The main characteristics of any method of analytical chemistry are. Introduction

ANALYTICAL CHEMISTRY, the science of determining the chemical composition of substances and materials and, to some extent, the chemical structure of compounds. Analytical chemistry develops the general theoretical foundations of chemical analysis, develops methods for determining the components of a sample under study, and solves the problems of analyzing specific objects. The main goal of analytical chemistry is the creation of methods and tools that provide, depending on the task, accuracy, high sensitivity, rapidity and selectivity of analysis. Methods are also being developed to analyze micro-objects, to conduct local analysis (at a point, on the surface, and so on), analysis without destroying the sample, at a distance from it (remote analysis), continuous analysis (for example, in a stream), and also to establish, in the form of what chemical compound and in what physical form the determined component exists in the sample (material chemical analysis) and what phase it is included in (phase analysis). Important trends in the development of analytical chemistry are the automation of analyses, especially in the control of technological processes, and mathematization, in particular the widespread use of computers.

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

The so-called chemometrics, including the metrology of chemical analysis, occupies a significant place in the theoretical foundations of analytical chemistry. The theory of analytical chemistry also includes teachings on the selection and preparation of analytical samples, on drawing up an analysis scheme and the choice of methods, on the principles and ways of automating analysis, using computers, as well as the principles of rational use of the results of chemical analysis. A feature of analytical chemistry is the study of not general, but individual, specific properties and characteristics of objects, which ensures the selectivity of many analytical methods. Thanks to close links with the achievements of physics, mathematics, biology and various fields of technology (this is especially true of methods of analysis), analytical chemistry is turning into a discipline at the intersection of sciences. Other names of this discipline are often used - analytics, analytical science, etc.

In analytical chemistry, methods of separation, determination (detection) and hybrid methods of analysis are distinguished, usually combining the methods of the first two groups. Methods of determination are conveniently subdivided into chemical methods of analysis (gravimetric analysis, titrimetric analysis, electrochemical methods of analysis, kinetic methods of analysis), physical methods of analysis (spectroscopic, nuclear physics, etc.), biochemical methods of analysis, and biological method of analysis. Chemical methods are based on chemical reactions (the interaction of matter with matter), physical methods are based on physical phenomena (the interaction of matter with radiation, energy flows), biological methods use the response of organisms or their fragments to changes in the environment.

Almost all determination methods are based on the dependence of any measurable properties of substances on their composition. Therefore, an important area of ​​analytical chemistry is the search for and study of such dependencies in order to use them to solve analytical problems. In this case, it is almost always necessary to find an equation for the relationship between a property and composition, develop methods for registering a property (analytical signal), eliminate interference from other components, and eliminate the interfering influence of various factors (for example, temperature fluctuations). The value of the analytical signal is converted into units characterizing the amount or concentration of the components. Measured properties can be, for example, mass, volume, light absorption, current strength.

Much attention is paid to the theory of methods of analysis. The theory of chemical methods is based on ideas about several basic types of chemical reactions widely used in analysis (acid-base, redox, complex formation), and several important processes (precipitation, dissolution, extraction). Attention to these issues is due to the history of the development of analytical chemistry and the practical significance of the corresponding methods. Since, however, the share of chemical methods is decreasing, while the share of physical, biochemical, and biological methods is growing, it is of great importance to improve the theory of the methods of the latter groups and to integrate the theoretical aspects of individual methods into the general theory of analytical chemistry.

History of development. Tests of materials were carried out in ancient times; for example, ores were examined to determine their suitability for smelting, various products - to determine the content of gold and silver in them. Alchemists of the 14th-16th centuries performed a huge amount of experimental work on the study of the properties of substances, laying the foundation for chemical methods of analysis. In the 16-17 centuries (the period of iatrochemistry), new chemical methods for detecting substances appeared, based on reactions in solution (for example, the discovery of silver ions by the formation of a precipitate with chloride ions). R. Boyle, who introduced the concept of "chemical analysis", is considered the founder of scientific analytical chemistry.

Until the middle of the 19th century, analytical chemistry was the main branch of chemistry. During this period, many chemical elements were discovered, the constituent parts of some natural substances were isolated, the laws of constancy of composition and multiple ratios, and the law of conservation of mass were established. The Swedish chemist and mineralogist T. Bergman developed a scheme for systematic qualitative analysis, actively used hydrogen sulfide as an analytical reagent, and proposed flame analysis methods to obtain pearls. In the 19th century, systematic qualitative analysis was improved by the German chemists G. Rose and K. Fresenius. The same century was marked by huge successes in the development of quantitative analysis. A titrimetric method was created (French chemist F. Decroisille, J. Gay-Lussac), gravimetric analysis was significantly improved, and methods for analyzing gases were developed. The development of methods for the elemental analysis of organic compounds (Yu. Liebig) was of great importance. At the end of the 19th century, a theory of analytical chemistry took shape, which was based on the theory of chemical equilibrium in solutions with the participation of ions (mainly W. Ostwald). By that time, methods for analyzing ions in aqueous solutions had taken the predominant place in analytical chemistry.

In the 20th century, methods for microanalysis of organic compounds were developed (F. Pregl). A polarographic method was proposed (J. Geyrovsky, 1922). Many physical methods have appeared, for example, mass spectrometric, X-ray, nuclear physics. Of great importance was the discovery of chromatography (M. S. Tsvet, 1903) and the creation of various variants of this method, in particular partition chromatography (A. Martin and R. Sing, 1941).

In Russia and the USSR, the textbook Analytical Chemistry by I. A. Menshutkin was of great importance for analytical chemistry (it went through 16 editions). M.A. Ilyinsky and L.A. Chugaev introduced organic analytical reagents into practice (late 19th - early 20th century), N.A. Tananaev developed the drop method of qualitative analysis (simultaneously with the Austrian chemist F. Feigl, 1920s). In 1938 N.A. Izmailov and M. S. Schreiber were the first to describe thin layer chromatography. Russian scientists made a great contribution to the study of complex formation and its analytical use (I.P. Alimarin, A.K. Babko), to the theory of the action of organic analytical reagents, to the development of mass spectrometry, methods of photometry, atomic absorption spectrometry (B.V. . Lvov), in the analytical chemistry of individual elements, especially rare and platinum, and a number of objects - substances of high purity, minerals, metals and alloys.

The demands of practice have always stimulated the development of analytical chemistry. Thus, in the 1940s-1970s, in connection with the need to analyze high-purity nuclear, semiconductor and other materials, such sensitive methods as radioactive analysis, spark mass spectrometry, chemical spectral analysis, and stripping voltammetry were created, providing the determination of up to 10 - 7 -10 -8% impurities in pure substances, i.e. 1 part of an impurity per 10-1000 billion parts of the main substance. For the development of ferrous metallurgy, especially in connection with the transition to high-speed BOF steel production, rapid analysis has become decisive. The use of so-called quantometers - photoelectric devices for multi-element optical spectral or X-ray analysis - allows analysis during melting.

The need to analyze complex mixtures of organic compounds has led to the intensive development of gas chromatography, which makes it possible to analyze the most complex mixtures containing several tens or even hundreds of substances. Analytical chemistry has greatly contributed to the mastery of the energy of the atomic nucleus, the study of space and the ocean, the development of electronics, and the progress of the biological sciences.

Subject of study. An important role is played by the development of the theory of sampling of analyzed materials; Typically, sampling issues are resolved jointly with specialists in the substances under study (for example, with geologists, metallurgists). Analytical chemistry is developing methods of sample decomposition - dissolution, fusion, sintering, etc., which should provide a complete "opening" of the sample and prevent loss of the determined components and contamination from the outside. The tasks of analytical chemistry include the development of techniques for such general operations of analysis as volume measurement, filtration, and calcination. One of the tasks of analytical chemistry is to determine the directions for the development of analytical instrumentation, the creation of new circuits and instrument designs (which most often serves as the final stage in the development of an analysis method), as well as the synthesis of new analytical reagents.

For quantitative analysis, the metrological characteristics of methods and instruments are very important. In this regard, analytical chemistry studies the problems of calibration, manufacture and use of reference samples (including standard samples) and other means of ensuring the correctness of the analysis. An important place is occupied by the processing of analysis results, especially computer processing. To optimize the conditions of analysis, information theory, pattern recognition theory and other branches of mathematics are used. Computers are used not only for processing results, but also for controlling instruments, accounting for interference, calibration, and planning of experiments; there are analytical tasks that can be solved only with the help of computers, for example, the identification of molecules of organic compounds using expert systems.

Analytical chemistry defines general approaches to the choice of ways and methods of analysis. Methods for comparing methods are being developed, the conditions for their interchangeability and combinations, principles and ways of automating analysis are determined. For the practical use of analysis, it is necessary to develop ideas about its result as an indicator of product quality, the doctrine of express control of technological processes, and the creation of economical methods. Of great importance for analysts working in various sectors of the economy are the unification and standardization of methods. A theory is being developed to optimize the amount of information needed to solve analytical problems.

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

Separation of mixtures is usually resorted to in cases where direct detection or detection methods do not provide the correct result due to the interfering influence of other components of the sample. Particularly important is the so-called relative concentration, the separation of small amounts of analyte components from significantly larger amounts of the main components of the sample. Separation of mixtures can be based on differences in the thermodynamic or equilibrium characteristics of the components (ion exchange constants, stability constants of complexes) or kinetic parameters. For separation, mainly chromatography, extraction, precipitation, distillation, as well as electrochemical methods, such as electrodeposition, are used. Methods of determination - the main group of methods of analytical chemistry. The methods of quantitative analysis are based on the dependence of any measurable property, most often physical, on the composition of the sample. This dependence must be described in a certain and known way. Hybrid methods of analysis are rapidly developing, combining separation and determination. For example, gas chromatography with various detectors is the most important method for analyzing complex mixtures of organic compounds. For the analysis of mixtures of non-volatile and thermally unstable compounds, high-performance liquid chromatography is more convenient.

For analysis, a variety of methods are needed, since each of them has its own advantages and limitations. Thus, extremely sensitive radioactivation and mass spectral methods require complex and expensive equipment. Simple, accessible and very sensitive kinetic methods do not always provide the desired reproducibility of results. When evaluating and comparing methods, when choosing them for solving specific problems, many factors are taken into account: metrological parameters, scope of possible use, availability of equipment, analyst qualifications, traditions, etc. The most important among these factors are metrological parameters such as detection limit or concentration range (quantities), in which the method gives reliable results, and the accuracy of the method, i.e., the correctness and reproducibility of the results. In a number of cases, "multicomponent" methods are of great importance, which make it possible to determine a large number of components at once, for example, atomic emission and X-ray spectral analysis, and chromatography. The role of such methods is growing. Ceteris paribus, methods of direct analysis are preferred, i.e., not associated with the chemical preparation of the sample; however, such preparation is often necessary. For example, preconcentration of the test component allows one to determine its lower concentrations, eliminate the difficulties associated with the inhomogeneous distribution of the component in the sample and the absence of reference samples.

A special place is occupied by methods of local analysis. An essential role among them is played by X-ray spectral microanalysis (electron probe), mass spectrometry of secondary ions, Auger spectroscopy, and other physical methods. They are of great importance, in particular, in the analysis of surface layers of solid materials or inclusions in rocks.

A specific group consists of methods of elemental analysis of organic compounds. Organic matter is decomposed in one way or another, and its components in the form of the simplest inorganic compounds (CO 2 , H 2 O, NH 3, etc.) are determined by conventional methods. The use of gas chromatography made it possible to automate elemental analysis; for this, C-, H-, N-, S-analyzers and other automatic devices are produced. Analysis of organic compounds by functional groups (functional analysis) is performed by various chemical, electrochemical, spectral (NMR or IR spectroscopy) or chromatographic methods.

In phase analysis, i.e., the determination of chemical compounds that form separate phases, the latter are first isolated, for example, using a selective solvent, and then the resulting solutions are analyzed by conventional methods; very promising physical methods of phase analysis without prior phase separation.

Practical value. Chemical analysis provides control of many technological processes and product quality in various industries, plays a huge role in the search and exploration of minerals, in the mining industry. With the help of chemical analysis, the purity of the environment (soil, water and air) is controlled. Achievements in analytical chemistry are used in various branches of science and technology: nuclear energy, electronics, oceanology, biology, medicine, forensics, archeology, and space research. The economic importance of chemical analysis is great. Thus, the exact determination of alloying additives in metallurgy allows saving valuable metals. The transition to continuous automatic analysis in medical and agrochemical laboratories makes it possible to dramatically increase the speed of analyzes (blood, urine, soil extracts, and so on) and reduce the number of laboratory employees.

Lit .: Fundamentals of analytical chemistry: In 2 books / Edited by Yu. A. Zolotov. M., 2002; Analytical chemistry: In 2 vols. M., 2003-2004.

The course of physical and colloidal chemistry, including physicochemical methods of analysis and methods of separation and purification, plays an essential role in the training of specialists in the field of environmental engineering. The main sections of physical chemistry - chemical kinetics and chemical thermodynamics - serve as the theoretical basis for other sections of chemistry, as well as chemical technology and methods for separating and purifying substances. Measurements of the physicochemical properties of substances form the basis of many modern instrumental (physicochemical) methods for analyzing and monitoring the state of the environment. Since most natural objects are colloidal systems, it is necessary to study the basics of colloidal chemistry.

The dangers of environmental contamination by products - harmful substances can be significantly reduced by careful cleaning of products. Chemical cleaning methods include treatment with reagents that neutralize harmful components. It is necessary to know the rate and completeness of reactions, their dependence on external conditions, to be able to calculate the concentration of reagents that provide the required degree of purification. Physicochemical purification methods are also widely used, including rectification, extraction, sorption, ion exchange, and chromatography.

The study of the course of physical and colloidal chemistry by students of environmental specialties (No. No.) includes the development of a theoretical (lecture) course, seminars on analytical chemistry, including physical and chemical methods of analysis, methods of separation and purification, chromatography and sections of colloidal chemistry, laboratory work and practical exercises , as well as independent work, including the completion of three homework assignments. In the course of laboratory and practical work, students acquire the skills of conducting physical and chemical experiments, plotting graphs, mathematical processing of measurement results and error analysis. When performing laboratory, practical and homework assignments, students acquire the skills of working with reference literature.

Seminars on analytical and colloidal chemistry

Seminar 1. The subject of analytical chemistry. Classification of methods of analysis. Metrology. Classical methods of quantitative analysis.

Specialists working in the field of engineering ecology need sufficiently complete information about the chemical composition of raw materials, production products, production wastes and the environment - air, water and soil; special attention should be paid to identifying harmful substances and determining their quantities. This problem is solved analytical chemistry - the science of determining the chemical composition of substances. Chemical analysis is the main and necessary means of controlling environmental pollution.

A super-brief study of this section of chemistry cannot qualify an analytical chemist, its goal is to familiarize with the minimum amount of knowledge sufficient to set specific tasks for chemists, focusing on the capabilities of certain analysis methods, and to understand the meaning of the results of analysis.

Classification of analysis methods

Distinguish between qualitative and quantitative analysis. The first determines the presence of certain components, the second - their quantitative content. When studying the composition of a substance, a qualitative analysis always precedes a quantitative analysis, since the choice of a quantitative analysis method depends on the qualitative composition of the object under study. Analysis methods are divided into chemical and physico-chemical. Chemical methods of analysis are based on the transformation of the analyte into new compounds with certain properties. By the formation of characteristic compounds of elements, the composition of the substance is established.

The qualitative analysis of inorganic compounds is based on ionic reactions and makes it possible to detect elements in the form of cations and anions. For example, Cu 2+ ions can be identified by the formation of a bright blue 2+ complex ion. When analyzing organic compounds, C, H, N, S, P, Cl and other elements are usually determined. Carbon and hydrogen are determined after the combustion of the sample, recording the released carbon dioxide and water. There are a number of techniques for detecting other elements.

Qualitative analysis is divided into fractional and systematic.

Fractional analysis is based on the use of specific and selective reactions, with the help of which it is possible to detect the desired ions in any sequence in individual portions of the test solution. Fractional analysis makes it possible to quickly determine the limited number of ions (from one to five) contained in a mixture whose composition is approximately known.

Systematic analysis is a specific sequence of detection of individual ions after all other ions that interfere with the determination have been found and removed from the solution.

Separate groups of ions are isolated using the similarities and differences in the properties of ions using the so-called group reagents - substances that react in the same way with a whole group of ions. Groups of ions are divided into subgroups, and those, in turn, are divided into individual ions, which are detected using the so-called. analytical reactions characteristic of these ions. Such reactions are necessarily accompanied by an analytical sign, that is, an external effect - precipitation, gas evolution, a change in the color of the solution.

Analytical reaction has the property of specificity, selectivity and sensitivity.

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

The selectivity (or selectivity) of the reaction is determined by the fact that a similar external effect is possible only with a limited number of ions with which the reaction gives a positive effect. The degree of selectivity (selectivity) is the greater, the smaller the number of ions with which the reaction gives a positive effect.

The sensitivity of the reaction is characterized by a number of interrelated values: the limit of detection and the limit of dilution. For example, the limit of detection in a microcrystalloscopic reaction to the Ca 2+ ion by the action of sulfuric acid is 0.04 μg of Ca 2+ in a drop of solution. The limiting dilution (V before, ml) is calculated by the formula: V before \u003d V 10 2 / C min, where V is the volume of the solution (ml). The limiting dilution shows in what volume of the solution (in ml) 1 g of the ion to be determined is contained. For example, in the reaction of the K + ion with sodium hexanitrosocobaltate - Na 3, a yellow crystalline precipitate K 2 Na is formed. The sensitivity of this reaction is characterized by a limiting dilution of 1:50,000. This means that using this reaction, you can open a potassium ion in a solution containing at least 1 g of potassium in 50,000 ml of water.

Chemical methods of qualitative analysis are of practical importance only for a small number of elements. For multi-element, molecular, as well as functional (determination of the nature of functional groups) analysis, physicochemical methods are used.

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

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

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

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

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

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

The analyzed components can be atoms and ions, isotopes of elements, molecules, functional groups and radicals, phases.

Classification according to the nature of the determined particles:

1.isotopic (physical)

2. elemental or atomic

3. molecular

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

5. phase - analysis of inclusions in heterogeneous objects, such as minerals.

Other types of analysis classification:

Gross and local.

Destructive and non-destructive.

Contact and remote.

discrete and continuous.

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

Depending on the task, there are 3 groups of methods of analytical chemistry:

• 1) detection methods allow you to determine which elements or substances (analytes) are present in the sample. They are used for qualitative analysis;
• 2) methods of determination allow to establish the quantitative content of analytes in the sample and are used for quantitative analysis;
• 3) separation methods make it possible to isolate the analyte and separate the interfering components. They are used in qualitative and quantitative analysis. There are various methods of quantitative analysis: chemical, physicochemical, physical, etc.

Chemical methods are based on the use of chemical reactions (neutralization, redox, complexation and precipitation) in which the analyte enters. In this case, a qualitative analytical signal is a visual external effect of the reaction - a change in the color of the solution, the formation or dissolution of a precipitate, the release of a gaseous product. In quantitative determinations, the volume of the evolved gaseous product, the mass of the precipitate formed, and the volume of a reagent solution with a precisely known concentration, spent on interaction with the analyte, are used as an analytical signal.

Physical methods do not use chemical reactions, but measure any physical properties (optical, electrical, magnetic, thermal, etc.) of the analyte, which are a function of its composition.

Physico-chemical methods use a change in the physical properties of the analyzed system as a result of chemical reactions. Physicochemical methods also include chromatographic methods of analysis based on the processes of sorption-desorption of a substance on a solid or liquid sorbent under dynamic conditions, and electrochemical methods (potentiometry, voltammetry, conductometry).

Physical and physico-chemical methods are often combined under the general name of instrumental methods of analysis, since analytical instruments and apparatuses are used for analysis that record physical properties or their change. When carrying out a quantitative analysis, an analytical signal is measured - a physical quantity associated with the quantitative composition of the sample. If quantitative analysis is carried out using chemical methods, then the determination is always based on a chemical reaction.

There are 3 groups of quantitative analysis methods:

• - Gas analysis
• - Titrimetric analysis
• - Gravimetric analysis

The most important among the chemical methods of quantitative analysis are gravimetric and titrimetric methods, which are called classical methods of analysis. These methods are standard for evaluating the correctness of a definition. Their main field of application is the precision determination of large and medium quantities of substances.

Classical methods of analysis are widely used in the chemical industry to control the progress of the technological process, the quality of raw materials and finished products, and industrial waste. Based on these methods, pharmaceutical analysis is also carried out - determining the quality of drugs and medicines that are produced by chemical and pharmaceutical enterprises.

Its subject as a science is the improvement of existing and the development of new methods of analysis, their practical application, the study of the theoretical foundations of analytical methods.

Depending on the task, analytical chemistry is subdivided into qualitative analysis, aimed at determining whether what or what kind substance, in what form it is in the sample, and quantitative analysisaimed at determining How many a given substance (elements, ions, molecular forms, etc.) is in the sample.

Determination of the elemental composition of material objects is called elemental analysis. The establishment of the structure of chemical compounds and their mixtures at the molecular level is called molecular analysis. One of the types of molecular analysis of chemical compounds is structural analysis aimed at studying the spatial atomic structure of substances, establishing empirical formulas, molecular weights, etc. The tasks of analytical chemistry include determining the characteristics of organic, inorganic and biochemical objects. The analysis of organic compounds by functional groups is called functional analysis.

History

Analytical chemistry has existed since there was chemistry in its modern sense, and many of the techniques used in it date back to an even earlier era, the era of alchemy, one of the main tasks of which was precisely the determination of the composition of various natural substances and the study of the processes of their mutual transformations. But, with the development of chemistry as a whole, the methods of work used in it have also been significantly improved, and, along with its purely auxiliary significance of one of the auxiliary departments of chemistry, analytical chemistry at present has the value of a completely independent department of chemical knowledge with very serious and important theoretical questions. A very important influence on the development of analytical chemistry was modern physical chemistry, which enriched it with a number of completely new methods of work and theoretical foundations, which include the doctrine of solutions (see), the theory of electrolytic dissociation, the law of mass action (see Chemical equilibrium) and the whole doctrine of chemical affinity.

Methods of analytical chemistry

Comparison of analytical chemistry methods

Aggregate traditional methods determination of the composition of a substance by its sequential chemical decomposition was called "wet chemistry" ("wet analysis"). These methods have relatively low accuracy, require relatively low qualifications of analysts, and have now been almost completely superseded by modern methods. instrumental methods(optical, mass spectrometric, electrochemical, chromatographic and other physical and chemical methods) determination of the composition of a substance. However, wet chemistry has its advantage over spectrometric methods - it allows using standardized procedures (systematic analysis) to directly determine the composition and various oxidation states of elements such as iron (Fe + 2 , Fe + 3), titanium, etc.

Analytical methods can be divided into gross and local. Gross methods of analysis usually require a separated, detailed substance (representative sample). Local Methods determine the composition of a substance in a small volume in the sample itself, which makes it possible to draw up "maps" of the distribution of the chemical properties of the sample over its surface and / or depth. It should also highlight the methods direct analysis, that is, not associated with the preliminary preparation of the sample. Sample preparation is often necessary (eg crushing, pre-concentration or separation). When preparing samples, interpreting results, estimating the number of analyzes, statistical methods are used.

Qualitative chemical analysis methods

To determine the qualitative composition of any substance, it is necessary to study its properties, which, from the point of view of analytical chemistry, can be of two kinds: the properties of the substance as such, and its properties in chemical transformations.

The former include: the physical state (solid, liquid, gas), its structure in the solid state (amorphous or crystalline substance), color, smell, taste, etc. feelings of a person, it is possible to establish the nature of this substance. In most cases, however, it is necessary to transform a given substance into some new one with clearly expressed characteristic properties, using for this purpose some specially selected compounds called reagents.

The reactions used in analytical chemistry are extremely diverse and depend on the physical properties and degree of complexity of the composition of the substance under study. In the case when a obviously pure, homogeneous chemical compound is subject to chemical analysis, the work is carried out relatively easily and quickly; when one has to deal with a mixture of several chemical compounds, the question of its analysis, therefore, becomes more complicated, and in the production of work it is necessary to adhere to a certain definite system in order not to overlook a single element entering the substance. There are two kinds of reactions in analytical chemistry: wet way reactions(in solutions) and dry reactions..

Reactions in solutions

In qualitative chemical analysis, only such reactions in solutions are used that are easily perceived by the human senses, and the moment of occurrence of the reaction is recognized by one of the following phenomena:

1. the formation of a water-insoluble precipitate,
2. changing the color of the solution
3. gas release.

Precipitation in chemical analysis reactions it depends on the formation of some water-insoluble substance; if, for example, sulfuric acid or its water-soluble salt is added to a solution of a barium salt, a white powdery precipitate of barium sulfate is formed:

BaCl 2 + H 2 SO 4 \u003d 2HCl + BaSO 4 ↓

Keeping in mind that some other metals, for example, lead, capable of forming an insoluble sulfate salt PbSO 4, can give a similar reaction of the formation of a white precipitate under the action of sulfuric acid, to be completely sure that it is this or that metal, it is necessary to produce more verification reactions, subjecting the precipitate formed in the reaction to an appropriate study.

In order to successfully carry out the reaction of precipitation formation, in addition to the selection of the appropriate reagent, it is also necessary to observe a number of very important conditions regarding the strength of the solutions of the studied salt and reagent, the proportion of both, temperature, duration of interaction, etc. When considering precipitation formed in chemical reactions analysis, it is necessary to pay attention to their appearance, that is, to the color, structure (amorphous and crystalline precipitates), etc., as well as to their properties in relation to the effect of heating, acids or alkalis, etc. on them. When weak solutions interact it is sometimes necessary to wait for the formation of a precipitate up to 24-48 hours, provided that they are kept at a certain certain temperature.

The reaction of precipitate formation, regardless of its qualitative significance in chemical analysis, is often used to separate certain elements from each other. To this end, a solution containing compounds of two or more elements is treated with an appropriate reagent capable of converting some of them into insoluble compounds, and then the precipitate formed is separated from the solution (filtrate) by filtration, further examining them separately. If we take, for example, salts of potassium chloride and barium chloride and add sulfuric acid to them, then an insoluble precipitate of barium sulfate BaSO 4 is formed, and potassium sulfate K 2 SO 4 soluble in water, which can be separated by filtration. When separating the precipitate of a water-insoluble substance from the solution, care must first be taken to ensure that it obtains an appropriate structure that allows the work of filtration to be carried out without difficulty, and then, having collected it on the filter, it is necessary to thoroughly wash it from foreign impurities. According to the studies of W. Ostwald, it must be borne in mind that when using a certain amount of water for washing, it is more expedient to wash the sediment many times with small portions of water than vice versa - several times with large portions. As for the success of the reaction of separating an element in the form of an insoluble precipitate, then, based on the theory of solutions, W. Ostwald found that for a sufficiently complete separation of an element in the form of an insoluble precipitate, it is always necessary to take an excess of the reagent used for precipitation .

Changing the color of the solution is one of the very important features in the reactions of chemical analysis and is very important, especially in connection with the processes of oxidation and reduction, as well as in work with chemical indicators (see below - alkalimetry and acidimetry).

Examples color reactions the following can serve in qualitative chemical analysis: potassium thiocyanate KCNS gives a characteristic blood-red coloration with iron oxide salts; with ferrous oxide salts, the same reagent does not give anything. If any oxidizing agent, for example, chlorine water, is added to a solution of ferric chloride FeCl 2, of a slightly green color, the solution turns yellow due to the formation of ferric chloride, which is the highest oxidation state of this metal. If you take orange potassium dichromate K 2 Cr 2 O 7 and add a little sulfuric acid and some reducing agent, for example, wine alcohol, to it in a solution, the orange color changes to dark green, corresponding to the formation of the lowest oxidation state of chromium in the form of a salt chromium sulfate Cr 3 (SO 4) 3.

Depending on the course of chemical analysis, these processes of oxidation and reduction often have to be carried out in it. The most important oxidizing agents are: halogens, nitric acid, hydrogen peroxide, potassium permanganate, potassium dichromate; the most important reducing agents are: hydrogen at the time of isolation, hydrogen sulfide, sulfurous acid, tin chloride, hydrogen iodide.

Outgassing reactions in solutions in the production of high-quality chemical analysis, most often they do not have independent significance and are auxiliary reactions; most often you have to meet with the release of carbon dioxide CO 2 - under the action of acids on carbonic salts, hydrogen sulfide - during the decomposition of sulfide metals with acids, etc.

Reactions by the dry route

These reactions are used in chemical analysis, mainly in the so-called. "preliminary test", when testing precipitates for purity, for verification reactions and in the study of minerals. The most important reactions of this kind consist in testing a substance in relation to:

1. its fusibility when heated,
2. the ability to color the non-luminous flame of a gas burner,
3. volatility when heated,
4. ability to oxidize and reduce.

For the production of these tests, in most cases, a non-luminous flame of a gas burner is used. The main components of lighting gas (hydrogen, carbon monoxide, swamp gas, and other hydrocarbons) are reducing agents, but when it is burned in air (see Combustion), a flame is formed, in various parts of which one can find the conditions necessary for reduction or oxidation, and equal to heating to a more or less high temperature.

Fusibility test It is carried out mainly in the study of minerals, for which a very small fragment of them, reinforced in a thin platinum wire, is introduced into the part of the flame that has the highest temperature, and then using a magnifying glass, they observe how rounded the edges of the sample are.

Flame color test is produced by introducing a small sample of sepia a small sample of the substance on a platinum wire, first into the base of the flame, and then into the part of it with the highest temperature.

Volatility test It is produced by heating a sample of a substance in an assay cylinder or in a glass tube sealed at one end, and the volatile substances turn into vapors, which then condense in the colder part.

Dry oxidation and reduction can be produced in balls of fused borax ( 2 4 7 + 10 2 ) The test substance is introduced in a small amount into balls obtained by melting these salts on platinum wire, and then they are heated in the oxidizing or reducing part of the flame. Restoration can be done in a number of other ways, namely: heating on a stick charred with soda, heating in a glass tube with metals - sodium, potassium or magnesium, heating in charcoal with a blowpipe, simple heating.

Element classification

The classification of elements adopted in analytical chemistry is based on the same division of them as is customary in general chemistry - into metals and non-metals (metalloids), the latter being considered most often in the form of the corresponding acids. To produce a systematic qualitative analysis, each of these classes of elements is divided in turn into groups with some common group features.

Metals in analytical chemistry are divided into two departments, which in turn are divided into five groups:

1. Metals whose sulfur compounds are soluble in water- the distribution of metals of this department into groups is based on the properties of their carbonic salts. 1st group: potassium, sodium, rubidium, cesium, lithium. Sulfur compounds and their carbonic salts are soluble in water. There is no common reagent for the precipitation of all metals of this group in the form of insoluble compounds. 2nd group: barium, strontium, calcium, magnesium. Sulfur compounds are soluble in water, carbonic salts are insoluble. A common reagent that precipitates all the metals of this group in the form of insoluble compounds is ammonium carbonate.
2. Metals whose sulfur compounds are insoluble in water- to divide this department into three groups, they use the ratio of their sulfur compounds to weak acids and to ammonium sulfide. 3rd group: aluminum , chromium , iron , manganese , zinc , nickel , cobalt .

Aluminum and chromium do not form sulfur compounds in water; the remaining metals form sulfur compounds, which, like their oxides, are soluble in weak acids. From an acidic solution, hydrogen sulfide does not precipitate them, ammonium sulfide precipitates oxides or sulfur compounds. Ammonium sulfide is a common reagent for this group, and an excess of its sulfur compounds does not dissolve. 4th group: silver, lead, bismuth, copper, palladium, rhodium, ruthenium, osmium. Sulfur compounds are insoluble in weak acids and are precipitated by hydrogen sulfide in an acidic solution; they are also insoluble in ammonium sulfide. Hydrogen sulfide is a common reagent for this group. 5th group: tin, arsenic, antimony, gold, platinum. Sulfur compounds are also insoluble in weak acids and are precipitated by hydrogen sulfide from an acidic solution. But they are soluble in ammonium sulfide and form water-soluble sulfasalts with it.

Non-metals (metalloids) have to be discovered in chemical analysis always in the form of the acids they form or their corresponding salts. The basis for dividing acids into groups is the properties of their barium and silver salts in relation to their solubility in water and partly in acids. Barium chloride is a common reagent for the 1st group, silver nitrate in a nitrate solution - for the 2nd group, barium and silver salts of the 3rd group of acids are soluble in water. 1st group: in a neutral solution, barium chloride precipitates insoluble salts; silver salts are insoluble in water, but soluble in nitric acid. These include acids: chromic, sulphurous, sulphurous, aqueous, carbonic, silicic, sulfuric, fluorosilicic (barium salts insoluble in acids), arsenic and arsenic. 2nd group: in a solution acidified with nitric acid, silver nitrate precipitates. These include acids: hydrochloric, hydrobromic and hydroiodic, hydrocyanic, hydrogen sulfide, iron and iron cyanide and iodine. 3rd group: nitric acid and chloric acid, which are not precipitated by either silver nitrate or barium chloride.

However, it must be borne in mind that the reagents indicated for acids are not general reagents that could be used to separate acids into groups. These reagents can only give an indication of the presence of an acidic or other group, and in order to discover each individual acid, one has to use their particular reactions. The above classification of metals and non-metals (metalloids) for the purposes of analytical chemistry was adopted in the Russian school and laboratories (according to N. A. Menshutkin), in Western European laboratories another classification was adopted, based, however, essentially on the same principles.

Theoretical foundations of reactions

The theoretical foundations of the reactions of qualitative chemical analysis in solutions must be sought, as already indicated above, in the departments of general and physical chemistry about solutions and chemical affinity. One of the first, most important issues is the state of all minerals in aqueous solutions, in which, according to the theory of electrolytic dissociation, all substances belonging to the classes of salts, acids and alkalis dissociate into ions. Therefore, all reactions of chemical analysis occur not between whole molecules of compounds, but between their ions. For example, the reaction of sodium chloride NaCl and silver nitrate AgNO 3 occurs according to the equation:

Na + + Cl - + Ag + + (NO 3) - = AgCl↓ + Na + + (NO 3) - sodium ion + chloride ion + silver ion + nitric acid anion = insoluble salt + nitric acid anion

Consequently, silver nitrate is not a reagent for sodium chloride or hydrochloric acid, but only for chlorine ion. Thus, for each salt in solution, from the point of view of analytical chemistry, its cation (metal ion) and anion (acid residue) must be considered separately. For a free acid, hydrogen ions and an anion must be considered; finally, for each alkali, a metal cation and an hydroxyl anion. And in essence, the most important task of qualitative chemical analysis is to study the reactions of various ions and ways of opening them and separating them from each other.

To achieve the latter goal, by the action of appropriate reagents, ions are converted into insoluble compounds that precipitate from solution in the form of precipitation, or they are separated from solutions in the form of gases. In the same theory of electrolytic dissociation, one must look for explanations of the action of chemical indicators, which often find application in chemical analysis. According to the theory of W. Ostwald, all chemical indicators are among the relatively weak acids, partially dissociated in aqueous solutions. Moreover, some of them have colorless whole molecules and colored anions, others, on the contrary, have colored molecules and a colorless anion or an anion of a different color; exposed to the influence of free hydrogen ions of acids or hydroxyl ions of alkali, chemical indicators can change the degree of their dissociation, and at the same time their color. The most important indicators are:

1. Methyl orange, which in the presence of free hydrogen ions (acid reaction) gives a pink color, and in the presence of neutral salts or alkalis gives a yellow color;
2. Phenolphthalein - in the presence of hydroxyl ions (alkaline reaction) gives a characteristic red color, and in the presence of neutral salts or acids it is colorless;
3. Litmus - reddens under the influence of acids, and turns blue under the influence of alkalis, and, finally,
4. Curcumin - under the influence of alkalis turns brown, and in the presence of acids again takes on a yellow color.

Chemical indicators have a very important application in bulk chemical analysis (see below). In the reactions of qualitative chemical analysis, one often also encounters the phenomenon of hydrolysis, that is, the decomposition of salts under the influence of water, and the aqueous solution acquires a more or less strong alkaline or acid reaction.

Progress of qualitative chemical analysis

In a qualitative chemical analysis, it is important to determine not only what elements or compounds are included in the composition of a given substance, but also in what, approximately, relative quantities are these constituents. For this purpose, it is always necessary to proceed from certain quantities of the analyte (it is usually sufficient to take 0.5-1 gram) and, in the course of analysis, to compare the magnitude of individual precipitation with each other. It is also necessary to use solutions of reagents of a certain strength, namely: normal, semi-normal, one-tenth normal.

Each qualitative chemical analysis is divided into three parts:

1. preliminary test,
2. discovery of metals (cations),
3. discovery of non-metals (metalloids) or acids (anions).

With regard to the nature of the analyte, four cases may occur:

1. a solid non-metallic substance,
2. a solid substance in the form of a metal or an alloy of metals,
3. liquid (solution)

When analyzing solid non-metallic substance first of all, an external examination and microscopic examination is carried out, as well as a preliminary test by the above methods of analysis in a dry form. The sample of the substance is dissolved, depending on its nature, in one of the following solvents: water, hydrochloric acid, nitric acid and aqua regia (a mixture of hydrochloric and nitric acids). Substances that are unable to dissolve in any of the indicated solvents are transferred into solution by some special methods, such as: fusion with soda or potash, boiling with a soda solution, heating with certain acids, etc. The resulting solution is subjected to systematic analysis with preliminary isolation of metals and acids by groups and further dividing them into separate elements, using their own particular reactions.

When analyzing metal alloy a certain sample of it is dissolved in nitric acid (in rare cases in aqua regia), and the resulting solution is evaporated to dryness, after which the solid residue is dissolved in water and subjected to systematic analysis.

If the substance is liquid First of all, attention is drawn to its color, smell and reaction to litmus (acid, alkaline, neutral). To make sure that there are no solids in the solution, a small portion of the liquid is evaporated on a platinum plate or watch glass. After these preliminary tests, the liquid is apalized by conventional methods.

Analysis gases produced by some special methods indicated in the quantitative analysis.

Methods of quantitative chemical analysis

Quantitative chemical analysis aims to determine the relative amount of individual constituents of a chemical compound or mixture. The methods used in it depend on the qualities and composition of the substance, and therefore quantitative chemical analysis must always be preceded by qualitative chemical analysis.

Two different methods can be used to produce quantitative analysis: gravimetric and volumetric. With the weight method, the bodies to be determined are isolated in the form of, if possible, insoluble or hardly soluble compounds of a known chemical composition, and their weight is determined, on the basis of which it is possible to find the amount of the desired element by calculation. In volumetric analysis, the volumes of titrated (containing a certain amount of reagent) solutions used for analysis are measured. In addition, a number of special methods of quantitative chemical analysis differ, namely:

1. electrolytic, based on the isolation of individual metals by electrolysis,
2. colorimetric, produced by comparing the color intensity of a given solution with the color of a solution of a certain strength,
3. organic analysis, consisting in the combustion of organic matter into carbon dioxide CO 2 and water H 2 0 and in determining the amount of their relative content in the substance of carbon and hydrogen,
4. gas analysis, consisting in the determination by some special methods of the qualitative and quantitative composition of gases or their mixtures.

A very special group is medical chemical analysis, embracing a number of different methods for examining blood, urine and other waste products of the human body.

Weighted quantitative chemical analysis

Methods of weight quantitative chemical analysis are of two kinds: direct analysis method And method of indirect (indirect) analysis. In the first case, the component to be determined is isolated in the form of some insoluble compound, and the weight of the latter is determined. Indirect analysis is based on the fact that two or more substances subjected to the same chemical treatment undergo unequal changes in their weight. Having, for example, a mixture of potassium chloride and sodium nitrate, one can determine the first of them by direct analysis, precipitating chlorine in the form of silver chloride and weighing it. If there is a mixture of potassium and sodium chloride salts, one can determine their ratio by an indirect method by precipitating all the chlorine, in the form of silver chloride, and determining its weight, followed by calculation.

Volumetric chemical analysis

Electrolysis analysis

Colorimetric Methods

Elemental organic analysis

Gas analysis

Classification of methods of analytical chemistry

• Elemental analysis methods
  • X-ray spectral analysis (X-ray fluorescence)
  • Neutron activation analysis ( English) (see radioactive analysis)
  • Auger electron spectrometry (EOS) ( English); see Auger effect
  • Analytical atomic spectrometry is a set of methods based on the transformation of analyzed samples into the state of individual free atoms, the concentrations of which are then measured spectroscopically (sometimes this also includes X-ray fluorescence analysis, although it is not based on sample atomization and is not associated with atomic vapor spectroscopy).
    • MS - mass spectrometry with registration of masses of atomic ions
      • ICP-MS - inductively coupled plasma mass spectrometry (see inductively coupled plasma in mass spectrometry)
      • LA-ICP-MS - mass spectrometry with inductively coupled plasma and laser ablation
      • LIMS - laser spark mass spectrometry; see laser ablation (example of commercial implementation: LAMAS-10M)
      • SIMS - Secondary Ion Mass Spectrometry (SIMS)
      • TIMS - Thermal Ionization Mass Spectrometry (TIMS)
      • Particle Accelerator High Energy Mass Spectrometry (AMS)
    • AAS - atomic absorption spectrometry
      • ETA-AAS - atomic absorption spectrometry with electrothermal atomization (see atomic absorption spectrometers)
      • CVR - Resonator Decay Time Spectroscopy (CRDS)
      • VRLS - intracavity laser spectroscopy
    • AES - atomic emission spectrometry
      • spark and arc as sources of radiation (see spark discharge; electric arc)
      • ICP-AES - inductively coupled plasma atomic emission spectrometry
      • LIES - laser spark emission spectrometry (LIBS or LIPS); see laser ablation
    • APS - atomic fluorescence spectrometry (see fluorescence)
      • ICP-AFS - inductively coupled plasma atomic fluorescence spectrometry (devices from Baird)
      • LAFS - laser atomic fluorescence spectrometry
      • Hollow cathode APS (commercial example: AI3300)
    • AIS - Atomic Ionization Spectrometry
      • LAIS (LIIS) - laser atomic ionization or laser-intensified ionization spectroscopy (eng. Laser Enhanced Ionization, LEI )
      • RIMS - laser resonance ionization mass spectrometry
      • OG - optogalvanic (LOGS - laser optogalvanic spectroscopy)

• Other methods of analysis
  • titrimetry, volumetric analysis
  • weight analysis - gravimetry, electrogravimetry
  • spectrophotometry (usually absorption) of molecular gases and condensed matter
    • electron spectrometry (visible spectrum and UV spectrometry); see electron spectroscopy
    • vibrational spectrometry (IR spectrometry); see vibrational spectroscopy
  • Raman spectroscopy; see Raman effect
  • luminescent analysis
  • mass spectrometry with registration of masses of molecular and cluster ions, radicals
  • ion mobility spectrometry (

V.F. Yustratov, G.N. Mikileva, I.A. Mochalova

ANALYTICAL CHEMISTRY

Quantitative chemical analysis

Tutorial

For university students

2nd edition, revised and enlarged

higher professional education for interuniversity use

as a textbook in analytical chemistry for students studying in the areas of training 552400 "Food Technology", 655600 "Production of food from plant materials",

655900 "Technology of raw materials, products of animal origin"

and 655700 "Technology of food products

special purpose and public catering "

Kemerovo 2005

UDC 543.062 (07)

V.F. Yustratov, G.N. Mikileva, I.A. Mochalova

Edited by V.F. Yustratova

Reviewers:

V.A. Nevostruev, head Department of Analytical Chemistry

Kemerovo State University, Dr. of Chem. sciences, professor;

A.I. Gerasimov, Associate Professor, Department of Chemistry and Technology

inorganic substances of the Kuzbass State Technical

University, Ph.D. chem. Sciences

Kemerovo Technological Institute

Food Industry

Yustratova V.F., Mikileva G.N., Mochalova I.A.

Yu90 Analytical chemistry. Quantitative chemical analysis: Proc. allowance. - 2nd ed., revised. and additional - / V.F. Yustratov, G.N. Mikileva, I.A. Mochalova; Ed. V.F. Yustratova; Kemerovo Technological Institute of Food Industry - Kemerovo, 2005. - 160 p.

ISBN 5-89289-312-X

The basic concepts and sections of analytical chemistry are outlined. All stages of quantitative chemical analysis from sampling to obtaining results and methods for their processing are considered in detail. The manual includes a chapter on instrumental methods of analysis, as the most promising. The use of each of the described methods in the technochemical control of the food industry is indicated.

The textbook is compiled in accordance with state educational standards in the areas of "Food Technology", "Food Production from Vegetable Raw Materials and Products of Animal Origin", "Technology of Food Products for Special Purposes and Public Catering". Contains methodological recommendations for students on taking notes of lectures and working with a textbook.

Designed for students of all forms of learning.

UDC 543.062 (07)

BBC 24.4 i 7

ISBN 5-89289-312-X

© V.F. Yustratov, G.N. Mikileva, I.A. Mochalova, 1994

© V.F. Yustratov, G.N. Mikileva, I.A. Mochalova, 2005, addition

© KemTIPP, 1994

FOREWORD

The textbook is intended for students of technological specialties of universities of the food profile. Second edition, revised and enlarged. When processing the material, the advice and comments of the head of the Department of Analytical Chemistry of the Voronezh State Technological Academy, Honored Worker of Science and Technology of the Russian Federation, Doctor of Chemical Sciences, Professor Ya.I. Korenman. The authors express their deep gratitude to him.

Over the past ten years since the publication of the first edition, new textbooks on analytical chemistry have appeared, but none of them fully complies with the State Educational Standards in the areas of "Food Technology", "Food Production from Vegetable Raw Materials", "Technology of Raw Materials and products of animal origin”, “Technology of food products for special purposes and public catering”.

In the manual, the material is presented in such a way that the student sees the "task of analytical chemistry" as a whole: from sampling to obtaining analysis results, methods of processing them and analytical metrology. A brief history of the development of analytical chemistry, its role in food production is given; the basic concepts of qualitative and quantitative chemical analyzes, ways of expressing the composition of solutions and preparing solutions, formulas for calculating the results of analysis are given; theory of titrimetric analysis methods: neutralization (acid-base titration), redoximetry (redox titration), complexometry, precipitation and gravimetry. The application of each of them in the food industry is indicated. When considering titrimetric methods of analysis, a structural-logical scheme is proposed that simplifies their study.

When presenting the material, the modern nomenclature of chemical compounds, modern generally accepted concepts and ideas are taken into account, new scientific data are used to argue the conclusions.

The manual additionally includes a chapter on instrumental methods of analysis, as the most promising, and shows current trends in the development of analytical chemistry.

According to the form of presentation, the text of the manual is adapted for students of I-II courses, who still lack the skills of independent work with educational literature.

Sections 1, 2, 5 were written by V.F. Yustratova, sections 3, 6, 8, 9 - G.N. Mikileva, section 7 - I.A. Mochalova, section 4 - G.N. Mikileva and I.A. Mochalova.

ANALYTICAL CHEMISTRY AS A SCIENCE

Analytical chemistry is one of the branches of chemistry. If we give the most complete definition of analytical chemistry as a science, then we can use the definition proposed by Academician I.P. Alimarin.

"Analytical chemistry is a science that develops the theoretical foundations of the analysis of the chemical composition of substances, develops methods for identifying and detecting, determining and separating chemical elements, their compounds, as well as methods for establishing the chemical structure of compounds."

This definition is quite voluminous and difficult to remember. In high school textbooks, more concise definitions are given, the meaning of which is as follows.

Analytical chemistryis the science of methods for determining the chemical composition and structure of substances (systems).

1.1. From the history of the development of analytical chemistry

Analytical chemistry is a very ancient science.

As soon as goods and materials appeared in society, the most important of which were gold and silver, it became necessary to check their quality. Cupellation, the test by fire, was the first widely used technique for the analysis of these metals. This quantitative technique involves weighing the analyte before and after heating. The mention of this operation is found in tablets from Babylon dated 1375-1350. BC.

Scales have been known to mankind since before the times of ancient civilization. Weights found for scales date back to 2600 BC.

According to the generally accepted point of view, the Renaissance can be considered the starting point, when individual analytical techniques took shape in scientific methods.

But the term "analysis" in the modern sense of the word was introduced by the English chemist Robert Boyle (1627-1691). He first used the term in 1654.

The rapid development of analytical chemistry began at the end of the 17th century. in connection with the emergence of manufactories, the rapid growth of their number. This gave rise to a variety of problems that could only be solved using analytical methods. The need for metals, in particular iron, increased greatly, which contributed to the development of the analytical chemistry of minerals.

Chemical analysis was elevated to the status of a separate branch of science - analytical chemistry - by the Swedish scientist Thornburn Bergman (1735-1784). Bergman's work can be considered the first textbook of analytical chemistry, which provides a systematic overview of the processes used in analytical chemistry, grouped according to the nature of the analyzed substances.

The first well-known book devoted entirely to analytical chemistry is The Complete Chemical Assay Office, written by Johann Goetling (1753-1809) and published in 1790 in Jena.

A huge number of reagents used for qualitative analysis is systematized by Heinrich Rose (1795-1864) in his book "A Guide to Analytical Chemistry". Separate chapters of this book are devoted to some elements and known reactions of these elements. Thus, in 1824, Rose was the first to describe the reactions of individual elements and gave a scheme of systematic analysis, which has survived in its main features to the present day (for systematic analysis, see section 1.6.3).

In 1862, the first issue of the "Journal of Analytical Chemistry" was published - a journal devoted exclusively to analytical chemistry, which is published to this day. The magazine was founded by Fresenius and published in Germany.

The foundations of weight (gravimetric) analysis - the oldest and most logical method of quantitative analysis - were laid by T. Bergman.

Methods of volumetric analysis began to be widely included in analytical practice only in 1860. Description of these methods appeared in textbooks. By this time, devices (devices) for titration had been developed and a theoretical substantiation of these methods was given.

The main discoveries that made it possible to make a theoretical substantiation of volumetric methods of analysis include the law of conservation of the mass of matter, discovered by M.V. Lomonosov (1711-1765), a periodic law discovered by D.I. Mendeleev (1834-1907), the theory of electrolytic dissociation developed by S. Arrhenius (1859-1927).

The foundations of volumetric methods of analysis have been laid for almost two centuries, and their development is closely related to the demands of practice, first of all, the problems of bleaching fabrics and the production of potash.

Many years have been spent on the development of convenient, accurate instruments, the development of operations for grading volumetric glassware, manipulations when working with precision glassware, and methods for fixing the end of titration.

It is not surprising that even in 1829 Berzelius (1779-1848) believed that volumetric methods of analysis could only be used for approximate estimates.

For the first time now generally accepted terms in chemistry "pipette"(Fig. 1) (from the French pipe - pipe, pipette - tubes) and "burette"(Fig. 2) (from the French burette - bottle) are found in the publication of J.L. Gay-Lussac (1778-1850), published in 1824. Here he also described the titration operation in the form it is done now.

Rice. 1. Pipettes Fig. 2. Burettes

The year 1859 turned out to be significant for analytical chemistry. It was in this year that G. Kirchhoff (1824-1887) and R. Bunsen (1811-1899) developed spectral analysis and turned it into a practical method of analytical chemistry. Spectral analysis was the first of the instrumental methods of analysis, which marked the beginning of their rapid development. See section 8 for more details on these analysis methods.

At the end of the 19th century, in 1894, the German physical chemist V.F. Ostwald published a book on the theoretical foundations of analytical chemistry, the fundamental theory of which was the theory of electrolytic dissociation, on which chemical methods of analysis are still based.

Started in the 20th century (1903) was marked by the discovery of the Russian botanist and biochemist M.S. The color of the phenomenon of chromatography, which was the basis for the development of various variants of the chromatographic method, the development of which continues to this day.

In the twentieth century analytical chemistry developed quite successfully. There was a development of both chemical and instrumental methods of analysis. The development of instrumental methods was due to the creation of unique devices that allow recording the individual properties of the analyzed components.

Russian scientists have made a great contribution to the development of analytical chemistry. First of all, the names of N.A. Tananaeva, I.P. Alimarina, A.K. Babko, Yu.A. Zolotov and many others.

The development of analytical chemistry has always taken place taking into account two factors: the developing industry formed a problem that needed to be solved, on the one hand; on the other hand, the discoveries of science adapted to the solution of problems of analytical chemistry.

This trend continues to this day. Computers and lasers are widely used in analysis, new methods of analysis are emerging, automation and mathematization are being introduced, methods and means of local non-destructive, remote, continuous analysis are being created.

1.2. General problems of analytical chemistry

General tasks of analytical chemistry:

1. Development of the theory of chemical and physico-chemical methods of analysis, scientific substantiation, development and improvement of techniques and research methods.

2. Development of methods for separating substances and methods for concentrating microimpurities.

3. Improvement and development of methods for the analysis of natural substances, the environment, technical materials, etc.

4. Ensuring chemical-analytical control in the process of conducting various research projects in the field of chemistry and related fields of science, industry and technology.

5. Maintenance of chemical-technological and physical-chemical production processes at a given optimal level based on systematic chemical-analytical control of all parts of industrial production.

6. Creation of methods for automatic control of technological processes, combined with control systems based on the use of electronic computing, recording, signaling, blocking and control machines, instruments and devices.

It can be seen from the foregoing that the possibilities of analytical chemistry are wide. This allows it to be used to solve a wide variety of practical problems, including in the food industry.

1.3. The role of analytical chemistry in the food industry

Methods of analytical chemistry allow solving the following problems in the food industry:

1. Determine the quality of raw materials.

2. Control the process of food production at all its stages.

3. Control the quality of products.

4. Analyze production waste for the purpose of their disposal (further use).

5. Determine in raw materials and food products substances that are toxic (harmful) to the human body.

1.4. Analysis method

Analytical chemistry studies methods of analysis, various aspects of their development and application. According to the recommendations of the authoritative international chemical organization IUPAC *, the method of analysis is the principles underlying the analysis of a substance, i.e. the type and nature of the energy that causes perturbation of the chemical particles of matter. The principle of analysis is in turn determined by the phenomena of nature on which chemical or physical processes are based.

In the educational literature on chemistry, the definition of the method of analysis, as a rule, is not given. But since it is important enough, it must be formulated. In our opinion, the most acceptable definition is the following:

The method of analysis is the sum of the rules and techniques for performing analysis, which make it possible to determine the chemical composition and structure of substances (systems).

1.5. Classification of analysis methods

In analytical chemistry, there are several types of classification of methods of analysis.

1.5.1. Classification based on the chemical and physical properties of the analyzed substances (systems)

Within this classification, the following groups of analysis methods are considered:

1. Chemical methods of analysis.

This group of methods of analysis includes those in which the results of the analysis are based on a chemical reaction occurring between substances. At the end of the reaction, the volume of one of the participants in the reaction or the mass of one of the reaction products is recorded. Then the results of the analysis are calculated.

2. Physical methods of analysis.

Physical methods of analysis are based on the measurement of the physical properties of the analyzed substances. Most widely, these methods fix optical, magnetic, electrical, and thermal properties.

3. Physical and chemical methods of analysis.

They are based on the measurement of some physical property (parameter) of the analyzed system, which changes under the influence of a chemical reaction occurring in it.

* IUPAC - International Union of Pure and Applied Chemistry. Scientific institutions of many countries are members of this organization. The Russian Academy of Sciences (as the successor to the Academy of Sciences of the USSR) has been a member of it since 1930.

In modern chemistry, physical and physico-chemical methods of analysis are called instrumental analysis methods. "Instrumental" means that this method of analysis can be carried out only with the use of an "instrument" - a device capable of recording and evaluating physical properties (see Section 8 for details).

4. Separation methods.

When analyzing complex mixtures (and this is the majority of natural objects and food products), it may be necessary to separate the analyte from interfering components.

Sometimes in the analyzed solution of the determined component is much less than can be determined by the chosen method of analysis. In this case, before determining such components, it is necessary to preconcentrate them.

concentration- this is an operation, after which the concentration of the determined component can increase from n to 10 n times.

Separation and concentration operations are often combined. At the stage of concentration in the analyzed system, some property can clearly manifest itself, the fixation of which will allow us to solve the problem of the amount of the analyte in the mixture. The method of analysis may begin with a separation operation, sometimes it also includes concentration.

1.5.2. Classification based on the mass of a substance or volume

solution taken for analysis

A classification demonstrating the possibilities of modern methods of analysis is presented in Table. 1. It is based on the mass of substances or volume of solution taken for analysis.

Table 1

Classification of methods of analysis depending on the mass of the substance

or volume of solution taken for analysis

1.6. Qualitative Analysis

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.

The task of qualitative analysis is to establish the chemical composition of the analyzed object.

Analyzed object can be an individual substance (simple or very complex, such as bread), as well as a mixture of substances. As part of an object, its various components may be of interest. It is possible to determine which ions, elements, molecules, phases, groups of atoms the analyzed object consists of. In food products, ions are most often determined, simple or complex substances that are either useful (Ca 2+, NaCl, fat, protein, etc.) or harmful to the human body (Cu 2+ , Pb 2+ , pesticides, etc. ). This can be done in two ways: identification And discovery.

Identification- establishing the identity (identity) of the chemical compound under study with a known substance (standard) by comparing their physical and chemical properties .

For this, certain properties of the given reference compounds are preliminarily studied, the presence of which is assumed in the analyzed object. For example, chemical reactions are carried out with cations or anions (these ions are standards) in the study of inorganic substances, or the physical constants of reference organic substances are measured. Then perform the same tests with the test compound and compare the results.

Detection- checking the presence in the analyzed object of certain main components, impurities, etc. .

Qualitative chemical analysis is mostly based on the transformation of the analyte into some new compound with characteristic properties: a color, a certain physical state, a crystalline or amorphous structure, a specific smell, etc. These characteristic properties are called analytical features.

A chemical reaction, during which analytical signs appear, is called high-quality analytical reaction.

Substances used in analytical reactions are called reagents or reagents.

Qualitative analytical reactions and, accordingly, the reagents used in them, depending on the field of application, are divided into group (general), characteristic and specific.

Group reactions allow you to isolate from a complex mixture of substances under the influence of a group reagent whole groups of ions that have the same analytical feature. For example, ammonium carbonate (NH 4) 2 CO 3 belongs to group reagents, since with Ca 2+, Sr 2+, Ba 2+ ions it forms white carbonates insoluble in water.

characteristic called such reactions in which reagents interacting with one or a small number of ions participate. The analytical feature in these reactions, most often, is expressed in a characteristic color. For example, dimethylglyoxime is a characteristic reagent for the Ni 2+ ion (pink precipitate) and for the Fe 2+ ion (water-soluble red compound).

The most important in qualitative analysis are specific reactions. specific a reaction to a given ion is such a reaction that makes it possible to detect it under experimental conditions in a mixture with other ions. Such a reaction is, for example, an ion detection reaction, proceeding under the action of alkali when heated:

Ammonia released can be identified by a specific, easily recognizable odor and other properties.

1.6.1. Reagent brands

Depending on the specific area of ​​application of reagents, a number of requirements are imposed on them. One of them is the requirement for the amount of impurities.

The amount of impurities in chemical reagents is regulated by special technical documentation: state standards (GOST), technical conditions (TU), etc. The composition of impurities can be different, and it is usually indicated on the factory label of the reagent.

Chemical reagents are classified according to the degree of purity. Depending on the mass fraction of impurities, the reagent is assigned a brand. Some brands of reagents are presented in Table. 2.

table 2

Reagent brands

Usually, in the practice of chemical analysis, reagents are used that meet the qualification "analytical grade" and "chemically pure". The purity of the reagents is indicated on the label of the original packaging of the reagent. Some industries introduce their own additional purity qualifications for reagents.

1.6.2. Methods for Performing Analytical Reactions

Analytical reactions can be performed "wet" And "dry" ways. When performing a reaction "wet" by the interaction of the analyte and the corresponding reagents occurs in solution. For its implementation, the test substance must be previously dissolved. The solvent is usually water or, if the substance is insoluble in water, another solvent. Wet reactions occur between simple or complex ions, therefore, when applied, it is these ions that are detected.

"Dry" method of performing reactions means that the test substance and reagents are taken in the solid state and the reaction between them is carried out by heating them to a high temperature.

Examples of reactions performed by the "dry" way are the reactions of coloring the flame with salts of certain metals, the formation of colored pearls (glasses) of sodium tetraborate (borax) or sodium and ammonium hydrogen phosphate when fusing them with salts of certain metals, as well as fusing the solid under study with "fluxes", for example: mixtures of solid Na 2 CO 3 and K 2 CO 3, or Na 2 CO 3 and KNO 3.

The reactions carried out by the "dry" way also include the reaction that occurs when the test solid is triturated with some solid reagent, as a result of which the mixture acquires a color.

1.6.3. Systematic analysis

Qualitative analysis of the object can be carried out by two different methods.

Systematic analysis - this is a method of conducting qualitative analysis according to the scheme, when the sequence of operations for adding reagents is strictly defined.

1.6.4. Fractional Analysis

An analysis method based on the use of reactions that can be used to detect the desired ions in any sequence in individual portions of the initial solution, i.e. without resorting to a specific ion detection scheme, is called fractional analysis.

1.7. Quantitative Analysis

The task of quantitative analysis is to determine the content (mass or concentration) of a particular component in the analyzed object.

Important concepts of quantitative analysis are the concepts of "determined substance" and "working substance".

1.7.1. Substance being identified. working substance

A chemical element, ion, simple or complex substance, the content of which is determined in a given sample of the analyzed product, is commonly called "identifiable substance" (O.V.).

The substance with which this determination is carried out is called working substance (RV).

1.7.2. Ways of expressing the composition of a solution used in analytical chemistry

1. The most convenient way to express the composition of a solution is the concentration . Concentration is a physical quantity (dimensional or dimensionless) that determines the quantitative composition of a solution, mixture or melt. When considering the quantitative composition of a solution, most often, they mean the ratio of the amount of solute to the volume of the solution.

The most common is the molar concentration of equivalents. Its symbol, written, for example, for sulfuric acid is C eq (H 2 SO 4), the unit of measurement is mol / dm 3.

(1)

There are other designations for this concentration in the literature. For example, C (1 / 2H 2 SO 4). The fraction in front of the sulfuric acid formula indicates which part of the molecule (or ion) is equivalent. It is called the equivalence factor, denoted by f equiv. For H 2 SO 4 f equiv = 1/2. The equivalence factor is calculated based on the stoichiometry of the reaction. The number showing how many equivalents are contained in the molecule is called the equivalence number and is denoted by Z*. f equiv \u003d 1 / Z *, therefore, the molar concentration of equivalents is also denoted in this way: C (1 / Z * H 2 SO 4).

2. In the conditions of analytical laboratories, when it takes a long time to perform a series of single analyzes using one calculation formula, a correction factor, or correction K, is often used.

Most often, the correction refers to the working substance. The coefficient shows how many times the concentration of the prepared solution of the working substance differs from the concentration expressed in round numbers (0.1; 0.2; 0.5; 0.01; 0.02; 0.05), one of which may be in calculation formula:

. (2)

K is written as numbers with four decimal places. From the record: K \u003d 1.2100 to C eq (HCl) \u003d 0.0200 mol / dm 3 it follows that C eq (HCl) \u003d 0.0200 mol / dm 3 is the standard molar concentration of HCl equivalents, then the true is calculated by formula:

3. Titer is the mass of the substance contained in 1 cm 3 of the volume of the solution.

Titer most often refers to a solution of the working substance.

(3)

The unit of titer is g/cm 3 , the titer is calculated to the sixth decimal place. Knowing the titer of the working substance, it is possible to calculate the molar concentration of the equivalents of its solution.

(4)

4. The titer of the working substance according to the analyte- this is the mass of the substance to be determined, equivalent to the mass of the working substance contained in 1 cm 3 of the solution.

(5)

(6)

5. The mass fraction of the solute is equal to the ratio of the mass of the solute A to the mass of the solution:

. (7)

6. Volume fraction solute is equal to the ratio of the volume of solute A to the total volume of the solution:

. (8)

Mass and volume fractions are dimensionless quantities. But most often the expressions for calculating the mass and volume fractions are written as:

; (9)

. (10)

In this case, the unit for w and j is a percentage.

Attention should be paid to the following circumstances:

1. When performing an analysis, the concentration of the working substance must be accurate and expressed as a number containing four decimal places if the concentration is molar equivalents; or a number containing six decimal places if it is a caption.

2. In all calculation formulas adopted in analytical chemistry, the unit of volume is cm 3. Since the glassware used in the analysis for measuring volumes allows you to measure the volume with an accuracy of 0.01 cm 3, it is with this accuracy that the numbers expressing the volumes of the solutions of analytes and working substances involved in the analysis should be recorded.

1.7.3. Methods for preparing solutions

Before proceeding with the preparation of the solution, the following questions should be answered.

1. For what purpose is the solution prepared (for use as an RV, to create a certain pH value of the medium, etc.)?

2. In what form is it most appropriate to express the concentration of the solution (in the form of molar concentration of equivalents, mass fraction, titer, etc.)?

3. With what accuracy, i.e. up to which decimal place should the number expressing the selected concentration be determined?

4. What volume of solution should be prepared?

5. Based on the nature of the substance (liquid or solid, standard or non-standard), which method of preparing the solution should be used?

The solution can be prepared in the following ways:

1. Accurate hitch.

If substance from which to prepare the solution, is standard, i.e. meets certain (listed below) requirements, then the solution can be prepared by an accurate sample. This means that the sample weight is calculated and measured on an analytical balance with an accuracy of four decimal places.

The requirements for standard substances are as follows:

a) the substance must have a crystalline structure and correspond to a certain chemical formula;

c) the substance must be stable during storage in solid form and in solution;

d) a large molar mass equivalent of the substance is desirable.

2. From the fix channel.

A variation of the method of preparing a solution for an accurate sample is the method of preparing a solution from fixanal. The role of an accurate sample is performed by the exact amount of the substance in the glass ampoule. It should be borne in mind that the substance in the ampoule can be standard (see paragraph 1) and non-standard. This circumstance affects the methods and duration of storage of solutions of non-standard substances prepared from fixanals.

FIXANAL(standard-titer, norm-dose) is a sealed ampoule, in which it is in dry form or in the form of a solution of 0.1000, 0.0500 or another number of moles of substance equivalents.

To prepare the required solution, the ampoule is broken over a funnel equipped with a special punching device (strike). Its contents are quantitatively transferred into a volumetric flask of the required capacity and the volume is adjusted with distilled water to the ring mark.

A solution prepared by an accurate sample or from fixanal is called titrated, standard or standard solution I, because its concentration after preparation is accurate. Write it as a number with four decimal places if it is a molar concentration of equivalents, and with six decimal places if it is a title.

3. By approximate weight.

If the substance from which the solution is to be prepared does not meet the requirements for standard substances, and there is no suitable fixanal, then the solution is prepared by an approximate weight.

Calculate the mass of the substance that must be taken to prepare the solution, taking into account its concentration and volume. This mass is weighed on technical scales with an accuracy of the second decimal place, dissolved in a volumetric flask. Get a solution with an approximate concentration.

4. By diluting a more concentrated solution.

If a substance is produced by the industry in the form of a concentrated solution (it is clear that it is non-standard), then its solution with a lower concentration can only be prepared by diluting the concentrated solution. When preparing a solution in this way, it should be remembered that the mass of the solute must be the same both in the volume of the prepared solution and in the part of the concentrated solution taken for dilution. Knowing the concentration and volume of the solution to be prepared, calculate the volume of the concentrated solution to be measured, taking into account its mass fraction and density. Measure the volume with a graduated cylinder, pour into a volumetric flask, dilute to the mark with distilled water, and mix. The solution prepared in this way has an approximate concentration.

The exact concentration of solutions prepared by an approximate sample and by diluting a concentrated solution is established by carrying out a gravimetric or titrimetric analysis, therefore, solutions prepared by these methods, after their exact concentrations are determined, are called solutions with a fixed titer, standardized solutions or standard solutions II.

1.7.4. Formulas used to calculate the mass of a substance needed to prepare a solution

If a solution with a given molar concentration of equivalents or a titer is prepared from dry substance A, then the calculation of the mass of the substance that must be taken to prepare the solution is carried out according to the following formulas:

; (11)

. (12)

Note. The unit of measurement of volume is cm 3.

The calculation of the mass of a substance is carried out with such accuracy, which is determined by the method of preparation of the solution.

The calculation formulas used in the preparation of solutions by the dilution method are determined by the type of concentration to be obtained and the type of concentration to be diluted.

1.7.5. Scheme of Analysis

The main requirement for analysis is that the results obtained correspond to the true content of the components. The results of the analysis will satisfy this requirement only if all the analysis operations are performed correctly, in a certain sequence.

1. The first step in any analytical determination is sampling for analysis. As a rule, an average sample is taken.

Average sample- this is a part of the analyzed object, small in comparison with its entire mass, the average composition and properties of which are identical (the same) in all respects to its average composition.

Sampling methods for different types of products (raw materials, semi-finished products, finished products from different industries) are very different from each other. When sampling, they are guided by the rules described in detail in the technical manuals, GOSTs and special instructions on the analysis of this type of product.

Depending on the type of product and type of analysis, the sample can be taken in the form of a certain volume or a certain mass.

Sampling- this is a very responsible and important preparatory operation of the analysis. An incorrectly selected sample can completely distort the results, in which case it is generally meaningless to perform further analysis operations.

2. Sample preparation for analysis. A sample taken for analysis is not always prepared in some special way. For example, when determining the moisture content of flour, bread and bakery products by the arbitration method, a certain sample of each product is weighed and placed in an oven. Most often, the analysis is subjected to solutions obtained by appropriate processing of the sample. In this case, the task of sample preparation for analysis is reduced to the following. The sample is subjected to such processing, in which the amount of the analyzed component is preserved, and it completely goes into solution. In this case, it may be necessary to eliminate foreign substances that may be in the analyzed sample along with the component to be determined.

Sample preparation for analysis, as well as sampling, are described in the regulatory and technical documentation, according to which raw materials, semi-finished products and finished products are analyzed. Of the chemical operations that are included in the procedure for preparing a sample for analysis, we can name one that is often used in the preparation of samples of raw materials, semi-finished products, finished products in the food industry - this is the ashing operation.

Ashing is the process of converting a product (material) into ash. A sample is prepared by ashing when determining, for example, metal ions. The sample is burned under certain conditions. The remaining ash is dissolved in a suitable solvent. A solution is obtained, which is subjected to analysis.

3. Obtaining analytical data. During the analysis, the prepared sample is affected by a reagent substance or some kind of energy. This leads to the appearance of analytical signals (color change, the appearance of new radiation, etc.). The appeared signal can be: a) registered; b) consider the moment when it is necessary to measure a certain parameter in the analyzed system, for example, the volume of the working substance.

4. Processing of analytical data.

A) The obtained primary analytical data is used to calculate the results of the analysis.

There are different ways to convert analytical data into analysis results.

1. Calculation method. This method is used very often, for example, in quantitative chemical analysis. After completing the analysis, the volume of the working substance spent on the reaction with the analyte is obtained. Then this volume is substituted into the appropriate formula and the result of the analysis is calculated - the mass or concentration of the analyte.

2. Method of calibration (calibration) graph.

3. Method of comparison.

4. Method of additions.

5. Differential method.

These methods of processing analytical data are used in instrumental methods of analysis, during the study of which it will be possible to get to know them in detail.

B) The obtained results of the analysis must be processed according to the rules of mathematical statistics, which are discussed in section 1.8.

5. Determining the socio-economic significance of the analysis result. This stage is final. Having completed the analysis and received the result, it is necessary to establish a correspondence between the quality of the product and the requirements of the regulatory documentation for it.

1.7.6. Method and technique of analysis

In order to be able to move from the theory of any method of analytical chemistry to a specific method of performing an analysis, it is important to distinguish between the concepts of "method of analysis" and "method of analysis".

When it comes to the method of analysis, this means that the rules are considered, following which one can obtain analytical data and interpret them (see section 1.4).

Analysis Method- this is a detailed description of all operations for performing the analysis, including taking and preparing samples (indicating the concentrations of all test solutions).

In the practical application of each method of analysis, many methods of analysis are developed. They differ in the nature of the analyzed objects, the method of taking and preparing samples, the conditions for carrying out individual analysis operations, etc.

For example, in a laboratory workshop on quantitative analysis, among others, laboratory work is performed "Permanganometric determination of Fe 2+ in Mohr's salt solution", "Iodometric determination of Cu 2+", "Dichromatometric determination of Fe 2+". The methods for their implementation are completely different, but they are based on the same method of analysis "Redoximetry".

1.7.7. Analytical characteristics of analysis methods

In order for methods or methods of analysis to be compared or evaluated with each other, which plays an important role in their choice, each method and method has its own analytical and metrological characteristics. The analytical characteristics include the following: sensitivity coefficient (limit of detection), selectivity, duration, performance.

Limit of detection(C min., p) is the lowest content at which the presence of the determined component with a given confidence probability can be detected by this method. Confidence probability - P is the proportion of cases in which the arithmetic mean of the result for a given number of determinations will be within certain limits.

In analytical chemistry, as a rule, a confidence level of P = 0.95 (95%) is used.

In other words, P is the probability of a random error occurring. It shows how many experiments out of 100 give results that are considered correct within the specified accuracy of the analysis. With P \u003d 0.95 - 95 out of 100.

Selectivity of the analysis characterizes the possibility of determining this component in the presence of foreign substances.

Versatility- the ability to detect many components from one sample at the same time.

Analysis duration- the time spent on its implementation.

Analysis performance- the number of parallel samples that can be analyzed per unit of time.

1.7.8. Metrological characteristics of analysis methods

Evaluating the methods or techniques of analysis from the point of view of the science of measurements - metrology - the following characteristics are noted: the interval of determined contents, correctness (accuracy), reproducibility, convergence.

Interval of determined contents- this is the area provided by this technique, in which the values ​​​​of the determined quantities of components are located. At the same time, it is also customary to note lower limit of determined contents(C n) - the smallest value of the determined content, limiting the range of determined contents.

Correctness (accuracy) of analysis- is the proximity of the obtained results to the true value of the determined value.

Reproducibility and convergence of results analysis are determined by the scatter of repeated analysis results and are determined by the presence of random errors.

Convergence characterizes the dispersion of results under fixed conditions of the experiment, and reproducibility- under changing conditions of the experiment.

All analytical and metrological characteristics of the method or method of analysis are reported in their instructions.

Metrological characteristics are obtained by processing the results obtained in a series of repeated analyzes. Formulas for their calculation are given in section 1.8.2. They are similar to formulas used for static processing of analysis results.

1.8. Errors (errors) in the analysis

No matter how carefully one or another quantitative determination is carried out, the result obtained, as a rule, differs somewhat from the actual content of the determined component, i.e. the result of the analysis is always obtained with some inaccuracy - an error.

Measurement errors are classified as systematic (certain), random (uncertain) and gross or misses.

Systematic errors- these are errors that are constant in value or vary according to a certain law. They can be methodical, depending on the specifics of the method of analysis used. They may depend on the instruments and reagents used, on incorrect or insufficiently careful performance of analytical operations, on the individual characteristics of the person performing the analysis. Systematic errors are difficult to notice, as they are constant and appear during repeated determinations. To avoid errors of this kind, it is necessary to eliminate their source or introduce an appropriate correction into the measurement result.

Random errors are called errors that are indefinite in magnitude and sign, in the appearance of each of which no regularity is observed.

Random errors occur in any measurement, including any analytical determination, no matter how carefully it is carried out. Their presence is reflected in the fact that repeated determinations of one or another component in a given sample, performed by the same method, usually give slightly different results.

Unlike systematic errors, random errors cannot be taken into account or eliminated by introducing any corrections. However, they can be significantly reduced by increasing the number of parallel determinations. The influence of random errors on the result of the analysis can be theoretically taken into account by processing the results obtained in a series of parallel determinations of this component using the methods of mathematical statistics.

Availability gross errors or misses manifests itself in the fact that among relatively close results, one or several values ​​are observed that stand out noticeably in magnitude from the general series. If the difference is so large that we can talk about a gross error, then this measurement is immediately discarded. However, in most cases, one cannot immediately recognize that other result as incorrect only on the basis of “jumping out” from the general series, and therefore additional research is necessary.

There are options when it makes no sense to conduct additional studies, and at the same time it is undesirable to use incorrect data to calculate the overall result of the analysis. In this case, the presence of gross errors or misses is determined according to the criteria of mathematical statistics.

Several such criteria are known. The simplest of these is the Q-test.

1.8.1. Determining the presence of gross errors (misses)

In chemical analysis, the content of a component in a sample is determined, as a rule, by a small number of parallel determinations (n ​​£ 3). To calculate the errors of definitions in this case, they use the methods of mathematical statistics developed for a small number of definitions. The results of this small number of determinations are considered as randomly selected - sampling- from all conceivable results of the general population under the given conditions.

For small samples with the number of measurements n<10 определение грубых погрешностей можно оценивать при помощи range of variation by Q-criterion. To do this, make the ratio:

where X 1 - suspiciously distinguished result of the analysis;

X 2 - the result of a single definition, closest in value to X 1 ;

R - range of variation - the difference between the largest and smallest values ​​of a series of measurements, i.e. R = X max. - X min.

The calculated value of Q is compared with the tabular value of Q (p, f). The presence of a gross error is proved if Q > Q(p, f).

The result, recognized as a gross error, is excluded from further consideration.

The Q-criterion is not the only indicator whose value can be used to judge the presence of a gross error, but it is calculated faster than others, because. allows you to immediately eliminate gross errors without performing other calculations.

The other two criteria are more accurate, but require a full calculation of the error, i.e. the presence of a gross error can be said only by performing a complete mathematical processing of the analysis results.

Gross errors can also be identified:

A) standard deviation. The result X i is recognized as a gross error and discarded if

. (14)

B) Accuracy of direct measurement. The result X i is discarded if

. (15)

About quantities indicated by signs , see section 1.8.2.

1.8.2. Statistical processing of analysis results

Statistical processing of the results has two main tasks.

The first task is to present the result of the definitions in a compact form.

The second task is to evaluate the reliability of the obtained results, i.e. the degree of their correspondence to the true content of the determined component in the sample. This problem is solved by calculating the reproducibility and accuracy of the analysis using the formulas below.

As already noted, reproducibility characterizes the scatter of repeated analysis results and is determined by the presence of random errors. The reproducibility of the analysis is evaluated by the values ​​of standard deviation, relative standard deviation, variance.

The overall scatter characteristic of the data is determined by the value of the standard deviation S.

(16)

Sometimes, when assessing the reproducibility of an assay, the relative standard deviation Sr is determined.

The standard deviation has the same unit as the mean, or true value m of the quantity being determined.

The method or technique of analysis is the better reproducible, the lower the absolute (S) and relative (Sr) deviation values ​​for them.

The scatter of the analysis data about the mean is calculated as the variance S 2 .

(18)

In the presented formulas: Xi - individual value of the quantity obtained during the analysis; - arithmetic mean of the results obtained for all measurements; n is the number of measurements; i = 1…n.

The correctness or accuracy of the analysis is characterized by the confidence interval of the average value of p, f. This is the area within which, in the absence of systematic errors, the true value of the measured quantity is found with a confidence probability P.

, (19)

where p, f - confidence interval, i.e. confidence limits within which the value of the determined quantity X may lie.

In this formula, t p, f is the Student's coefficient; f is the number of degrees of freedom; f = n - 1; P is the confidence level (see 1.7.7); t p, f - given tabular.

Standard deviation of the arithmetic mean. (twenty)

The confidence interval is calculated either as an absolute error in the same units in which the result of the analysis is expressed, or as a relative error DX o (in %):

. (21)

Therefore, the result of the analysis can be represented as:

. (23)

The processing of analysis results is greatly simplified if the true content (m) of the analyte is known when performing analyzes (control samples or standard samples). Calculate the absolute (DX) and relative (DX o, %) errors.

DX \u003d X - m (24)

(25)

1.8.3. Comparison of two average results of the analysis performed

different methods

In practice, there are situations when an object needs to be analyzed by different methods, in different laboratories, by different analysts. In these cases, average results differ from each other. Both results characterize some approximation to the true value of the desired quantity. In order to find out whether both results can be trusted, it is determined whether the difference between them is statistically significant, i.e. "too big. The average values ​​of the desired value are considered compatible if they belong to the same general population. This can be solved, for example, by the Fisher criterion (F-criterion).

where are the dispersions calculated for different series of analyses.

F ex - is always greater than one, because it is equal to the ratio of the larger variance to the smaller one. The calculated value of F ex is compared with the table value of F table. (confidence probability P and the number of degrees of freedom f for experimental and tabular values ​​should be the same).

When comparing F ex and F table options are possible.

A) F ex >F tab. The discrepancy between the variances is significant and the considered samples differ in reproducibility.

B) If F ex is significantly less than F table, then the difference in reproducibility is random and both variances are approximate estimates of the same general population variance for both samples.

If the difference between the variances is small, you can find out if there is a statistically significant difference in the average results of the analysis obtained by different methods. To do this, use the Student's coefficient t p, f. Calculate the weighted average standard deviation and t ex.

; (27)

(28)

where are the average results of the compared samples;

n 1 , n 2 - the number of measurements in the first and second samples.

Compare t ex with t table with the number of degrees of freedom f = n 1 +n 2 -2.

If at the same time t ex > t table, then the discrepancy between is significant, the samples do not belong to the same general population and the true values ​​in each sample are different. If t ex< t табл, можно все данные рассматривать как единую выборочную совокупность для (n 1 +n 2) результатов.

TEST QUESTIONS

1. What does analytical chemistry study?

2. What is the analysis method?

3. What groups of methods of analysis are considered by analytical chemistry?

4. What methods can be used to perform qualitative analysis?

5. What are analytical features? What can they be?

6. What is a reagent?

7. What reagents are needed to perform a systematic analysis?

8. What is fractional analysis? What reagents are needed for its implementation?

9. What do the letters “chemically pure”, “ch.d.a.” mean? on the chemical label?

10. What is the task of quantitative analysis?

11.What is the working substance?

12. In what ways can a working substance solution be prepared?

13. What is a standard substance?

14. What do the terms “standard solution I”, “standard solution II” mean?

15. What is the titer and titer of the working substance according to the analyte?

16. How is the molar concentration of equivalents briefly indicated?