Abstract: Analytical chemistry

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

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

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

AT chemical methods carry out a chemical reaction and measure either the mass of the product obtained - gravimetric (weight) methods, or the volume of the reagent used for interaction with the substance - titrimetric, gas volumetric (volumetric) methods.

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

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

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

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

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

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

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

In the theoretical fundamentals of analytical occupies a significant place, including statistical. processing results. Analytical theory also includes the doctrine of selection and preparation, of drawing up an analysis scheme and the choice of methods, principles and ways of automating analysis, the use of computers, and the foundations of national economies. using the results of chem. analysis. A feature of the analytical is the study of not general, but individual, specific. sv-in and characteristics of objects, which ensures the selectivity of many others. analyte methods. Thanks to close ties with the achievements of physics, mathematics, biology, etc. areas of technology (this is especially true of methods of analysis) analytical transformation. into a discipline at the intersection of sciences.

Almost all determination methods are based on the dependence of c.-l. measurable properties in-in from their composition. So important direction analytical - finding and studying such dependencies in order to use them to solve the analyte. tasks. At the same time, it is almost always necessary to find the level of connection between St. and the composition, develop ways to register St. Islands (analytical signal), eliminate interference from other components, eliminate the interfering influence of decomp. factors (eg, fluctuations of t-ry). The value of the analyte. the signal is converted into units characterizing the number or components. Measured to be, for example, mass, volume, light absorption.

Much attention is paid to the theory of methods of analysis. Theory of chem. and partially physical.-chemical. methods is based on ideas about several fundamentals. types of chem. p-tions, widely used in the analysis (acid-base, redox.,), and several important processes(-, ). Attention to these issues is due to the history of the development of analytical and practical. the significance of the respective methods. Since, however, the proportion of chem. methods decreases, and the share of fiz.-chem. and physical methods is growing, the improvement of the theory of methods of the last two groups and the integration of theoretical. aspects of individual methods in general theory analytical .

History of development. Tests of materials were carried out in ancient times, for example. investigated to establish their suitability for melting, decomp. products - to determine the content of Au and Ag in them. Alchemists 14th-16th centuries for the first time applied and performed a huge amount of experiments. studies st-in-in, initiating the chem. analysis methods. In the 16-17 centuries. (period ) new chem. ways detection in-in, based on p-tions in the solution (eg, the discovery of Ag + by the formation of a precipitate with Cl -). R. Boyle, who introduced the concept of "chemical analysis", is considered the founder of scientific analytical.

Until the 1st floor. 19th century analytical was osn. section. During this period, many were opened. chem. elements, the constituent parts of certain natures are distinguished. in-in, established and multiple relations, . T. Bergman developed a systematic scheme. analysis, introduced H 2 S as an analyte. , proposed methods of analysis in a flame to obtain pearls, etc. In the 19th century systematic qualities. the analysis was improved by G. Rose and K. Fresenius. The same century was marked by huge successes in the development of quantities. analysis. Titrimetric was created. method (F. Decroisil, J. Gay-Lussac), significantly improved gravimetric. analysis, methods developed. Of great importance was the development of methods org. compounds (Yu. Liebig). In con. 19th century there was an analytical theory, which was based on the doctrine of chem. in solutions with participation (ch. arr. W. Ostwald). By this time, methods of analysis in aqueous solutions occupied a predominant place in the analytical.

In the 20th century methods of microanalysis org. compounds (F. Pregl). Polarographic was proposed. method (J. Geyrovsky, 1922). Appeared a lot of fiz.-chem. and physical methods, eg. mass spectrometric, x-ray, nuclear physics. Of great importance was the discovery (M.S. Tsvet, 1903) and then the creation of its various variants, in particular, distribution. (A. Martin and R. Sint, 1941).

In Russia and in the USSR great importance for the development of the analytical had the work of N.A. Menshutkin (his textbook on analytics went through 16 editions). M.A. Ilyinsky, and especially L.A. Chugaev put into practice org. analyte (late 19th-early 20th centuries), N.A. Tananaev developed the drip method of qualities. analysis (simultaneously with F. Feigl, 20s of the 20th century). In 1938, N. A. Izmailov and M. S. Schreiber first described. In the 1940s Plasma sources have been proposed for atomic emission analysis. Huge contribution Soviet scientists also included its analyte in the study. use (I.P. Alimarin, A.K. BabkoKh in the theory of the action of org. analytics, in the development of methods of photometric analysis, atomic absorption, in the analytical of individual elements, especially rare and platinum, and a number of objects - in-in high purity, mineral raw materials, and .

The demands of practice have always stimulated the development of the analytical. So, in the 40-70s. 20th century In connection with the need to analyze high-purity nuclear, semiconductor, and other materials, such sensitive methods as spark mass spectrometry, chemical-spectral analysis, and voltammetry were created, which ensure the determination of up to 10 -7 - 10 -8% of impurities in pure in-wah, i.e. 1 part of the impurity per 10-1000 billion parts of the main. in-va. For the development of black steel, especially in connection with the transition to high-speed converter steel production, the rapidity of analysis has become decisive. The use of the so-called. quantometers-photoelectric. devices for multi-element optical. spectral or X-ray analysis allows analysis during melting for several times. minutes.

The need to analyze complex mixtures of org. compounds led to intensive development, edges allows you to analyze the most complex mixtures containing several. tens and even hundreds. Analytical in means. contributed to the mastery of energy, the study of space and the ocean, the development of electronics, and progress. Sciences.

Subject of study. Important role plays the development of the theory of selection of analyzed materials; Usually, sampling issues are resolved jointly with specialists in the studied substances (for example, with geologists, metallurgists). Analytical develops methods of decomposition - fusion, etc., to-rye should provide a complete "opening" of the sample and prevent the loss of the determined components and contamination from the outside. The tasks of the analytical include the development of techniques for such general operations of analysis as the measurement of volumes, calcination.

One of the tasks analytical chemistry - definition directions of development of the analyt. instrumentation, the creation of new schemes and designs of devices (which most often serves as the final stage in the development of an analysis method), as well as the synthesis of new analytes. reagents.

For quantities. analysis are very important metrological. characteristics of methods and devices. In this regard, the analytical studies the problems of calibration, manufacture and use of comparison samples (including ) and other media in ensuring the correctness of the analysis. Creatures. the place is occupied by the processing of the results of the analysis, including with the use of a computer. For the conditions of analysis, information theory is used, mat. utility theory, pattern recognition theory, and other branches of mathematics. Computers are used not only for processing results, but also for controlling instruments, accounting for interference, calibration,; there are analytes. tasks that can be solved only with the help of a computer, for example. org. connections using art theory. intelligence (see Automated analysis).

Methods of determination-osn. group of analytical methods. At the heart of quantity methods. analysis lies the dependence of c.-l. measurable property, most often physical, from the composition of the sample. This dependence must be described in a certain and known way.

For analysis, a variety of methods are needed, since each of them has its own advantages and limitations. Yes, extremely sensitive. radioactivation and mass spectral methods require complex and expensive equipment. Simple, affordable 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. factors: metrological. parameters, the scope of possible use, the availability of equipment, the qualifications of the analyst, traditions, etc. The most important among these factors are such metrological. parameters, such as the detection limit or range (number), in which the method gives reliable results, and the accuracy of the method, i.e. correctness and reproducibility of results. In a number of cases, "multicomponent" methods are of great importance, allowing one to determine immediately big number components, eg. atomic emission and x-ray

4.2. CHROMATOGRAPHIC METHODS

4.3. CHEMICAL METHODS

4.4. ELECTROCHEMICAL METHODS

4.5. SPECTROSCOPIC METHODS

4.6. MASS SPECTROMETRIC METHODS

4.7. METHODS OF ANALYSIS BASED ON RADIOACTIVITY

4.8. THERMAL METHODS

4.9. BIOLOGICAL METHODS OF ANALYSIS

5. CONCLUSION

6. LIST OF USED LITERATURE

INTRODUCTION

Chemical analysis serves as a means of monitoring production and product quality in a number of industries National economy. On the results of the analysis in varying degrees based mineral exploration. Analysis is the main means of controlling contamination environment. Finding out chemical composition soil, fertilizers, feed and agricultural products is important for the normal functioning of the agro-industrial complex. Chemical analysis is indispensable in medical diagnostics and biotechnology. The development of many sciences depends on the level of chemical analysis, the equipment of the laboratory with methods, instruments and reagents.

The scientific basis of chemical analysis is analytical chemistry, a science that has been a part, and sometimes the main part, of chemistry for centuries.

Analytical chemistry is the science of determining the chemical composition of substances and partly their chemical structure. Methods of analytical chemistry allow answering questions about what a substance consists of, what components are included in its composition. These methods often make it possible to find out in what form a given component is present in a substance, for example, to determine the oxidation state of an element. Sometimes it is possible to estimate the spatial arrangement of components.

When developing methods, you often have to borrow ideas from related fields of science and adapt them to your goals. The task of analytical chemistry includes the development of the theoretical foundations of methods, the establishment of the limits of their applicability, the assessment of metrological and other characteristics, the creation of methods for the analysis of various objects.

Methods and means of analysis are constantly changing: new approaches are involved, new principles and phenomena are used, often from distant fields of knowledge.

The analysis method is understood as a fairly universal and theoretically justified method for determining the composition, regardless of the component being determined and the object being analyzed. When they talk about the method of analysis, they mean the underlying principle, the quantitative expression of the relationship between the composition and any measured property; selected implementation techniques, including interference detection and elimination; devices for practical implementation and methods for processing measurement results. The analysis technique is detailed description analysis of this object using the selected method.

There are three functions of analytical chemistry as a field of knowledge:

1. solution general issues analysis,

2. development of analytical methods,

3. solution of specific problems of analysis.

It can also be distinguished qualitative and quantitative analyses. The first decides the question of which components the analyzed object includes, the second gives information about the quantitative content of all or individual components.

2. CLASSIFICATION OF METHODS

All existing methods analytical chemistry can be divided into methods of sampling, decomposition of samples, separation of components, detection (identification) and determination. There are hybrid methods that combine separation and definition. Detection and definition methods have much in common.

The methods of determination are of the greatest importance. They can be classified according to the nature of the measured property or the way the corresponding signal is registered. Methods of determination are divided into chemical , physical and biological. Chemical methods are based on chemical (including electrochemical) reactions. This includes methods called physicochemical. Physical methods are based on physical phenomena and processes, biological methods are based on the phenomenon of life.

The main requirements for analytical chemistry methods are: correctness and good reproducibility of results, low detection limit of the required components, selectivity, rapidity, ease of analysis, and the possibility of its automation.

When choosing an analysis method, it is necessary to clearly know the purpose of the analysis, the tasks that need to be solved, and evaluate the advantages and disadvantages of the available analysis methods.

3. ANALYTICAL SIGNAL

After the selection and preparation of the sample, the stage of chemical analysis begins, at which the component is detected or its amount is determined. For this purpose, they measure analytical signal. In most methods, the analytical signal is the average of measurements of a physical quantity at the final stage of the analysis, functionally related to the content of the analyte.

If it is necessary to detect any component, it is usually fixed appearance analytical signal - the appearance of a precipitate, color, lines in the spectrum, etc. The appearance of an analytical signal must be reliably recorded. When determining the amount of a component, it is measured magnitude analytical signal - sediment mass, current strength, spectrum line intensity, etc.

4. METHODS OF ANALYTICAL CHEMISTRY

4.1. METHODS OF MASKING, SEPARATION AND CONCENTRATION

Masking.

Masking is inhibition or complete suppression chemical reaction in the presence of substances capable of changing its direction or speed. In this case, no new phase is formed. There are two types of masking - thermodynamic (equilibrium) and kinetic (non-equilibrium). In thermodynamic masking, conditions are created under which the conditional reaction constant is reduced to such an extent that the reaction proceeds insignificantly. The concentration of the masked component becomes insufficient to reliably fix the analytical signal. Kinetic masking is based on increasing the difference between the reaction rates of the masked and the analyte with the same reagent.

Separation and concentration.

The need for separation and concentration may be due to the following factors: the sample contains components that interfere with the determination; the concentration of the analyte is below the detection limit of the method; the components to be determined are unevenly distributed in the sample; there are no standard samples for calibrating instruments; the sample is highly toxic, radioactive and expensive.

Separation- this is an operation (process), as a result of which the components that make up the initial mixture are separated from one another.

concentration- this is an operation (process), as a result of which the ratio of the concentration or amount of microcomponents to the concentration or amount of the macrocomponent increases.

Precipitation and co-precipitation.

Precipitation is generally used to separate inorganic substances. Precipitation of microcomponents by organic reagents, and especially their co-precipitation, provide a high concentration factor. These methods are used in combination with methods of determination that are designed to obtain an analytical signal from solid samples.

Separation by precipitation is based on the different solubility of the compounds, mainly in aqueous solutions.

Co-precipitation is the distribution of a microcomponent between a solution and a precipitate.

Extraction.

Extraction is a physicochemical process of distributing a substance between two phases, most often between two immiscible liquids. It is also a process of mass transfer with chemical reactions.

Extraction methods are suitable for concentration, extraction of microcomponents or macrocomponents, individual and group isolation of components in the analysis of various industrial and natural objects. The method is simple and fast to perform, provides high efficiency separation and concentration and is compatible with different methods definitions. Extraction allows you to study the state of substances in solution at various conditions, to determine the physico-chemical characteristics.

Sorption.

Sorption is well used for separation and concentration of substances. Sorption methods usually provide good separation selectivity, high values concentration coefficients.

Sorption- the process of absorption of gases, vapors and dissolved substances by solid or liquid absorbers on a solid carrier (sorbents).

Electrolytic separation and cementation.

The most common method of electrolysis, in which the separated or concentrated substance is isolated on solid electrodes in the elemental state or in the form of some kind of compound. Electrolytic isolation (electrolysis) based on precipitation electric shock at controlled potential. The most common variant of cathodic deposition of metals. The electrode material can be carbon, platinum, silver, copper, tungsten, etc.

electrophoresis is based on differences in the speeds of movement of particles of different charges, shapes and sizes in an electric field. The speed of movement depends on the charge, field strength and particle radius. There are two types of electrophoresis: frontal (simple) and zone (on a carrier). In the first case, a small volume of a solution containing the components to be separated is placed in a tube with an electrolyte solution. In the second case, the movement occurs in a stabilizing medium that keeps the particles in place after the electric field is turned off.

Method grouting consists in the reduction of components (usually small amounts) on metals with sufficiently negative potentials or almagamas of electronegative metals. During cementation, two processes occur simultaneously: cathodic (separation of the component) and anodic (dissolution of the cementing metal).

MOSCOW AUTOMOTIVE AND ROAD INSTITUTE (STATE TECHNICAL UNIVERSITY)

Department of Chemistry

I approve the head. department professor

I.M. Papisov "___" ____________ 2007

A.A. LITMANOVICH, O.E. LITMANOVYCH

ANALYTICAL CHEMISTRY Part 1: Qualitative Chemical Analysis

Toolkit

for students of the second year of the specialty "Engineering environmental protection"

MOSCOW 2007

Litmanovich A.A., Litmanovich O.E. Analytical Chemistry: Part 1: Qualitative Chemical Analysis: Methodological Guide / MADI

(GTU) - M., 2007. 32 p.

The main chemical laws of the qualitative analysis of inorganic compounds and their applicability for determining the composition of environmental objects are considered. The manual is intended for students of the specialty "Environmental Engineering".

© Moscow Automobile and Road Institute (state Technical University), 2008

CHAPTER 1. SUBJECT AND OBJECTIVES OF ANALYTICAL CHEMISTRY. ANALYTICAL REACTIONS

1.1. Subject and tasks of analytical chemistry

Analytical chemistry- the science of methods for studying the composition of substances. These methods determine which chemical elements, in what form and in what quantity are contained in the object under study. In analytical chemistry, two large sections are distinguished - qualitative and quantitative analysis. The tasks set by analytical chemistry are solved with the help of chemical and instrumental methods (physical, physicochemical).

In chemical methods of analysis the element to be determined is converted into a compound having such properties, with the help of which it is possible to establish the presence of this element or to measure its amount. One of the main ways to measure the amount of a formed compound is to determine the mass of a substance by weighing on an analytical balance - a gravimetric method of analysis. Methods of quantitative chemical analysis and instrumental methods of analysis will be discussed in part 2 methodological manual in analytical chemistry.

An urgent direction in the development of modern analytical chemistry is the development of methods for analyzing environmental objects, waste and waste water, gas emissions from industrial enterprises and road transport. Analytical control makes it possible to detect the excess of the content of especially harmful components in discharges and emissions, helps to identify sources of environmental pollution.

Chemical analysis is based on the fundamental laws of general and inorganic chemistry with which you are already familiar. Theoretical basis chemical analysis include: knowledge of the properties of aqueous solutions; acid-base equilibria in aqueous

solutions; redox equilibria and properties of substances; patterns of complexation reactions; conditions for the formation and dissolution of the solid phase (precipitates).

1.2. analytical reactions. Conditions and methods for their implementation

Qualitative chemical analysis is carried out using analytical reactions accompanied by noticeable external changes: for example, gas evolution, color change, formation or dissolution of a precipitate, in some cases - the appearance of a specific odor.

Basic requirements for analytical reactions:

1) High sensitivity, characterized by the value of the detection limit (Cmin) - the lowest concentration of the component in the solution sample, at which this analysis technique allows you to confidently detect this component. The absolute minimum value of the mass of a substance that can be detected by analytical reactions is from 50 to 0.001 μg (1 μg = 10–6 g).

2) Selectivity- characterized by the ability of the reagent to react with as few components (elements) as possible. In practice, they try to detect ions under conditions under which the selective reaction becomes specific, i.e. allows you to detect this ion in the presence of other ions. As examples of specific reactions(of which there are few) are as follows.

a) The interaction of ammonium salts with an excess of alkali when heated:

NH4Cl + NaOH → NH3 + NaCl + H2O . (one)

The released ammonia is easy to recognize by its characteristic odor (“ammonia”) or by a change in the color of a wet indicator paper brought to the neck of the test tube. Reaction

allows you to detect the presence of ammonium ions NH4 + in the analyzed solution.

b) Interaction of ferrous salts with potassium hexacyanoferrate (III) K3 with the formation of a precipitate of blue color(Turnbull blue, or Prussian blue). Reaction (well familiar to you on the topic "Corrosion of metals" in the course

These reactions make it possible to detect Fe2+ and Fe3+ ions in the analyzed solution.

Specific reactions are convenient in that the presence of unknown ions can be determined fractional method- in separate samples of the analyzed solution containing other ions.

3) The speed of the reaction ( high speed ) and ease of implementation.

The high reaction rate ensures the achievement of thermodynamic equilibrium in the system for a short time(practically at the rate of mixing components during reactions in solution).

When performing analytical reactions, it is necessary to remember what determines the shift in the equilibrium of the reaction in the right direction and its flow to a large depth of transformation. For reactions occurring in aqueous solutions of electrolytes, the shift in thermodynamic equilibrium is affected by the concentration of ions of the same name, the pH of the medium, and the temperature. In particular, temperature depends the value of equilibrium constants - constants

dissociation for weak electrolytes and solubility products (PR) for sparingly soluble salts, bases

These factors determine the depth of the reaction, the yield of the product and the accuracy of the determination of the analyte (or the very possibility of detecting a certain ion at a small amount and concentration of the analyte).

The sensitivity of some reactions increases in an aqueous organic solution, for example, when acetone or ethanol is added to an aqueous solution. For example, in an aqueous ethanol solution, the solubility of CaSO4 is much lower than in an aqueous solution (the SP value is lower), which makes it possible to unambiguously detect the presence of Ca2+ ions in the analyzed solution at much lower concentrations than in an aqueous solution, and also to most completely free the solution from these ions (precipitation with H2 SO4 ) to continue the analysis of the solution.

In qualitative chemical analysis, a rational sequence is developed in the separation and detection of ions - a systematic course (scheme) of analysis. In this case, ions are isolated from the mixture in groups, based on their equal relation to the action of certain group reagents.

One portion of the analyzed solution is used, from which groups of ions are sequentially isolated in the form of precipitation and solutions, in which individual ions are then detected . The use of group reagents makes it possible to decompose the complex problem of qualitative analysis into a number of simpler ones. The ratio of ions to the action of certain

group reagents is the basis analytical classification of ions.

1.3. preliminary analysis an aqueous solution containing a mixture of salts, by color, smell, pH value

The presence of a color in a clear solution proposed for analysis may indicate the presence of one or several ions at once (Table 1). The intensity of the color depends on the concentration of the ion in the sample, and the color itself can change if

metal cations form more stable complex ions than complex cations with H2O molecules as ligands, for which the color of the solution is indicated in Table. one .

pH measurement of the proposed solution ( if the solution is prepared in water, and not in a solution of alkali or acid) also

Aqueous solutions of some salts may have specific odors depending on the pH of the solution due to the formation of unstable (decomposing) or volatile compounds. By adding NaOH solutions to the sample solution or

strong acid (HCl, H2 SO4 ), you can gently smell the solution (Table 3).

Table 3

The reason for the smell (see Table 3) is the well-known property of reactions in electrolyte solutions - the displacement of weak acids or bases (often aqueous solutions of gaseous substances) from their salts by strong acids and bases, respectively.

CHAPTER 2. QUALITATIVE CHEMICAL ANALYSIS OF CATIONS

2.1. Acid-base method for classifying cations by analytical groups

The simplest and least “harmful” acid-base (basic) method of qualitative analysis is based on the ratio of cations to acids and bases. The classification of cations is carried out according to the following criteria:

a) solubility of chlorides, sulfates and hydroxides; b) basic or amphoteric character of hydroxides;

c) the ability to form stable complex compounds with ammonia (NH3) - ammoniates (i.e. amino complexes).

All cations are divided into six analytical groups using 4 reagents: 2M HCl solution, 1M H2SO4 solution, 2M NaOH solution and concentrated aqueous ammonia solution

NH4 OH (15-17%) (Table 4).

Table 4 Classification of cations by analytical groups

Obviously, the above list of cations is far from complete and includes the cations most frequently encountered in practice in the analyzed samples. In addition, there are other principles of classification by analytic groups.

2.2. Intragroup analysis of cations and analytical reactions for their detection

2.2.1. First group (Ag+ , Pb2+ )

Test solution containing Ag+, Pb2+ cations

↓ + 2M HCl solution + C 2 H5 OH (to reduce the solubility of PbCl2)

If PC > PR, are formed white precipitates of a mixture of chlorides,

which are separated from the solution (the solution is not analyzed):

Ag+ + Cl– ↔ AgCl↓ and Pb2+ + 2Cl– ↔ PbCl2 ↓ (3)

Obviously, at low concentrations of precipitated cations, the concentration of Cl– anions should be relatively high

↓ To sediment part + H2 O (distilled) + boiling

↓ To the 2nd part of the sediment of the mixture of chlorides + 30%

Analytical chemistry

the science of methods for studying the composition of matter. It consists of two main sections: qualitative analysis and quantitative analysis. a set of methods for establishing the qualitative chemical composition of bodies - the identification of atoms, ions, molecules that make up the analyzed substance. The most important characteristics of each qualitative analysis method are: specificity and sensitivity. Specificity characterizes the possibility of detecting the desired element in the presence of other elements, such as iron in the presence of nickel, manganese, chromium, vanadium, silicon, etc. Sensitivity determines the smallest amount of the element that can be detected by this method; sensitivity is expressed for modern methods by values ​​of the order of 1 mcg(one millionth of a gram).

Quantitative analysis - a set of methods for determining the quantitative composition of bodies, i.e., the quantitative ratios in which chemical elements or individual compounds are found in the analyzed substance. The most important characteristic of each method of quantitative analysis is, along with specificity and sensitivity, accuracy. The accuracy of the analysis is expressed by the value of the relative error, which in most cases should not exceed 1-2%. Sensitivity in quantitative analysis is expressed as a percentage.

Many modern methods have a very high sensitivity. Thus, the presence of copper in silicon can be determined by the method of radioactive analysis with an accuracy of 2 × 10 -8%.

Due to some specific features in A. x. it is customary to highlight the analysis of organic substances (see below).

A special place in A. x. occupies based on the totality of methods of qualitative and quantitative, inorganic and organic analysis in their application to a particular object. Technical analysis includes analytical control of production processes, raw materials, finished products, water, air, exhaust gases, etc. Especially great is the need for "express" methods of technical analysis, requiring 5-15 min. for a separate definition.

Determining the suitability of a product for human needs has as ancient a history as its production itself. Initially, such a definition was aimed at establishing the reasons for the inconsistency of the obtained properties of the products with the desired or necessary ones. This applied to food products - such as bread, beer, wine, etc., for which taste, smell, color were used (these test methods, called organoleptic, are also used in the modern food industry). Raw materials and products of ancient metallurgy - ores, metals and alloys, which were used for the manufacture of tools of production (copper, bronze, iron) or for decoration and commodity exchange (gold, silver), were tested for their density, mechanical properties through test melts. A combination of such methods for testing noble alloys is still used in assay analysis. The good quality of dyes, ceramics, soap, leather, fabrics, glass, and medicines was determined. In the process of such an analysis, individual metals (gold, silver, copper, tin, iron), alkalis, and acids began to be distinguished.

During the alchemical period in the development of chemistry (see Alchemy), which was characterized by the development of experimental work, the number of distinguishable metals, acids, alkalis increased, the concept of salt, sulfur as a combustible substance, etc. arose. During the same period, many instruments for chemical research were invented , weighing of the studied and used substances was applied (14-16 centuries).

The main significance of the alchemical period for the future A. x. consisted in the fact that purely chemical methods of distinguishing between individual substances were discovered; so, in the 13th century. it was found that "strong vodka" (nitric acid) dissolves silver, but does not dissolve gold, and "aqua regia" (a mixture of nitric and hydrochloric acids) also dissolves gold. The alchemists laid the foundation for chemical definitions; before that, to distinguish between substances, their physical properties were used.

During the period of iatrochemistry (16th-17th centuries), the proportion of chemical research methods increased even more, especially the methods of "wet" qualitative research of substances transferred into solutions: for example, silver and hydrochloric acid were recognized by the reaction of their formation of a precipitate in a nitric acid medium; used reactions with the formation of colored products, such as iron with tannins.

The beginning of the scientific approach to chemical analysis was laid by the English scientist R. Boyle (17th century), when, after separating chemistry from alchemy and medicine and embarking on the soil of chemical atomism, he introduced the concept of a chemical element as an indecomposable component of various substances. According to Boyle, the subject of chemistry is the study of these elements and how they combine to form chemical compounds and mixtures. Boyle called the decomposition of substances into elements "analysis". The whole period of alchemy and iatrochemistry was largely a period of synthetic chemistry; many inorganic and some organic compounds have been obtained. But since synthesis was closely connected with analysis, it was analysis that was the leading direction in the development of chemistry at that time. New substances were obtained in the process of more and more refined decomposition of natural products.

Thus, almost until the middle of the 19th century. Chemistry developed mainly as A. x.; the efforts of chemists were aimed at developing methods for determining qualitatively different principles (elements), at establishing the quantitative laws of their interaction.

Of great importance in chemical analysis was the differentiation of gases, which were previously considered one substance; This research was initiated by the Dutch scientist van Helmont (17th century), who discovered carbon dioxide. The greatest success in these studies was achieved by J. Priestley, C. V. Scheele, and A. L. Lavoisier (18th century). Experimental chemistry received a solid foundation in the law of conservation of the mass of substances in chemical operations established by Lavoisier (1789). True, even earlier this law was expressed in a more general form by M. V. Lomonosov (1758), and the Swedish scientist T. A. Bergman used the conservation of the mass of substances for the purposes of chemical analysis. It is Bergman who is credited with creating a systematic course of qualitative analysis, in which the studied substances transferred to a dissolved state are then divided into groups using precipitation reactions with reagents and further divided into even smaller groups up to the possibility of determining each element separately. As the main group reagents, Bergman proposed hydrogen sulfide and alkalis, which are still used today. He also systematized qualitative analysis "dry way", by heating substances, which leads to the formation of "pearls" and plaques of various colors.

Further improvement of systematic qualitative analysis was carried out by the French chemists L. Vauquelin and L. J. Tenard, the German chemists G. Rose and K. R. Fresenius, and the Russian chemist N. A. Menshutkin. In the 20-30s. 20th century the Soviet chemist N. A. Tananaev, based on a significantly expanded set of chemical reactions, proposed a fractional method of qualitative analysis, in which there is no need for a systematic course of analysis, division into groups and the use of hydrogen sulfide.

Quantitative analysis was originally based on the precipitation reactions of the elements being determined in the form of poorly soluble compounds, the mass of which was then weighed. This weight (or gravimetric) method of analysis has also improved considerably since the time of Bergmann, mainly due to the improvement of weights and weighing techniques and the use of various reagents, in particular organic ones, which form the least soluble compounds. In the 1st quarter of the 19th century. The French scientist J. L. Gay-Lussac proposed a volumetric method of quantitative analysis (volumetric), in which, instead of weighing, the volumes of solutions of interacting substances are measured. This method, also called the titration method or titrimetric method, is still the main method of quantitative analysis. It has significantly expanded both due to an increase in the number of chemical reactions used in it (precipitation, neutralization, complexation, oxidation-reduction reactions), and due to the use of many indicators (substances that indicate by changes in their color the end of the reaction between interacting solutions), etc. means of indication (by determining the various physical properties of solutions, such as electrical conductivity or refractive index).

The analysis of organic substances containing carbon and hydrogen as the main elements by burning and determining the products of combustion - carbon dioxide and water - was first carried out by Lavoisier. It was further improved by J. L. Gay-Lussac and L. J. Tenard and J. Liebig. In 1911, the Austrian chemist F. Pregl developed a technique for the microanalysis of organic compounds, which requires only a few mg original substance. In view of the complex construction of molecules of organic substances, their large sizes (polymers), pronounced isomerism, organic analysis includes not only elemental analysis - determining the relative amounts of individual elements in a molecule, but also functional - determining the nature and number of individual characteristic atomic groups in a molecule. Functional analysis is based on the characteristic chemical reactions and physical properties of the compounds under study.

Almost until the middle of the 20th century. the analysis of organic substances, due to its specificity, developed in its own ways, different from inorganic analysis, and was not included in the academic courses in A. x. The analysis of organic substances was considered as part of organic chemistry. But then, with the emergence of new, mainly physical, methods of analysis, the widespread use of organic reagents in inorganic analysis, both of these branches of A. x. began to converge and now represent a single common scientific and educational discipline.

A. x. as a science it includes the theory of chemical reactions and the chemical properties of substances, and as such it coincided with it in the first period of the development of general chemistry. However, in the second half of the 19th century, when the “wet method,” that is, analysis in solutions, mainly aqueous solutions, occupied a dominant position in chemical analysis, the subject of A. x. began to study only those reactions that give an analytically valuable characteristic product - an insoluble or colored compound that occurs during a fast reaction. In 1894, the German scientist W. Ostwald first outlined the scientific foundations of A. x. as a theory of chemical equilibrium of ionic reactions in aqueous solutions. This theory, supplemented by the results of the entire subsequent development of ionic theory, became the basis of A. x.

The work of the Russian chemists M. A. Ilyinsky and L. A. Chugaev (late 19th century - early 20th century) laid the foundation for the use of organic reagents, characterized by high specificity and sensitivity, in inorganic analysis.

Studies have shown that each inorganic ion is characterized by a chemical reaction with an organic compound containing a certain functional group (the so-called functional-analytical group). Starting from the 20s. 20th century In chemical analysis, the role of instrumental methods began to increase, again returning analysis to the study of the physical properties of the analyzed substances, but not those macroscopic properties that the analysis operated in the period before the creation of scientific chemistry, but atomic and molecular properties. Modern A. x. widely uses atomic and molecular emission and absorption spectra (visible, ultraviolet, infrared, X-ray, radio frequency and gamma spectra), radioactivity (natural and artificial), isotope mass spectrometry, electrochemical properties of ions and molecules, adsorption properties, etc. (see Colorimetry , Luminescence , Microchemical analysis , Nephelometry , Activation analysis , Spectral analysis , Photometry , Chromatography , Electron paramagnetic resonance , Electrochemical methods of analysis). The application of methods of analysis based on these properties is equally successful in inorganic and organic analysis. These methods significantly deepen the possibilities of deciphering the composition and structure of chemical compounds, their qualitative and quantitative determination; they allow you to bring the sensitivity of the determination to 10 -12 - 10 -15% of an impurity, require a small amount of the analyte, and can often serve for the so-called. non-destructive testing (i.e., not accompanied by the destruction of a sample of a substance), can serve as the basis for automating the processes of production analysis.

At the same time, the widespread use of these instrumental methods poses new challenges for A. x. as a science, requires the generalization of methods of analysis not only on the basis of the theory of chemical reactions, but also on the basis of the physical theory of the structure of atoms and molecules.

A. x., which plays an important role in the progress of chemical science, is also of great importance in the control of industrial processes and in agriculture. Development A. x. in the USSR is closely connected with the industrialization of the country and the subsequent general progress. Departments of chemical chemistry have been organized in many higher educational institutions to train highly qualified chemists-analysts. Soviet scientists are developing the theoretical foundations of A. x. and new methods for solving scientific and practical problems. With the emergence of such industries as the nuclear industry, electronics, the production of semiconductors, rare metals, cosmochemistry, at the same time there was a need to develop new fine and finest methods for controlling the purity of materials, where in many cases the impurity content should not exceed one atom per 1-10 million atoms produced product. All these problems are being successfully solved by Soviet analytical chemists. Old methods of chemical production control are also being improved.

Development A. x. as a special branch of chemistry, the publication of special analytical journals in all industrialized countries of the world also brought to life. Two such journals have been published in the USSR—Factory Laboratory (since 1932) and Journal of Analytical Chemistry (since 1946). There are also specialized international journals on individual sections of A. x., for example, journals on chromatography and electroanalytical chemistry. Specialists in A. x. they are prepared at special departments of universities, chemical-technological technical schools and vocational schools.

Lit.: Alekseev V.N., Course of Qualitative Chemical Semimicroanalysis, 4th ed., M. 1962: his own. Quantitative Analysis, 2nd ed. , M., 1958; Lyalikov Yu.S., Physical and chemical methods of analysis, 4th ed., M., 1964; Yuing G. D. . Instrumental methods of chemical analysis, trans. from English, M., 1960; Lurie Yu. Yu., Handbook of analytical chemistry, M., 1962.

Yu. A. Klyachko.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

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