do not interact with neighboring molecules. Solid or continuous spectra are emitted by incandescent liquids or solids. Under certain conditions, individual atoms or molecules can also emit them.

Banded spectra consist of closely spaced lines, which are clearly observed in spectra obtained on instruments with large dispersion. For analytical purposes, the ultraviolet, visible and near-infrared parts of the spectrum are often used. The ultraviolet region of the spectrum is conventionally divided into vacuum (10–185 nm), far (185–230 nm) and near (230–400 nm). The visible part of the spectrum (400–750 nm), unlike other areas 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, and molecular composition of a substance and its structure.

Atomic emission spectral analysis is a method of analysis based on emission spectra that occur when a sample is evaporated and excited in an arc, spark or flame. Excited atoms and ions spontaneously, spontaneously move from the excited E k to lower energy statesЕi . 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 quantum meters are equipped with a mini-computer, which makes it possible to carry out mass multi-element express analysis of materials of standard composition with an accuracy that is often not inferior to the accuracy of most chemical methods.

Flame photometry– one of the methods of atomic emission spectral analysis. This method consists of transferring the sample to be analyzed into a solution, which is then converted into an aerosol using a nebulizer and fed into a burner flame. The solvent evaporates, and the elements, when 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 electric light sources in that the gas fuel and oxidizer gas coming from the cylinder produce 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: primarily alkaline elements, for the determination of which there are practically no rapid 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 a plasma torch arc 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

extensive methods of analytical chemistry. Preliminary preparation of the analyzed sample is similar to this operation in flame photometry: transferring the sample into solution, spraying and feeding aerosols into the flame. The solvent evaporates, the salts decompose, and the metals go 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 an arc spectrum of the element being determined, is directed through the flame to the slit of the spectrometer, with the help of which the analytical spectral line is isolated and the degree of absorption of its intensity by the vapors of the element being determined is measured.

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

The use of electrothermal atomizers instead of 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. Using this method, not only the tasks performed by atomic absorption analysis are solved, it allows you to determine individual atoms in a gaseous environment. For example, by exciting atomic fluorescence with a laser beam, it is possible to determine sodium in the upper layers of the atmosphere at a distance

100 km from Earth.

14.2. Methods based on the interaction of substances

with magnetic field

Brief Introduction to Magnetism. In a magnetic system (macro or microscopic) there are always two magnetic charges of different sign, but equal in absolute value, separated by a certain distance. Such a magnetic system is a magnetic dipole and, when placed in an external magnetic field with intensity H, tends to position itself parallel to the lines of force of the applied field. The force orienting a free dipole in a magnetic field can either pull it into a 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 dH/dx coincide or do not coincide. Unlike electric charges, 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, the model of which can be a loop with current. In this case, the resulting magnetic moment μ is directly proportional to the current strength and the loop area.

Let us 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., can be expressed through the magnetic moment or magnetization of the entire body M. In real conditions, due to the thermal movements 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 than the arithmetic sum μi and depending on the temperature T, and its direction may not coincide with the direction H.

To characterize a specific substance, the concept of specific magnetization σ = M/t (t is body mass) was 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 coefficient of proportionality in the relation 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 denoted 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 it is acted upon by a force

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 consists 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 per unit volume of the substance.

In an isotropic medium, all three vectors are collinear, so we can introduce the scalar

μ = Н В =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 external influences. 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 will be magnetization, and the response function will be magnetic susceptibility. Typically, changes in magnetic susceptibility are used to judge the most important processes occurring in a 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 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 the 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 peculiarities of the behavior of the systems under consideration in magnetic fields, the strength of which is constant or changes over 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 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 a dependence of the field strength on time, 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.

Static magnetic susceptibility method. The feasibility of using an experimental research method involving a magnetic field depends significantly on the behavior of the substance in a magnetic field. Based on their magnetic properties, all bodies are divided into diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic. The diamagnetic susceptibility of an atom is proportional to the number of electrons and the sum of the squares of the radii of the electrons’ orbitals, taken with the opposite sign, in accordance with Lenz’s law, according to which when the magnetic flux changes in

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Topic 14. Physical methods of analysis

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

The molecular susceptibility of a chemical compound can be expressed as

χМ = ∑ N i χi + λ,

where N i is the number of atoms of the i-th element in the molecule of the compound; χi – atomic susceptibility of a given element; λ is a correction 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 susceptibility of all components, taking into account their share in the composition of the sample.

Let us consider a substance characterized by many non-interacting magnetic moments. In the absence of an external magnetic field, under the influence 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 retraction of the body due to interaction in the region of the strong field. This phenomenon is called paramagnetism. Due to the competing influence of thermal motion at T ≠ 0, ordering is never complete, and the degree of ordering is proportional to H. Typically, for paramagnetic materials, the magnetic susceptibility is the sum of the dia- and paramagnetic contributions:

χ = χpair + χdia .

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

μ eff = 8χ М Т, for an ordinary paramagnet does not depend on T and is equal to 1÷6

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

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

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

their interaction with the external field is too small - the magnetic susceptibility of the nuclei is on 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 mass m with specific susceptibility χg, placed in a non-uniform field, the gradient of which has a direction perpendicular to the direction of the field (directions denoted by Z and X, respectively), is acted upon by a force

Fz = Hx dH dZ x χ g m,

which can be measured using scales.

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 region are placed in a 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 operating sensitivity of the scale is 5 mcg.

The range of areas of use of 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, precision of measurements and rapidity of obtaining results and makes the method easily implemented in automation systems for process control. Despite the widespread use 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 component being determined must be sufficiently reliable for registration. The accuracy when 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 small, since the measurement time, even with automatic registration, is at least several seconds. Often, due to small differences in the magnetic susceptibility of individual reaction products, the method does not allow their identification and determination.

Electron paramagnetic resonance (EPR) method. When you enter

When a paramagnetic substance is placed in an alternating magnetic field with a frequency υ, dispersion of magnetic permeability (i.e., the dependence of magnetic permeability on frequency υ) and absorption of external field energy are observed. In this case, the absorption is of a resonant nature. 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 frequency v is turned on, and the complex magnetic susceptibility χ = 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.

You can find resonance conditions and obtain ESR spectra by changing the radiation frequency or 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 energy levels of macroscopic systems under the influence of an alternating magnetic field of a resonant frequency.

The EPR method is used to obtain information about the processes of oxidation-reduction, 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 STS and DSTS constants, signal intensity or area can be used.

The types of particles responsible for signals in EPR spectra are the following: 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 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 rare earth elements and transuranics - 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 Raman spectrum (RS) of molecules of chemical compounds are among the important characteristics of substances. However, since the spectra are of a different nature, the intensity of the manifestation of the same vibrations 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 specified independently of the others, the molecule can be considered to have 3N degrees of freedom. Having specified 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 movement of the oxygen atom can be neglected, since it is located near the center of gravity of the molecule:

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

To describe the free movement of a molecule in three-dimensional space without changing its configuration, it is necessary to know the 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. Taking this into account, the only remaining independent form of motion of a molecule is its internal vibrations. The number of fundamental vibrations of a linear molecule will be 3N–5 (taking into account the rotation around the bond axis), nonlinear – 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. In Fig. Figure 14.1 shows all possible types of vibrations of a water molecule.

In order for an oscillation to appear in the infrared region, a change in the dipole moment is necessary when oscillating along the axis of symmetry or perpendicular to it, that is, 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. The symmetric 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), the antisymmetric vibration with a frequency of 3755.8 cm-1 is designated υ3.

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Internuclear distance

Rice. 14.2. Oscillatory states of a harmonic oscillator

When dividing vibrations into symmetric and antisymmetric, it should be emphasized that a symmetric stretching vibration does not change the dipole moment and therefore does not appear in the infrared region of the spectrum. Consequently, 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 actually vibrating molecules, the picture of motion 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 pulsed with frequencies coinciding with the frequencies of each of the main vibrations in turn. This is the essence of infrared spectroscopy, only the role of illumination is played by the frequency of the absorbed radiation, and the observation is made of changes in the dipole moment.

A complex molecule has a large number of vibrations, many of which can appear in the IR spectrum. Each such vibration involves the majority of the atoms of the molecule in motion, 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. Based on this feature, 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 assign individual frequencies to any of the vibrations possible for a molecule, although the combination of bands quite unambiguously indicates belonging to a particular molecular structure. In such cases, the bands are called fingerprints of the molecule in the spectrum.

The vibration frequencies of characteristic groups depend little on the structure of the molecule as a whole and are located in areas that usually do not overlap.

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Topic 14. Physical methods of analysis

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

Using IR spectroscopy, the following problems can be solved.

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

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

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

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

5. Study of the functionality 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 graphs, it is possible to determine the quartz content (less than 10 μg) in coal dust deposited on control filters over a certain time. Similar results can be obtained when determining 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 varieties of the method.

1. Actually X-ray spectral analysis. In this method, the sample is placed in an X-ray tube as an anticathode. The heated cathode emits a stream of electrons that bombard the anti-cathode. 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 electron energy in the anti-cathode 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.

Using special devices, it is possible to focus an 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 being studied. This microprobe method is used, for example, if it is 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 widespread.

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

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indicates the secondary X-ray emission of a 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 different wavelengths) of the X-ray tube is equal to or exceeds the energy required to knock out the K-electrons of the elements that make up the analyzed sample. In this case, the spectrum of secondary X-ray radiation contains characteristic X-ray lines. Excess energy from the tube's primary radiation (above and beyond that required to remove electrons) is released as photoelectron kinetic energy.

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

A schematic diagram of the installation for X-ray fluorescence analysis is shown in Fig. 14.3. The primary radiation of the X-ray tube hits 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 a wide variety of wavelengths reflected from the surface of the sample pass through collimator 3 - a system of parallel molybdenum plates designed to transmit parallel rays traveling in only one direction. Diverging rays from other directions are absorbed by the inner surface of the tubes. The rays coming from the sample are decomposed into a spectrum, that is, they are distributed over 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 crystal analyzer lattice.

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

A variety of substances are used as crystals.

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

Monochromatic rays reflected from the analyzer crystal pass through the collimator and are recorded by the receiver, which rotates synchronously with the analyzer crystal at twice the speed. Geiger counters, proportional or scintillation counters are used as receivers. The latter consists of crystal phosphorus - potassium iodide activated by thallium - which converts x-rays into visible radiation. The light, in turn, is converted into electrical impulses, which are then amplified and recorded by a recording device - a recorder. Curves are drawn on the paper tape of the recorder, the height of which characterizes the radiation intensity, and the position relative to the abscissa axis - the 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 quick and quite accurate.

The X-ray fluorescence method makes it possible to analyze samples containing individual elements (starting from an element with atomic mass 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. You can analyze samples of various states of aggregation - solid, liquid and gaseous. When analyzing solid materials, they are prepared into tablets, which are then exposed to radiation from an X-ray tube.

Some 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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Topic 14. Physical methods of analysis

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 measuring the radioactive emission of elements. Radioactivity analysis was known earlier. Thus, by measuring the natural radioactivity of uranium ores, the uranium content 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 bombardment of the analyzed sample with a stream of elementary particles. With such bombardment, nuclear reactions occur and radioactive isotopes of the elements that make up the analyzed sample are formed

Table 14.1

Limits of detection of elements by thermal neutron activation analysis

Elements	Weight – 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 compared to 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 emission energies of the resulting radioactive isotopes are different for individual elements, and therefore significant specificity of determination can be achieved. 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 for quantitative identification of traces of elements - the use of standards allows you to obtain 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, the analysis performers must be protected from radioactive radiation.

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

Nuclear reactors, in which a controlled chain reaction of fission of uranium nuclei occurs, can serve as sources of neutrons. Neutron generators are known, in which the reaction of deuterium with tritium, as well as other devices, are used to produce neutrons.

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 the following.

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

2. β-decay, in which the mass number of the element is maintained, but the charge of the nucleus changes by one - upward when the nucleus emits electrons and downward when positrons are emitted. The radiation has a continuous energy spectrum.

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

14.6. Selection of scheme and analysis method

To select a 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 what methods can be used to transfer the analyte into solution, what methods must be used to separate the components being determined and to what extent the present components will interfere with the separation, to what extent it is possible to prevent the interfering effect of the substances present during determination of certain components. When analyzing silicates, rocks, minerals, and often ores, it is necessary, as a rule, to determine almost all components, although in some cases a more narrow task may be set. For example, when studying an ore deposit, it is not necessary to conduct a complete analysis of all samples. To do this, it is enough to perform a complete analysis of a certain number of samples, but determining the main ore component (for example, iron or manganese during analysis

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Topic 14. Physical methods of analysis

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

Methods for transferring a sample into solution or methods for decomposing a sample depend entirely on the composition of the analyte. In general, it can be noted that when analyzing 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. When analyzing metals and alloys, acid decomposition is usually 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 be proposed. As an example of choosing an analysis scheme, we present a silicate analysis scheme.

	Silicate analysis scheme
Silicate (weight)
Fusion with KNaCO3
Water leaching and evaporation with HCl
SiO2	NH4OH precipitation
		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 the various components in the scheme, the influence of these components and their behavior during the analysis process according to such a scheme must be provided. Thus, if boron, fluorine and manganese are present in the silicate, then this scheme cannot be accepted without modification, because the following deviations may occur: 1) during evaporation with hydrochloric acid, losses of silicon and boron will be noticeable; 2) boron will partially precipitate along with silicic acid, and then will evaporate 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 ammonia solution; 4) some part of the boron will precipitate along with sesquihydroxides; 5) without the addition of an oxidizing agent, not all manganese precipitates along with sesquihydroxides during precipitation with an aqueous solution of ammonia, then it partially precipitates in the form of oxalate together with calcium oxalate; 6) when magnesium is precipitated with 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 present components contained in the analyzed sample. The choice of determination method also depends on the content of the component being determined and on the presence of other substances. Thus, when determining tenths of carbon in metals in the presence of thousandths and even several hundredths of a percent of sulfur, the determination can be made without taking sulfur into account. If the sulfur content exceeds 0.04%, then the influence of sulfur must be taken into account and eliminated. Test questions and exercises 1. What are the physical methods of analysis based on? 2. What is the advantage of physical methods of analysis over chemical and by 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 static magnetic susceptibility method based on? 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 oscillations in 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 using 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 sample composition used before selecting a method and analysis design? 20. Why do you need to create a sample analysis plan?

All existing methods of analytical chemistry can be divided into methods of sampling, sample decomposition, 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 quantity, 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 component being determined. This may be 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, color, line in the spectrum, etc. The appearance of an analytical signal must be reliably recorded. At a certain amount of a component, the magnitude of the analytical signal is measured: sediment mass, current strength, intensity of spectrum lines, etc. Then the content of the component is calculated using the functional relationship analytical signal - content: y=f(c), which is established by calculation or experiment and can be presented in the form of a formula, table or graph.

In analytical chemistry, a distinction is made between chemical, physical and physicochemical methods of analysis.

In chemical methods of analysis, the element or ion being determined is converted into some compound that has one or another characteristic properties, 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 obtain analyzes in a timely manner, while it is still possible to regulate the technological process. Therefore, along with chemical ones, physical and physicochemical methods of analysis are becoming increasingly widespread.

Physical methods analysis are based on the measurement of some

a system parameter that is a function of composition, for example, emission absorption spectra, electrical or thermal conductivity, potential of the electrode immersed in a solution, dielectric constant, refractive index, nuclear magnetic resonance, etc.

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

For the analysis of substances, physicochemical methods of analysis are widely used, based on chemical reactions, the occurrence 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.

Physico-chemical methods of analysis They are distinguished by high sensitivity and speed of execution, make it possible to automate chemical analytical determinations and are indispensable when analyzing small quantities of substances.

It should be noted that it is not always possible to draw a strict line 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 accurately measure the values ​​of certain parameters that characterize certain properties of a substance.