The value of methods of chemical analysis. Quantitative Analysis

Lecture plan:

1. General characteristics of physicochemical methods

2. General information about spectroscopic methods of analysis.

3. Photometric analysis method: photocolorimetry, colorimetry, spectrophotometry.

4. General information about nephelometric, luminescent, polarimetric methods of analysis.

5. Refractometric method of analysis.

6. General information about mass-spectral, radiometric analyses.

7. Electrochemical methods of analysis (potentiometry, conductometry, coulometry, amperometry, polarography).

8. Chromatographic method of analysis.

The essence of physico-chemical methods of analysis. Their classification.

Physico-chemical methods of analysis, like chemical methods, are based on carrying out one or another chemical reaction. In physical methods, chemical reactions are absent or are of secondary importance, although in spectral analysis the intensity of the lines always depends significantly on the chemical reactions in the carbon electrode or in the gas flame. Therefore, sometimes physical methods are included in the group of physicochemical methods, since there is no sufficiently strict unambiguous difference between physical and physicochemical methods, and the allocation of physical methods to a separate group is not of fundamental importance.

Chemical methods of analysis were unable to satisfy the diverse demands of practice, which had increased as a result of scientific and technological progress, the development of the semiconductor industry, electronics and computers, and the widespread use of pure and ultrapure substances in technology.

The use of physical and chemical methods of analysis is reflected in the technochemical control of food production, in research and production laboratories. These methods are characterized by high sensitivity and fast analysis. They are based on the use of physical and chemical properties of substances.

When performing analyzes by physicochemical methods, the equivalence point (the end of the reaction) is determined not visually, but with the help of instruments that record the change in the physical properties of the test substance at the equivalence point. For this purpose, devices with relatively complex optical or electrical circuits are usually used, so these methods are called methods. instrumental analysis.

In many cases, these methods do not require a chemical reaction to perform the analysis, unlike chemical methods of analysis. It is only necessary to measure the indicators of any physical properties of the analyzed substance: electrical conductivity, light absorption, light refraction, etc. Physicochemical methods allow continuous monitoring of raw materials, semi-finished products and finished products in industry.

Physicochemical methods of analysis began to be used later than chemical methods of analysis, when the relationship between the physical properties of substances and their composition was established and studied.

The accuracy of physicochemical methods varies greatly depending on the method. The highest accuracy (up to 0.001%) has coulometry, based on the measurement of the amount of electricity that is spent on the electrochemical oxidation or reduction of the ions or elements being determined. Most physicochemical methods have an error within 2-5%, which exceeds the error of chemical methods of analysis. However, such a comparison of errors is not entirely correct, since it refers to different concentration regions. With a low content of the determined component (about 10 -3% or less), classical chemical methods of analysis are generally unsuitable; at high concentrations, physicochemical methods successfully compete with chemical ones. Among the significant shortcomings of most physicochemical methods is the mandatory availability of standards and standard solutions.

Among the physicochemical methods, the most practical applications are:

1. spectral and other optical methods (refractometry, polarimetry);

2. electrochemical methods of analysis;

3. chromatographic methods of analysis.

In addition, there are 2 more groups of physico-chemical methods:

1. radiometric methods based on measuring the radioactive emission of a given element;

2. mass spectrometric methods of analysis based on the determination of the masses of individual ionized atoms, molecules and radicals.

The most extensive in terms of the number of methods and important in terms of practical value is the group of spectral and other optical methods. These methods are based on the interaction of substances with electromagnetic radiation. There are many different types of electromagnetic radiation: x-rays, ultraviolet, visible, infrared, microwave and radio frequency. Depending on the type of interaction of electromagnetic radiation with matter, optical methods are classified as follows.

On the measurement of the effects of polarization of the molecules of a substance are based refractometry, polarimetry.

Analyzed substances can absorb electromagnetic radiation and, based on the use of this phenomenon, a group is distinguished absorption optical methods.

The absorption of light by atoms of analytes is used in atomic absorption analysis. The ability to absorb light by molecules and ions in the ultraviolet, visible and infrared regions of the spectrum made it possible to create molecular absorption analysis (colorimetry, photocolorimetry, spectrophotometry).

The absorption and scattering of light by suspended particles in a solution (suspension) has led to the emergence of methods turbidimetry and nephelometry.

Methods based on measuring the intensity of radiation resulting from the release of energy by excited molecules and atoms of the analyzed substance are called emission methods. TO molecular emission methods include luminescence (fluorescence), to atomic emission- emission spectral analysis and flame photometry.

Electrochemical methods analyzes are based on the measurement of electrical conductivity ( conductometry); potential difference ( potentiometry); the amount of electricity passing through the solution coulometry); the dependence of the current on the applied potential ( voltammetry).

To the group chromatographic methods of analysis includes methods of gas and gas-liquid chromatography, distribution, thin-layer, adsorption, ion-exchange and other types of chromatography.

Spectroscopic methods of analysis: general information

The concept of the spectroscopic method of analysis, its varieties

Spectroscopic methods of analysis- physical methods based on the interaction of electromagnetic radiation with matter. The interaction leads to various energy transitions, which are recorded instrumentally in the form of radiation absorption, reflection and scattering of electromagnetic radiation.

Classification:

Emission spectral analysis is based on the study of emission (radiation) spectra or emission spectra of various substances. A variation of this analysis is flame photometry, based on measuring the intensity of atomic radiation excited by heating a substance in a flame.

Absorption spectral analysis is based on the study of the absorption spectra of the analyzed substances. If radiation is absorbed by atoms, then the absorption is called atomic, and if by molecules, then it is called molecular. There are several types of absorption spectral analysis:

1. Spectrophotometry - takes into account the absorption of light with a certain wavelength by the analyzed substance, i.e. absorption of monochromatic radiation.

2. Photometry - based on measuring the absorption of light by the analyzed substance is not strictly monochromatic radiation.

3. Colorimetry is based on measuring the absorption of light by colored solutions in the visible part of the spectrum.

4. Nephelometry is based on the measurement of the intensity of light scattered by solid particles suspended in solution, i.e. light scattered by the suspension.

Luminescence spectroscopy uses the glow of the object under study, which occurs under the action of ultraviolet rays.

Depending on in which part of the spectrum absorption or emission occurs, spectroscopy is distinguished in the ultraviolet, visible and infrared regions of the spectrum.

Spectroscopy is a sensitive method for determining more than 60 elements. It is used to analyze numerous materials, including biological media, plant materials, cements, glasses, and natural waters.

Photometric methods of analysis

Photometric methods of analysis are based on the selective absorption of light by the analyte or its combination with a suitable reagent. The absorption intensity can be measured by any method, regardless of the nature of the colored compound. The accuracy of the method depends on the method of measurement. There are colorimetric, photocolorimetric and spectrophotometric methods.

Photocolorimetric method of analysis.

The photocolorimetric method of analysis makes it possible to quantitatively determine the intensity of light absorption by the analyzed solution using photoelectrocolorimeters (sometimes they are simply called photocolorimeters). To do this, prepare a series of standard solutions and plot the dependence of the light absorption of the analyte on its concentration. This dependence is called a calibration curve. In photocolorimeters, the light fluxes passing through the solution have a wide absorption region - 30-50 nm, so the light here is polychromatic. This leads to loss of reproducibility, accuracy and selectivity of the analysis. The advantages of the photocolorimeter lie in the simplicity of design and high sensitivity due to the large luminosity of the radiation source - an incandescent lamp.

Colorimetric method of analysis.

The colorimetric method of analysis is based on measuring the absorption of light by a substance. In this case, the color intensity is compared, i.e. optical density of the test solution with the color (optical density) of a standard solution, the concentration of which is known. The method is very sensitive and is used to determine micro- and semi-micro quantities.

The analysis by colorimetric method requires much less time than by chemical analysis.

In visual analysis, equality of the intensity of staining of the analyzed and stained solution is achieved. This can be achieved in 2 ways:

1. equalize the color by changing the layer thickness;

2. select standard solutions of different concentrations (method of standard series).

However, it is visually impossible to quantify how many times one solution is colored more intensely than another. In this case, it is possible to establish only the same color of the analyzed solution when comparing it with the standard one.

Basic law of light absorption.

If the light flux, the intensity of which is I 0, is directed to a solution located in a flat glass vessel (cuvette), then one part of its intensity I r is reflected from the surface of the cuvette, the other part with intensity I a is absorbed by the solution and the third part with intensity I t passes through solution. There is a relationship between these values:

I 0 \u003d I r + I a + I t (1)

Because the intensity I r of the reflected part of the light flux when working with identical cuvettes is constant and insignificant, then it can be neglected in the calculations. Then equality (1) takes the form:

I 0 \u003d I a + I t (2)

This equality characterizes the optical properties of the solution, i.e. its ability to absorb or transmit light.

The intensity of the absorbed light depends on the number of colored particles in the solution, which absorb light more than the solvent.

The light flux, passing through the solution, loses part of the intensity - the greater, the greater the concentration and thickness of the solution layer. For colored solutions, there is a relationship called the Bouguer-Lambert-Beer law (between the degree of light absorption, the intensity of the incident light, the concentration of the colored substance and the layer thickness).

According to this law, the absorption of monochromatographic light passing through a layer of colored liquid is proportional to the concentration and thickness of its layer:

I \u003d I 0 10 - kCh,

Where I is the intensity of the light flux passing through the solution; I 0 is the intensity of the incident light; WITH- concentration, mol/l; h– layer thickness, cm; k is the molar absorption coefficient.

Molar absorption coefficient k is the optical density of a solution containing 1 mol/l absorbing substance, with a layer thickness of 1 cm. It depends on the chemical nature and physical state of the light-absorbing substance and on the wavelength of monochromatic light.

Standard series method.

The standard series method is based on obtaining the same color intensity of the test and standard solutions at the same layer thickness. The color of the test solution is compared with the color of a number of standard solutions. At the same color intensity, the concentrations of the test and standard solutions are equal.

To prepare a series of standard solutions, 11 test tubes of the same shape, size and glass are taken. Pour the standard solution from the burette in a gradually increasing amount, for example: into 1 test tube 0.5 ml, in the 2nd 1 ml, in the 3rd 1.5 ml, etc. - before 5 ml(in each next test tube 0.5 ml more than in the previous one). Equal volumes of a solution are poured into all test tubes, which gives a color reaction with the ion being determined. The solutions are diluted so that the liquid levels in all tubes are the same. The tubes are stoppered, the contents are thoroughly mixed and placed in a rack in increasing concentrations. In this way a color scale is obtained.

The same amount of reagent is added to the test solution in the same test tube, diluted with water to the same volume as in other test tubes. Close the cork, mix the contents thoroughly. The color of the test solution is compared with the color of standard solutions on a white background. Solutions should be well lit with diffused light. If the color intensity of the test solution coincides with the color intensity of one of the solutions on the color scale, then the concentrations of this and the test solutions are equal. If the color intensity of the test solution is intermediate between the intensity of two adjacent scale solutions, then its concentration is equal to the average concentration of these solutions.

The use of the method of standard solutions is advisable only for the mass determination of a substance. The prepared series of standard solutions has a relatively short time.

Method for equalizing the color intensity of solutions.

The method of equalizing the color intensity of the test and standard solutions is carried out by changing the layer height of one of the solutions. To do this, colored solutions are placed in 2 identical vessels: test and standard. Change the height of the solution layer in one of the vessels until the color intensity in both solutions is the same. In this case, determine the concentration of the test solution With research. , comparing it with the concentration of the standard solution:

From research \u003d C st h st / h research,

where h st and h research are the layer heights of the standard and test solutions, respectively.

Devices used to determine the concentrations of the studied solutions by equalizing the color intensity are called colorimeters.

There are visual and photoelectric colorimeters. In visual colorimetric determinations, the color intensity is measured by direct observation. Photoelectric methods are based on the use of photocells-photocolorimeters. Depending on the intensity of the incident light beam, an electric current is generated in the photocell. The strength of the current caused by exposure to light is measured with a galvanometer. The deflection of the arrow indicates the intensity of the color.

Spectrophotometry.

Photometric method is based on measuring the absorption of light of non-strictly monochromatic radiation by the analyzed substance.

If monochromatic radiation (radiation of one wavelength) is used in the photometric method of analysis, then this method is called spectrophotometry. The degree of monochromaticity of the flow of electromagnetic radiation is determined by the minimum interval of wavelengths, which is separated by the used monochromator (light filter, diffraction grating or prism) from a continuous flow of electromagnetic radiation.

TO spectrophotometry also include the field of measurement technology, which combines spectrometry, photometry and metrology and is engaged in the development of a system of methods and instruments for quantitative measurements of spectral coefficients of absorption, reflection, radiation, spectral brightness as characteristics of media, coatings, surfaces, emitters.

Stages of spectrophotometric research:

1) carrying out a chemical reaction to obtain systems suitable for spectrophotometric analysis;

2) measurements of the absorption of the resulting solutions.

The essence of the method of spectrophotometry

The dependence of the absorption of a solution of a substance on the wavelength on the graph is depicted in the form of an absorption spectrum of a substance, on which it is easy to distinguish the absorption maximum located at the wavelength of light that is maximally absorbed by the substance. Measurement of the optical density of solutions of substances on spectrophotometers is carried out at the wavelength of the maximum absorption. This makes it possible to analyze in one solution substances whose absorption maxima are located at different wavelengths.

In spectrophotometry in the ultraviolet and visible regions, electronic absorption spectra are used.

They characterize the highest energy transitions, which are capable of a limited range of compounds and functional groups. In inorganic compounds, electronic spectra are associated with a high polarization of the atoms that make up the molecule of the substance, and usually appear in complex compounds. In organic compounds, the appearance of electronic spectra is caused by the transition of electrons from the ground to excited levels.

The position and intensity of the absorption bands are strongly affected by ionization. During acid-type ionization, an additional lone pair of electrons appears in the molecule, which leads to an additional bathochromic shift (shift to the long-wavelength region of the spectrum) and an increase in the intensity of the absorption band.

The spectrum of many substances has several absorption bands.

For spectrophotometric measurements in the ultraviolet and visible regions, two types of instruments are used - non-registering(the result is observed on the instrument scale visually) and recording spectrophotometers.

Luminescent method of analysis.

Luminescence- the ability to self-luminescence, arising under various influences.

Classification of processes that cause luminescence:

1) photoluminescence (excitation by visible or ultraviolet light);

2) chemiluminescence (excitation due to the energy of chemical reactions);

3) cathodoluminescence (excitation by electron impact);

4) thermoluminescence (excitation by heating);

5) triboluminescence (excitation by mechanical action).

In chemical analysis, the first two types of luminescence matter.

Classification of luminescence by the presence of afterglow. It can stop immediately with the disappearance of excitation - fluorescence or continue for a certain time after the cessation of the exciting effect - phosphorescence. The phenomenon of fluorescence is mainly used, so the method is named fluorimetry.

Application of fluorimetry: analysis of traces of metals, organic (aromatic) compounds, vitamins D, B 6 . Fluorescent indicators are used for titration in cloudy or dark-colored media (titration is carried out in the dark, illuminating the titrated solution, where the indicator is added, with the light of a fluorescent lamp).

Nephelometric analysis.

Nephelometry was proposed by F. Kober in 1912 and is based on measuring the intensity of light scattered by a suspension of particles using photocells.

With the help of nephelometry, the concentration of substances that are insoluble in water, but form stable suspensions, is measured.

For nephelometric measurements, nephelometers, similar in principle to colorimeters, with the only difference being that with nephelometry

When conducting photonephelometric analysis first, based on the results of determining a series of standard solutions, a calibration graph is built, then the test solution is analyzed and the concentration of the analyte is determined from the graph. To stabilize the resulting suspensions, a protective colloid is added - a solution of starch, gelatin, etc.

Polarimetric analysis.

Electromagnetic oscillations of natural light occur in all planes perpendicular to the direction of the beam. The crystal lattice has the ability to transmit rays only in a certain direction. Upon exiting the crystal, the beam oscillates only in one plane. A beam whose oscillations are in the same plane is called polarized. The plane in which vibrations occur is called oscillation plane polarized beam, and the plane perpendicular to it - plane of polarization.

The polarimetric method of analysis is based on the study of polarized light.

Refractometric method of analysis.

The basis of the refractometric method of analysis is the determination of the refractive index of the substance under study, since an individual substance is characterized by a certain refractive index.

Technical products always contain impurities that affect the refractive index. Therefore, the refractive index can in some cases serve as a characteristic of the purity of the product. For example, varieties of purified turpentine are distinguished by refractive indices. So, the refractive indices of turpentine at 20 ° for yellow, denoted by n 20 D (the entry means that the refractive index was measured at 20 ° C, the wavelength of the incident light is 598 mmk), are equal to:

First class Second class Third class

1,469 – 1,472 1,472 – 1,476 1,476 – 1,480

The refractometric method of analysis can be used for binary systems, for example, to determine the concentration of a substance in aqueous or organic solutions. In this case, the analysis is based on the dependence of the refractive index of the solution on the concentration of the solute.

For some solutions there are tables of dependence of refractive indices on their concentration. In other cases, they are analyzed using the calibration curve method: a series of solutions of known concentrations are prepared, their refractive indices are measured, and a plot of refractive indices versus concentration is plotted, i.e. build a calibration curve. It determines the concentration of the test solution.

refractive index.

When a beam of light passes from one medium to another, its direction changes. He breaks. The refractive index is equal to the ratio of the sine of the angle of incidence to the sine of the angle of refraction (this value is constant and characteristic of a given medium):

n = sinα / sinβ,

where α and β are the angles between the direction of the rays and the perpendicular to the interface of both media (Fig. 1)

The refractive index is the ratio of the speeds of light in air and in the medium under study (if a beam of light falls from air).

The refractive index depends on:

1. The wavelength of the incident light (as the wavelength increases, the indicator

refraction decreases).

2. temperature (with increasing temperature, the refractive index decreases);

3. pressure (for gases).

The index of refraction indicates the wavelengths of the incident light and the temperature of the measurement. For example, the entry n 20 D means that the refractive index is measured at 20°C, the wavelength of the incident light is 598 microns. In technical handbooks, the refractive indices are given at n 20 D.

Determination of the refractive index of a liquid.

Before starting work, the surface of the prisms of the refractometer is washed with distilled water and alcohol, the correctness of the zero point of the device is checked, and the refractive index of the liquid under study is determined. To do this, the surface of the measuring prism is carefully wiped with a cotton swab moistened with the liquid under study, and a few drops of it are applied to this surface. The prisms are closed and, rotating them, direct the border of light and shade to the cross of the eyepiece threads. The compensator eliminates the spectrum. When reading the refractive index, three decimal places are taken on the refractometer scale, and the fourth is taken by eye. Then they shift the border of chiaroscuro, again combine it with the center of the sighting cross and make a second count. That. 3 or 5 readings are made, after which the working surfaces of the prisms are washed and wiped. The test substance is again applied to the surface of the measuring prism and a second series of measurements is carried out. From the data obtained, the arithmetic mean is taken.

Radiometric analysis.

Radiometric analysis h is based on the measurement of radiation from radioactive elements and is used for the quantitative determination of radioactive isotopes in the test material. In this case, either the natural radioactivity of the element being determined is measured, or the artificial radioactivity obtained using radioactive isotopes.

Radioactive isotopes are identified by their half-life or by the type and energy of the radiation emitted. In the practice of quantitative analysis, the activity of radioactive isotopes is most often measured by their α-, β-, and γ-radiation.

Application of radiometric analysis:

Study of the mechanism of chemical reactions.

The method of labeled atoms is used to investigate the effectiveness of various methods of applying fertilizers to the soil, the ways of penetration into the body of microelements applied to the leaves of a plant, etc. Radioactive phosphorus 32 P and nitrogen 13 N are especially widely used in agrochemical research.

Analysis of radioactive isotopes used for the treatment of oncological diseases and for the determination of hormones, enzymes.

Mass spectral analysis.

Based on the determination of the masses of individual ionized atoms, molecules and radicals as a result of the combined action of electric and magnetic fields. Registration of separated particles is carried out by electrical (mass spectrometry) or photographic (mass spectrography) methods. The determination is carried out on instruments - mass spectrometers or mass spectrographs.

Electrochemical methods of analysis.

Electrochemical methods of analysis and research are based on the study and use of processes occurring on the electrode surface or in the near-electrode space. Analytical signal- electrical parameter (potential, current strength, resistance), which depends on the concentration of the analyte.

Distinguish straight And indirect electrochemical methods. In direct methods, the dependence of the current strength on the concentration of the analyte is used. In indirect - the current strength (potential) is measured to find the end point of the titration (equivalence point) of the component being determined by the titrant.

Electrochemical methods of analysis include:

1. potentiometry;

2. conductometry;

3. coulometry;

4. amperometry;

5. polarography.

Electrodes used in electrochemical methods.

1. Reference electrode and indicator electrode.

Reference electrode- This is an electrode with a constant potential, insensitive to the ions of the solution. The reference electrode has a time-stable reproducible potential that does not change when a small current is passed, and the potential of the indicator electrode is reported relative to it. Silver chloride and calomel electrodes are used. The silver chloride electrode is a silver wire coated with a layer of AgCl and placed in a KCI solution. The electrode potential is determined by the concentration of chlorine ion in the solution:

The calomel electrode consists of metallic mercury, calomel and KCl solution. The electrode potential depends on the concentration of chloride ions and temperature.

Indicator electrode- this is an electrode that reacts to the concentration of the ions being determined. The indicator electrode changes its potential with a change in the concentration of "potential-determining ions". Indicator electrodes are divided into irreversible and reversible. Potential jumps of reversible indicator electrodes at interphase boundaries depend on the activity of participants in electrode reactions in accordance with thermodynamic equations; equilibrium is established fairly quickly. Irreversible indicator electrodes do not meet the requirements of reversible ones. In analytical chemistry, reversible electrodes are used, for which the Nernst equation is satisfied.

2. Metal electrodes: electron exchange and ion exchange.

Electron exchange electrode at the interfacial boundary, a reaction occurs with the participation of electrons. The electron exchange electrodes are divided into electrodes first kind and electrodes second kind. Electrodes of the first kind - a metal plate (silver, mercury, cadmium) immersed in a solution of a highly soluble salt of this metal. Electrodes of the second kind - a metal coated with a layer of a sparingly soluble compound of this metal and immersed in a solution of a highly soluble compound with the same anion (silver chloride, calomel electrodes).

Ion exchange electrodes- electrodes, the potential of which depends on the ratio of the concentrations of the oxidized and reduced forms of one or more substances in solution. Such electrodes are made of inert metals such as platinum or gold.

3. Membrane electrodes they are a porous plate impregnated with a liquid immiscible with water and capable of selective adsorption of certain ions (for example, solutions of Ni 2+, Cd 2+, Fe 2+ chelates in an organic solution). The operation of membrane electrodes is based on the occurrence of a potential difference at the phase boundary and the establishment of an exchange equilibrium between the membrane and the solution.

Potentiometric method of analysis.

The potentiometric method of analysis is based on measuring the potential of an electrode immersed in a solution. In potentiometric measurements, a galvanic cell is made up with an indicator electrode and a reference electrode and the electromotive force (EMF) is measured.

Varieties of potentiometry:

Direct potentiometry used to directly determine the concentration by the value of the potential of the indicator electrode, provided that the electrode process is reversible.

Indirect potentiometry is based on the fact that a change in the concentration of an ion is accompanied by a change in the potential at the electrode immersed in the titrated solution.

In potentiometric titration, an end point is found in terms of a potential jump, due to the replacement of an electrochemical reaction with another one in accordance with the values ​​of E ° (standard electrode potential).

The value of the potential depends on the concentration of the corresponding ions in the solution. For example, the potential of a silver electrode immersed in a silver salt solution changes with a change in the concentration of Ag + -ions in the solution. Therefore, by measuring the potential of an electrode immersed in a solution of a given salt of unknown concentration, it is possible to determine the content of the corresponding ions in the solution.

The electrode, by the potential of which the concentration of the ions to be determined in the solution is judged, is called indicator electrode.

The potential of the indicator electrode is determined by comparing it with the potential of another electrode, which is commonly called reference electrode. As a reference electrode, only such an electrode can be used, the potential of which remains unchanged when the concentration of the ions being determined changes. A standard (normal) hydrogen electrode is used as a reference electrode.

In practice, a calomel rather than a hydrogen electrode is often used as a reference electrode with a known value of the electrode potential (Fig. 1). The potential of the calomel electrode with a saturated solution of CO at 20 °C is 0.2490 V.

Conductometric method of analysis.

The conductometric method of analysis is based on measuring the electrical conductivity of solutions, which changes as a result of chemical reactions.

The electrical conductivity of a solution depends on the nature of the electrolyte, its temperature, and the concentration of the solute. The electrical conductivity of dilute solutions is due to the movement of cations and anions, which differ in different mobility.

With an increase in temperature, the electrical conductivity increases, as the mobility of the ions increases. At a given temperature, the electrical conductivity of an electrolyte solution depends on its concentration: as a rule, the higher the concentration, the greater the electrical conductivity! Therefore, the electrical conductivity of a given solution serves as an indicator of the concentration of a solute and is determined by the mobility of the ions.

In the simplest case of conductometric quantification, when the solution contains only one electrolyte, a graph is plotted as a function of the electrical conductivity of the analyte solution versus its concentration. Having determined the electrical conductivity of the test solution, the concentration of the analyte is found from the graph.

Thus, the electrical conductivity of barite water changes in direct proportion to the content of Ba(OH) 2 in the solution. This dependence is graphically expressed by a straight line. To determine the content of Ba(OH) 2 in barite water of unknown concentration, it is necessary to determine its electrical conductivity and, using the calibration graph, find the concentration of Ba(OH) 2 corresponding to this value of electrical conductivity. If a measured volume of gas containing carbon dioxide is passed through a solution of Ba (OH) 2, whose electrical conductivity is known, then CO 2 reacts with Ba (OH) 2:

Ba (OH) 2 + CO 2 → BaCO 3 + H 2 0

As a result of this reaction, the content of Ba(OH) 2 in the solution will decrease and the electrical conductivity of barite water will decrease. By measuring the electrical conductivity of barite water after it has absorbed CO 2 , one can determine how much the concentration of Ba(OH) 2 in the solution has decreased. By the difference in concentrations of Ba (OH) 2 in barite water, it is easy to calculate the amount of absorbed

The analysis of a substance can be carried out in order to establish its qualitative or quantitative composition. Accordingly, a distinction is made between qualitative and quantitative analysis.

Qualitative analysis allows you to establish what chemical elements the analyzed substance consists of and what ions, groups of atoms or molecules are included in its composition. When studying the composition of an unknown substance, a qualitative analysis always precedes a quantitative one, since the choice of a method for the quantitative determination of the constituent parts of the analyzed substance depends on the data obtained during its qualitative analysis.

Qualitative chemical analysis is mostly based on the transformation of the analyte into some new compound with characteristic properties: color, a certain physical state, crystalline or amorphous structure, a specific smell, etc. The chemical transformation that occurs in this case is called a qualitative analytical reaction, and the substances that cause this transformation are called reagents (reagents).

When analyzing a mixture of several substances with similar chemical properties, they are first separated and only then characteristic reactions are carried out for individual substances (or ions), therefore, qualitative analysis covers not only individual reactions for detecting ions, but also methods for their separation.

Quantitative analysis allows you to establish the quantitative ratio of the parts of a given compound or mixture of substances. Unlike qualitative analysis, quantitative analysis makes it possible to determine the content of individual components of the analyte or the total content of the analyte in the test product.

Methods of qualitative and quantitative analysis, allowing to determine the content of individual elements in the analyzed substance, are called elements of analysis; functional groups - functional analysis; individual chemical compounds characterized by a certain molecular weight - molecular analysis.

A set of various chemical, physical and physico-chemical methods for separating and determining individual structural (phase) components of heterogeneous systems that differ in properties and physical structure and are limited from each other by interfaces is called phase analysis.

Qualitative analysis methods

Qualitative analysis uses characteristic chemical or physical properties of the substance to establish the composition of the substance under investigation. There is absolutely no need to isolate the discovered elements in their pure form in order to detect their presence in the analyzed substance. However, the isolation of metals, nonmetals, and their compounds in pure form is sometimes used in qualitative analysis for their identification, although this way of analysis is very difficult. To detect individual elements, simpler and more convenient methods of analysis are used, based on chemical reactions characteristic of the ions of these elements and occurring under strictly defined conditions.

An analytical sign of the presence of the desired element in the analyzed compound is the release of a gas that has a specific odor; in the other - the precipitation, characterized by a certain color.

Reactions between solids and gases. Analytical reactions can take place not only in solutions, but also between solid and gaseous substances.

An example of a reaction between solids is the reaction of the release of metallic mercury when dry salts of it are heated with sodium carbonate. The formation of white smoke from the interaction of gaseous ammonia with hydrogen chloride can serve as an example of an analytical reaction involving gaseous substances.

The reactions used in qualitative analysis can be divided into the following groups.

1. Precipitation reactions, accompanied by the formation of precipitates of various colors. For example:

CaC2O4 - white

Fe43 - blue,

CuS - brown - yellow

HgI2 - red

MnS - flesh - pink

PbI2 - golden

The resulting precipitates may differ in a certain crystal structure, solubility in acids, alkalis, ammonia, etc.

2. Reactions accompanied by the formation of gases with a known odor, solubility, etc.

3. Reactions accompanied by the formation of weak electrolytes. Among such reactions, which result in the formation of: CH3COOH, H2F2, NH4OH, HgCl2, Hg(CN)2, Fe(SCN)3, etc. Reactions of the same type can be considered reactions of acid-base interaction, accompanied by the formation of neutral water molecules, reactions of the formation of gases and precipitates that are poorly soluble in water, and complex formation reactions.

4. Reactions of acid-base interaction, accompanied by the transition of protons.

5. Complexation reactions accompanied by the addition of various legends - ions and molecules - to the atoms of the complexing agent.

6. Complexation reactions associated with acid-base interaction

7. Oxidation reactions - reductions, accompanied by the transition of electrons.

8. Oxidation reactions - reductions associated with acid - base interaction.

9. Oxidation-reduction reactions associated with complex formation.

10. Oxidation reactions - reductions, accompanied by the formation of precipitation.

11. Ion exchange reactions occurring on cation exchangers or anion exchangers.

12. Catalytic reactions used in kinetic methods of analysis

Wet and dry analysis

The reactions used in qualitative chemical analysis are most often carried out in solutions. The analyte is first dissolved and then the resulting solution is treated with appropriate reagents.

To dissolve the analyte, distilled water, acetic and mineral acids, aqua regia, aqueous ammonia, organic solvents, etc. are used. The purity of the solvents used is an important condition for obtaining correct results.

The substance transferred into solution is subjected to systematic chemical analysis. A systematic analysis consists of a series of preliminary tests and sequentially performed reactions.

The chemical analysis of test substances in solutions is called wet analysis.

In some cases, substances are analyzed dry, without transferring them into solution. Most often, such an analysis is reduced to testing the ability of a substance to color a colorless flame of a burner in a characteristic color or to impart a certain color to a melt (the so-called pearl) obtained by heating a substance with sodium tetraborate (borax) or sodium phosphate ("phosphorus salt") in a platinum wire.

Chemical and physical method of qualitative analysis.

Chemical methods of analysis. Methods for determining the composition of substances based on the use of their chemical properties are called chemical methods of analysis.

Chemical methods of analysis are widely used in practice. However, they have a number of disadvantages. So, to determine the composition of a given substance, it is sometimes necessary to first separate the component to be determined from foreign impurities and isolate it in its pure form. The isolation of substances in pure form is often a very difficult and sometimes impossible task. In addition, in order to determine small amounts of impurities (less than 10-4%) contained in the analyte, it is sometimes necessary to take large samples.

Physical methods of analysis. The presence of one or another chemical element in the sample can be detected without resorting to chemical reactions, based directly on the study of the physical properties of the substance under study, for example, coloring a colorless burner flame in characteristic colors with volatile compounds of certain chemical elements.

Methods of analysis, by which it is possible to determine the composition of the substance under study, without resorting to the use of chemical reactions, are called physical methods of analysis. Physical methods of analysis include methods based on the study of optical, electrical, magnetic, thermal and other physical properties of the analyzed substances.

Among the most widely used physical methods of analysis are the following.

Spectral qualitative analysis. Spectral analysis is based on the observation of emission spectra (emission spectra, or radiation) of the elements that make up the analyte.

Luminescent (fluorescent) qualitative analysis. Luminescent analysis is based on the observation of luminescence (light emission) of analytes caused by the action of ultraviolet rays. The method is used to analyze natural organic compounds, minerals, medicines, a number of elements, etc.

To excite the luminescence, the test substance or its solution is irradiated with ultraviolet rays. In this case, the atoms of matter, having absorbed a certain amount of energy, pass into an excited state. This state is characterized by a larger supply of energy than the normal state of matter. During the transition of a substance from an excited to a normal state, luminescence occurs due to excess energy.

Luminescence that decays very quickly after cessation of irradiation is called fluorescence.

Observing the nature of the luminescent glow and measuring the intensity or brightness of the luminescence of a compound or its solutions, one can judge the composition of the substance under study.

In some cases, determinations are based on the study of fluorescence resulting from the interaction of the analyte with certain reagents. Fluorescent indicators are also known, which are used to determine the reaction of the medium by changing the fluorescence of the solution. Luminescent indicators are used in the study of colored media.

X-ray diffraction analysis. With the help of X-rays, it is possible to determine the size of atoms (or ions) and their mutual arrangement in the molecules of the sample under study, i.e., it is possible to determine the structure of the crystal lattice, the composition of the substance, and sometimes the presence of impurities in it. The method does not require chemical treatment of the substance and its large quantities.

Mass spectrometric analysis. The method is based on the determination of individual ionized particles deflected by an electromagnetic field to a greater or lesser extent depending on the ratio of their mass to charge (for more details, see Book 2).

Physical methods of analysis, having a number of advantages over chemical ones, in some cases make it possible to solve problems that cannot be resolved by methods of chemical analysis; using physical methods, it is possible to separate elements that are difficult to separate by chemical methods, as well as to conduct continuous and automatic recording of readings. Very often, physical methods of analysis are used along with chemical ones, which makes it possible to use the advantages of both methods. The combination of methods is of particular importance when determining negligible amounts (traces) of impurities in the analyzed objects.

Macro, semi-micro and micro methods

Analysis of large and small quantities of the test substance. In the old days, chemists used large quantities of the substance to be analyzed. In order to determine the composition of a substance, samples of several tens of grams were taken and dissolved in a large volume of liquid. This also required chemical glassware of the appropriate capacity.

At present, chemists manage in analytical practice with small amounts of substances. Depending on the amount of the analyte, the volume of solutions used for analysis, and mainly on the technique used to perform the experiment, analysis methods are divided into macro-, semi-micro- and micro-methods.

When performing a macro analysis, a few milliliters of a solution containing at least 0.1 g of the substance is taken to carry out the reaction, and at least 1 ml of the reagent solution is added to the test solution. The reactions are carried out in test tubes. During precipitation, voluminous precipitates are obtained, which are separated by filtration through funnels with paper filters.

Drop analysis

Technique for carrying out reactions in drop analysis. The so-called drop analysis, introduced into analytical practice by N. A. Tananaev, has acquired great importance in analytical chemistry.

When working with this method, the phenomena of capillarity and adsorption are of great importance, with the help of which it is possible to open and separate various ions in their joint presence. In drop analysis, individual reactions are carried out on porcelain or glass plates or on filter paper. In this case, a drop of the test solution and a drop of a reagent that causes a characteristic coloration or the formation of crystals are applied to the plate or paper.

When performing the reaction on filter paper, the capillary-adsorption properties of the paper are used. The liquid is absorbed by the paper, and the resulting colored compound is adsorbed on a small area of ​​the paper, thereby increasing the sensitivity of the reaction.

Microcrystalloscopic analysis

The microcrystalloscopic method of analysis is based on the detection of cations and anions by means of a reaction, as a result of which a compound is formed that has a characteristic crystal shape.

Previously, this method was used in qualitative microchemical analysis. Currently, it is also used in drip analysis.

To examine the resulting crystals in microcrystalloscopic analysis, a microscope is used.

Crystals of a characteristic shape are used when working with pure substances by introducing a drop of a solution or a crystal of a reagent into a drop of the test substance placed on a glass slide. After a while, clearly distinguishable crystals of a certain shape and color appear.

Powder grinding method

To detect some elements, the method of grinding a powdered analyte with a solid reagent in a porcelain plate is sometimes used. The element to be discovered is detected by the formation of characteristic compounds that differ in color or odor.

Methods of analysis based on heating and fusion of a substance

pyrochemical analysis. For the analysis of substances, methods based on heating the test solid or its fusion with appropriate reagents are also used. Some substances, when heated, melt at a certain temperature, others sublime, and precipitation characteristic of each substance appears on the cold walls of the device; some compounds, when heated, decompose with the release of gaseous products, etc.

When the analyte is heated in a mixture with the appropriate reagents, reactions occur, accompanied by a change in color, the release of gaseous products, and the formation of metals.

Spectral qualitative analysis

In addition to the method described above for observing the coloring of a colorless flame with the naked eye when a platinum wire with the analyzed substance is introduced into it, other methods for studying light emitted by incandescent vapors or gases are currently widely used. These methods are based on the use of special optical devices, the description of which is given in the physics course. In such spectral devices, the decomposition into a spectrum of light with different wavelengths occurs, emitted by a sample of a substance heated in a flame.

Depending on the method of observing the spectrum, spectral instruments are called spectroscopes, which are used to visually observe the spectrum, or spectrographs, in which spectra are photographed.

Chromatographic analysis method

The method is based on the selective absorption (adsorption) of individual components of the analyzed mixture by various adsorbents. Adsorbents are called solids, on the surface of which the adsorbed substance is absorbed.

The essence of the chromatographic method of analysis is briefly as follows. A solution of a mixture of substances to be separated is passed through a glass tube (adsorption column) filled with an adsorbent.

Kinetic methods of analysis

Methods of analysis based on measuring the reaction rate and using its magnitude to determine the concentration are combined under the general name of kinetic methods of analysis (K. B. Yatsimirsky).

Qualitative detection of cations and anions by kinetic methods is carried out quite quickly and relatively simply, without the use of complex instruments.

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, various methods are used, including physicochemical methods of 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 the physical properties of a substance on 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. Physicochemical methods of analysis have a high degree of sensitivity, as well as 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 in order to qualitatively and quantitatively determine 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 branches of 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 device. That is, the influence of the human factor is reduced 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 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 a color of a certain color, so their identification is possible in this way. Basically, these are substances such as: alkali 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, chromatographic methods become very convenient and effective. 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. It is used in mass production of 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.

Analytical chemistry and chemical analysis

Chemical analysis

chemical analysis called obtaining information about the composition and structure of substances, no matter how the information is obtained .

Some methods (methods) of analysis are based on carrying out chemical reactions with specially added reagents, in others, chemical reactions play an auxiliary role, and still others are not at all associated with the reactions. But the result of the analysis in any case is information about chemical the composition of matter, i.e., the nature and quantitative content of its constituent atoms and molecules. This circumstance is emphasized by using the adjective "chemical" in the phrase "chemical analysis".

The meaning of analysis. With the help of chemical-analytical methods, chemical elements were discovered, the properties of elements and their compounds were studied in detail, and the composition of many natural substances was determined. Numerous analyzes made it possible to establish the basic laws of chemistry (the law of composition constancy, the law of conservation of the mass of substances, the law of equivalents, etc.), and confirmed the atomic and molecular theory. Analysis has become a means of scientific research not only in chemistry, but also in geology, biology, medicine and other sciences. A significant part of the knowledge about nature that mankind has accumulated since the time of Boyle, it has received precisely through chemical analysis.

The capabilities of analysts increased dramatically in the second half of the 19th and especially in the 20th century, when many physical analysis methods. They made it possible to solve problems that could not be solved by classical methods. A striking example is the knowledge about the composition of the Sun and stars obtained at the end of the 19th century by the method of spectral analysis. An equally striking example at the turn of the 20th and 21st centuries was the decoding of the structure of one of the human genes. In this case, the initial information was obtained by mass spectrometry.

Analytical chemistry as a science

The science of "analytical chemistry" was formed in XVIII - XIX centuries. There are many definitions ("definitions") of this science. . The most concise and obvious is the following: Analytical chemistry - the science of determining the chemical composition of substances ” .

You can give a more precise and detailed definition:

Analytical chemistry is a science that develops a general methodology, methods and tools for studying the chemical composition (as well as structure) of substances and develops methods for analyzing various objects.

Object and directions of research. Analyst-Practitioners Research Specific Chemicals

Research in the field of analytical chemistry in Russia is mainly carried out in research institutes and universities. The objectives of these studies:

• development of the theoretical foundations of various methods of analysis;
• creation of new methods and techniques, development of analytical instruments and reagents;
• solving specific analytical problems of great economic or social importance. Examples of such problems are: the creation of methods for analytical control for nuclear power engineering and for the production of semiconductor devices (these tasks were successfully solved in the 50-70s of the twentieth century); the development of reliable methods for assessing man-made environmental pollution (this task is currently being solved).

1.2. Types of analysis

The types of analysis are very diverse. They can be classified in different ways: according to the nature of the information received, according to the objects of analysis and objects of determination, according to the required accuracy and duration of a single analysis, and also according to other criteria.

Classification according to the nature of the information received. Distinguish qualitative And quantitative analysis. In the first case find out what the given substance consists of, what are the constituent parts ( Components) are included in it. In the second case, the quantitative content of the components is determined, expressing it as a mass fraction, concentration, molar ratio of components, etc.

Classification by objects of analysis. Each area of ​​human activity has traditional objects of analysis. So, in industry, raw materials, finished products, semi-finished products, production wastes are studied. Objects agrochemical analysis are soil, fertilizer, feed, grain and other agricultural products. In medicine, they clinical analysis, its objects - blood, urine, gastric juice, various tissues, exhaled air and much more. Law enforcement officials are forensic analysis ( analysis of printing ink in detecting forgeries of documents; drug analysis; analysis of fragments found at the scene of a traffic accident, etc.). Taking into account the nature of the objects under study, other types of analysis are distinguished, for example, the analysis of drugs ( pharmaceutical analysis), natural and waste water ( hydrochemical analysis), analysis of petroleum products, building materials, etc.

Classification by objects of definition. Do not confuse similar terms - analyze And determine. These are not synonyms! So, if we are interested in whether there is iron in a person’s blood and what is its percentage, then blood is object of analysis, and iron definition object. Of course, iron can also become an object of analysis - if impurities of other elements are determined in a piece of iron. Definition objects name those components of the material under study, the quantitative content of which is required to be established. The objects of definition are no less diverse than the objects of analysis. Taking into account the nature of the component being determined, different types of analysis are distinguished (Table 1.). As can be seen from this table, the detection objects or definitions themselves (they are also called analytes) belong to different levels of matter structuring (isotopes, atoms, ions, molecules, groups of molecules of related structure, phases).

Table 1.

Classification of types of analysis by objects of definition or detection

Type of analysis	Object of determination or detection (analyte)	Example	Application area
Isotopic	Atoms with given values ​​of nuclear charge and mass number (isotopes)	137Cs, 90 Sr, 235U	Nuclear energy, environmental pollution control, medicine, archeology, etc.
elemental	Atoms with given values ​​of chargenucleus(elements)	cs, Sr,U, Cr, Fe, Hg	Everywhere
Real	Atoms (ions) of an element in a given oxidation state or in compounds of a given composition (element shape)	C r (III ), Fe 2+ , Hg as part of complex compounds	Chemical technology, environmental pollution control, geology, metallurgy, etc.
Molecular	Molecules with a given composition and structure	Benzene, glucose, ethanol	Medicine, environmental control, agrochemistry, chem. technology, criminalistics.
Structural group or functional	The sum of molecules with given structural characteristics and similar properties	Limit hydrocarbons, monosaccharides, alcohols	Chemical technology, food industry, medicine.
phase	A single phase or an element within a given phase	Graphite in steel, quartz in granite	Metallurgy, geology, construction materials technology.

During elemental analysis identify or quantify this or that element, regardless of its degree of oxidation or on the inclusion in the composition of certain molecules. The full elemental composition of the test material is determined in rare cases. It is usually sufficient to determine some elements that significantly affect the properties of the object under study.

Real analysis began to be singled out as an independent form recently, earlier it was considered as part of the elemental. The purpose of material analysis is to separately determine the content of different form-modes of the same element. For example, the content of chromium (III) and chromium (VI) in wastewater. In petroleum products, “sulphate sulfur”, “free sulfur” and “sulfide sulfur” are separately determined. Investigating the composition of natural waters, they find out what part of mercury exists in the form of strong complex and organoelement compounds, and what part - in the form of free ions. These tasks are much more difficult than those of elemental analysis.

Molecular analysis is especially important in the study of organic substances and materials of biogenic origin. An example would be the determination of benzene in gasoline or acetone in exhaled air. In such cases, it is necessary to take into account not only the composition, but also the structure of the molecules. Indeed, in the material under study there may be isomers and homologues of the determined component. Thus, the glucose content usually has to be determined in the presence of its isomers and other related compounds, such as sucrose.

Classification according to the accuracy, duration and cost of analyzes. A simplified, fast and cheap version of the analysis is called express analysis. It is often used here test methods . For example, any person (not an analyst) can evaluate the content of nitrates in vegetables (sugar in urine, heavy metals in drinking water, etc.) using a special test tool - indicator paper. The content of the desired component is determined using the color scale attached to the paper. The result will be visible to the “naked eye” and understandable to a non-specialist. Test methods do not require the delivery of a sample to the laboratory, any processing of the test material; these methods do not use expensive equipment and do not perform calculations. It is only important that the result of the test method does not depend on the presence of other components in the material under study, and for this it is necessary that the reagents with which the paper is impregnated during its manufacture would be specific. It is very difficult to ensure the specificity of test methods, and this type of analysis became widespread only in the last years of the 20th century. Of course, test methods cannot provide high accuracy of analysis, but it is not always required.

The direct opposite of express analysis - arbitration analy h. The main requirement for it is to ensure the greatest possible accuracy of the results. Arbitration analyzes are rarely carried out (for example, to resolve a conflict between the manufacturer and the consumer of some product). To perform such analyzes, the most qualified performers are involved, the most reliable and repeatedly proven methods are used. The execution time and cost of such an analysis are not of fundamental importance.

An intermediate place between express and arbitrage analysis in terms of accuracy, duration, cost and other indicators is occupied by routine tests. The main part of the analyzes performed in the factory and other control and analytical laboratories is of this type.

1.3 Methods of analysis

Classification of methods. The concept of "method of analysis" is used when they want to reveal the essence of this or that analysis, its basic principle. The method of analysis is a fairly universal and theoretically justified method of conducting analysis, which is fundamentally different from other methods in its purpose and basic principle, regardless of which component is determined and what is analyzed. The same method can be used to analyze different objects and to determine different analytes .

There are three main groups of methods (Fig. 1). Some of them are aimed primarily at separating the components of the mixture under study (subsequent analysis without this operation turns out to be inaccurate or even impossible). In the course of separation, the concentration of the components to be determined usually also occurs (see Chapter 8). An example would be extraction methods or ion exchange methods. Other methods are used in the course of qualitative analysis; they serve for reliable identification (identification) of the components of interest to us. The third, most numerous, are intended for the quantitative determination of components. The respective groups are called methods of separation and concentration, methods of identification and methods of determination. The methods of the first two groups, as a rule , play a supporting role. Most important for practice are determination methods.

Physico-chemical

Fig.1. Classification of analysis methods

In addition to the three main groups, there are hybrid methods. In Fig.1. they are not shown. In hybrid methods, separation, identification and determination of components are organically combined in one device (or in a single instrument complex). The most important of these methods is chromatographic analysis. In a special device (chromatograph), the components of the test sample (mixture) are separated, since they move at different speeds through a column filled with solid powder (sorbent). By the time of release of the component from the column, its nature is judged and thus all components of the sample are identified. The components leaving the column in turn fall into another part of the device, where a special device - a detector - measures and records the signals of all components. Often, signals are automatically assigned to one or another substance, as well as the content of each component of the sample is calculated. It is clear that chromatographic analysis cannot be considered only a method of separation of components, or only a method of quantitative determination, it is a hybrid method.

1.4. Analysis methods and requirements for them

Concepts should not be confused method And methods.

A methodology is a clear and detailed description of how an analysis should be performed by applying some method to a specific analytical problem.

Usually, a technique is developed by specialists, undergoes preliminary verification and metrological certification, is officially registered and approved. The name of the technique indicates the method used, the object of determination and the object of analysis

To pick up optimal(best) method, a number of practical requirements must be taken into account in each case.

1. T accuracy. This is the main requirement. It means that the relative or absolute error of the analysis should not exceed a certain limit value

2. Sensitivity. This word in colloquial speech is replaced by more stringent terms “limit of detection” and “lower limit of detectable concentrations". Highly sensitive techniques are those by which we can detect and determine the component even at its low content in the material under study. The lower the expected content, the more sensitive the technique required. .

3. Selectivity (selectivity). It is important that the result of the analysis is not affected by foreign substances that make up the sample.

4. expressiveness . We are talking about the duration of the analysis of one sample - from sampling to the issuance of a conclusion. The sooner the results are obtained, the better.

5.C cost. This characteristic of the technique does not require comments. Only relatively inexpensive assays can be used on a mass scale. The cost of analytical control in industry usually does not exceed 1% of the cost of production. Analyzes that are unique in their complexity and rarely performed are very expensive.

There are other requirements for the method - the safety of the analysis, the ability to carry out the analysis without the direct participation of a person, the stability of the results to random fluctuations in conditions, etc.

1.5. Main stages (stages) of quantitative analysis

The method of quantitative analysis can be mentally divided into several successive stages (stages), and almost any technique has the same stages. The corresponding analysis logic is shown in Figure 1.2. The main steps in performing a quantitative analysis are: statement of the analytical problem and choice of method, sampling, sample preparation, signal measurement, calculation and presentation of results.

Statement of the analytical problem and choice of methodology. The work of a specialist analyst usually begins with obtaining order for analysis. The appearance of such an order is usually caused by the professional activities of other specialists, the emergence of some Problems. Such a problem can be, for example, making a diagnosis, finding out the cause of a defect during the production of some product, determining the authenticity of a museum exhibit, the possibility of the presence of some toxic substance in tap water, etc. Based on the information received from a specialist (organic chemist, process engineer, geologist, dentist, prosecutor's office investigator, agronomist, archaeologist, etc.), the analyst must formulate analytical task. Naturally, it is necessary to take into account the possibilities and wishes of the "customer". In addition, it is necessary to collect additional information (primarily on the qualitative composition of the material that will have to be analyzed).

The formulation of the analytical problem requires a very high qualification of the analyst and is the most difficult part of the forthcoming research. It is not enough to determine what material will have to be analyzed and what exactly needs to be determined in it. It is necessary to understand at what concentration level the analysis will have to be carried out, what foreign components will be present in the samples, how often it will be necessary to analyze, how much time and money can be spent on one analysis, whether it will be possible to deliver the samples to the laboratory or will it be necessary to perform the analysis directly " on the object”, whether there will be restrictions on the mass and reproducibility properties of the studied material, etc. And most importantly, you need to understand: what accuracy of the analysis results will need to be ensured and how it will be possible to achieve such accuracy!

A clearly formulated analytical problem is the basis for choosing the optimal technique. The search is carried out using collections of normative documents (including standard methods), reference books, reviews on individual objects or methods. For example, if they are going to determine the content of oil products in wastewater by the photometric method, then they look at monographs devoted, firstly, to photometric analysis, secondly, to methods for analyzing wastewater, and thirdly, to different methods for determining oil products. There are series of books, each of which is devoted to the analytical chemistry of some element. Manuals on individual methods and on individual objects of analysis have been issued. If it was not possible to find suitable methods in reference books and monographs, the search is continued using abstract and scientific journals, Internet search engines, expert advice, etc. After selecting suitable methods, they choose the one that best meets the analytical task.

Often there are not only no standard methods for solving a specific problem, but there are no previously described technical solutions at all (particularly complex analytical problems, unique objects). Such a situation is often encountered when conducting scientific research. In these cases, one has to develop an analysis technique on one's own. But, when performing analyzes according to your own methodology, you should especially carefully check the correctness of the results obtained.

Sampling. Develop an analysis method that would allow measure the concentration of the component of interest to us directly in the object under study, it is quite rare. An example would be a carbon dioxide sensor in the air, which is installed in submarines and other enclosed spaces. sample- and deliver it to the analytical laboratory for further research. The sample must be representative(representative), that is, its properties and composition should approximately coincide with the properties and composition of the material under study as a whole. For gaseous and liquid objects of analysis, it is quite easy to take a representative sample, since they are homogeneous. You just need to choose the right time and place for selection. For example, when sampling water from reservoirs, it is taken into account that the water of the surface layer differs in its composition from the water from the bottom layer, the water near the coast is more polluted, the composition of the river water is not the same at different times of the year, etc. In large cities, atmospheric air samples are taken taking into account the direction of the wind and the location of sources of emissions of impurities. Sampling is also not a problem when pure chemicals, even solids or homogeneous fine powders, are being examined.

It is much more difficult to correctly select a representative sample of a heterogeneous solid (soil, ores, coal, grains, etc.). If you take soil samples in different places of the same field, or from different depths, or at different times, the results of the analysis of samples of the same type will not be the same. They can differ by several times, especially if the material itself was heterogeneous, consisted of particles of different composition and size.

The matter is complicated by the fact that sampling is often carried out not by the analyst himself, but by insufficiently qualified workers or, much worse, by persons interested in obtaining a certain result of the analysis. So, in the stories of M. Twain and Bret Garth, it is colorfully described how, before the sale of a gold-bearing area, the seller tried to select pieces of rock with obvious inclusions of gold for analysis, and the buyer - waste rock. It is not surprising that the results of the corresponding analyzes gave the opposite, but in both cases an incorrect characterization of the area under study.

To ensure the correctness of the analysis results for each group of objects, special rules and sampling schemes have been developed and adopted. An example would be soil analysis. In this case, one should select some large portions of the test material in different places of the test area and then combine them. It is calculated in advance how many sampling points should be, at what distance from each other these points should be located. It is indicated from what depth each portion of the soil should be taken, what mass it should be, etc. There is even a special mathematical theory that allows you to calculate the minimum mass of the combined sample, taking into account the particle size, heterogeneity of their composition, etc. The larger the mass of the sample, the more representative it is; therefore, for an inhomogeneous material, the total mass of the combined sample can reach tens or even hundreds of kilograms. The combined sample is dried, crushed, thoroughly mixed, and the amount of the test material is gradually reduced (there are special methods and devices for this purpose). But even after a multiple reduction, the mass of the sample can reach several hundred grams. The reduced sample in a hermetically sealed container is delivered to the laboratory. There, they continue grinding and mixing the material under study (in order to average the composition), and only then take a weighed portion of the average sample on an analytical balance for further analysis. sample preparation and subsequent signal measurement.

Sampling is the most important stage of the analysis, since the errors that occur at this stage are very difficult to correct or account for. Sampling errors are often the main contributor to the overall analysis error. If sampling is incorrect, even the perfect execution of subsequent operations will not be able to help - it will no longer be possible to obtain the correct result.

Sample preparation . This is the collective name for all the operations to which the sample delivered there is subjected in the laboratory before the measurement of the analytical signal. During sample preparation perform a variety of operations: evaporation, drying, calcination or combustion of the sample, its dissolution in water, acids or organic solvents, preliminary oxidation or reduction of the component to be determined with specially added reagents, removal or masking of interfering impurities. Often it is necessary to carry out the concentration of the determined component - from a sample of a large volume, the component is quantitatively transferred to a small volume of solution (concentrate), where the analytical signal is then measured. Sample components with similar properties during sample preparation try to separate from each other to make it easier to determine the concentration of each separately. Sample preparation requires more time and labor than other analysis operations; it is quite difficult to automate. It should be remembered that each operation sample preparation is an additional source of analysis errors. The fewer such operations, the better. Methods that do not include the stage at all are ideal. sample preparation(“I came, measured, calculated”), but there are relatively few such methods.

Analytical signal measurement requires the use of appropriate measuring instruments, primarily precision instruments (balances, potentiometers, spectrometers, chromatographs, etc.), as well as pre-calibrated measuring utensils. Measuring instruments must be certified (“verified”), that is, it must be known in advance what maximum error the signal measurement using this device can give. In addition to instruments, for signal measurement, in many cases, standards of known chemical composition are required (comparison samples, for example, state standard samples). They are used to calibrate the methodology (see Chapter 5), verify and adjust the instruments. The result of the analysis is also calculated using standards.

Calculation and presentation of results - the fastest and easiest stage of analysis. You just need to choose the appropriate method of calculation (according to one or another formula, according to the schedule, etc.). So, to determine uranium in uranium ore, the radioactivity of the sample is compared with the radioactivity of a standard sample (ore with a known uranium content), and then the uranium content in the sample is found by solving the usual proportion. However, this simple method is not always suitable, and the use of an inappropriate calculation algorithm can lead to serious errors. Some calculation methods are very complex and require the use of a computer. In the following chapters, the methods of calculation used in different methods of analysis, their advantages, and the conditions for the applicability of each method will be described in detail. The results of the analysis should be statistically processed. All data related to the analysis of this sample is reflected in the laboratory journal, and the result of the analysis is entered into a special protocol. Sometimes the analyst himself compares the results of the analysis of several substances with each other or with some standards and draws meaningful conclusions. For example, about the compliance or non-compliance of the quality of the material under study with the established requirements ( analytical control).

The vast majority of information about substances, their properties and chemical transformations was obtained using chemical or physicochemical experiments. Therefore, the main method used by chemists should be considered a chemical experiment.

The traditions of experimental chemistry have evolved over the centuries. Even when chemistry was not an exact science, in ancient times and in the Middle Ages, scientists and artisans sometimes accidentally, and sometimes purposefully, discovered ways to obtain and purify many substances that were used in economic activity: metals, acids, alkalis, dyes and etc. Alchemists contributed a lot to the accumulation of such information (see Alchemy).

Thanks to this, by the beginning of the 19th century. chemists were well versed in the basics of experimental art, in particular the methods of purification of various liquids and solids, which allowed them to make many important discoveries. Nevertheless, chemistry began to become a science in the modern sense of the word, an exact science, only in the 19th century, when the law of multiple ratios was discovered and the atomic-molecular theory was developed. Since that time, the chemical experiment began to include not only the study of the transformations of substances and methods of their isolation, but also the measurement of various quantitative characteristics.

A modern chemical experiment includes many different measurements. The equipment for setting up experiments and chemical glassware have also changed. In a modern laboratory, you will not find homemade retorts - they have been replaced by standard glass equipment produced by industry and adapted specifically for performing a particular chemical procedure. Work methods have also become standard, which in our time no longer have to be reinvented by every chemist. Description of the best of them, proven by many years of experience, can be found in textbooks and manuals.

Methods for studying matter have become not only more universal, but also much more diverse. An increasing role in the work of a chemist is played by physical and physicochemical research methods designed to isolate and purify compounds, as well as to establish their composition and structure.

The classical technique for purifying substances was extremely labor intensive. There are cases when chemists spent years of work on the isolation of an individual compound from a mixture. Thus, salts of rare earth elements could be isolated in pure form only after thousands of fractional crystallizations. But even after that, the purity of the substance could not always be guaranteed.

Modern chromatography methods allow you to quickly separate a substance from impurities (preparative chromatography) and check its chemical identity (analytical chromatography). In addition, classical but highly improved methods of distillation, extraction and crystallization, as well as such effective modern methods as electrophoresis, zone melting, etc., are widely used to purify substances.

The task facing the synthetic chemist after the isolation of a pure substance - to establish the composition and structure of its molecules - relates to a large extent to analytical chemistry. With the traditional technique of work, it was also very laborious. In practice, as the only method of measurement, elemental analysis was used before, which allows you to establish the simplest formula of the compound.

To determine the true molecular as well as the structural formula, it was often necessary to study the reactions of a substance with various reagents; isolate the products of these reactions individually, in turn establishing their structure. And so on - until, on the basis of these transformations, the structure of the unknown substance did not become obvious. Therefore, the establishment of the structural formula of a complex organic compound often took a very long time, and such work was considered full-fledged, which ended with a counter synthesis - the receipt of a new substance in accordance with the formula established for it.

This classical method was extremely useful for the development of chemistry in general. Nowadays, it is rarely used. As a rule, an isolated unknown substance after elemental analysis is subjected to a study using mass spectrometry, spectral analysis in the visible, ultraviolet and infrared ranges, as well as nuclear magnetic resonance. A substantiated derivation of a structural formula requires the use of a whole range of methods, and their data usually complement each other. But in a number of cases, conventional methods do not give an unambiguous result, and one has to resort to direct methods of establishing the structure, for example, to X-ray diffraction analysis.

Physicochemical methods are used not only in synthetic chemistry. They are of no less importance in the study of the kinetics of chemical reactions, as well as their mechanisms. The main task of any experiment on the study of the reaction rate is the accurate measurement of the time-varying, and, moreover, usually very small, concentration of the reactant. To solve this problem, depending on the nature of the substance, one can use chromatographic methods, various types of spectral analysis, and electrochemical methods (see Analytical chemistry).

The sophistication of technology has reached such a high level that it has become possible to accurately determine the rate of even “instantaneous”, as previously believed, reactions, for example, the formation of water molecules from hydrogen cations and anions. With an initial concentration of both ions equal to 1 mol/l, the time of this reaction is several hundred-billionths of a second.

Physicochemical research methods are also specially adapted for the detection of short-lived intermediate particles formed during chemical reactions. To do this, the devices are equipped with either high-speed recording devices or attachments that ensure operation at very low temperatures. Such methods successfully capture the spectra of particles whose lifetime under normal conditions is measured in thousandths of a second, such as free radicals.

In addition to experimental methods, calculations are widely used in modern chemistry. Thus, the thermodynamic calculation of a reacting mixture of substances makes it possible to accurately predict its equilibrium composition (see Chemical equilibrium).

Calculations of molecules based on quantum mechanics and quantum chemistry have become universally recognized and in many cases irreplaceable. These methods are based on a very complex mathematical apparatus and require the use of the most advanced electronic computers - computers. They allow you to create models of the electronic structure of molecules that explain the observable, measurable properties of low-stability molecules or intermediate particles formed during reactions.

Methods for studying substances developed by chemists and physical chemists are useful not only in chemistry, but also in related sciences: physics, biology, geology. Neither industry, nor agriculture, nor medicine, nor criminology can do without them. Physical and chemical instruments occupy a place of honor on spacecraft, which are used to study near-Earth space and neighboring planets.

Therefore, knowledge of the basics of chemistry is necessary for every person, regardless of his profession, and the further development of its methods is one of the most important directions of the scientific and technological revolution.