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

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

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

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 In a modern laboratory, you will not find home-made retorts - they have been replaced by standard glass equipment produced by industry and adapted specifically for performing a particular chemical procedure.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

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

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

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

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

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

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

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

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

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

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

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

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

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

Lecture 9. Fundamentals of quantitative analysis.

1. Classification of methods of chemical analysis.

2. Types of gravimetric determinations.

3. General characteristics of the gravimetric method of analysis.

4. Volumetric titrimetric method of analysis.

5. Calculations in titrimetric analysis.

6. Methods of titrimetric analysis.

D.Z. according to account Pustovalov pp. 181-218.

Classification of methods of chemical analysis.

Col and honest en a liz - Col.a. - a set of chemical, physico-chemical and physical methods for determining the quantitative ratio of the components that make up the analyte.

Quantitative analysis methods:

1) chemical (gravimetry, titrimetry, gas analysis);

2) physical and chemical method (photometry, electrochemical, chromatographic analysis);

3) physical-spectral: luminescent, etc.

Along with a qualitative analysis, Kol. a. is one of the main branches of analytical chemistry. According to the amount of the substance taken for analysis, macro-, semi-micro-, micro- and ultra-micro methods are distinguished. K. a. In macro methods, the sample mass is usually >100 mg, solution volume > 10 ml; in ultramicromethods - respectively 1-10 -1 mg and 10 -3 -10 -6 ml(see also Microchemical analysis, Ultramicrochemical analysis) . Depending on the object of study, inorganic and organic K. a. are distinguished, which, in turn, are divided into elemental, functional, and molecular analysis.. Elemental analysis allows you to determine the content of elements (ions), functional analysis - the content of functional (reactive) atoms and groups in the analyzed object. Molecular K. a. involves the analysis of individual chemical compounds characterized by a certain molecular weight. Of great importance is the so-called phase analysis - a set of methods for separating and analyzing individual structural (phase) components of heterogeneous systems. In addition to specificity and sensitivity (see Qualitative analysis), an important characteristic of K.'s methods and. - accuracy, that is, the value of the relative error of determination; accuracy and sensitivity in K. a. expressed as a percentage.

To the classical chemical methods of K. a. include: gravimetric analysis, based on an accurate measurement of the mass of the analyte, and volumetric analysis. The latter includes volumetric titrimetric analysis - methods for measuring the volume of a reagent solution consumed in a reaction with an analyte, and gas volume analysis - methods for measuring the volume of analyzed gaseous products (see Titrimetric analysis, Gas analysis) .

Along with classical chemical methods, physical and physicochemical (instrumental) methods of CA are widely used, based on the measurement of the optical, electrical, adsorption, catalytic, and other characteristics of analyzed substances, which depend on their quantity (concentration). Usually these methods are divided into the following groups: electrochemical (conductometry, polarography, potentiometry, etc.); spectral or optical (emission and absorption spectral analysis, photometry, colorimetry, nephelometry, luminescence analysis, etc.); X-ray (absorption and emission X-ray spectral analysis, X-ray phase analysis, etc.); chromatographic (liquid, gas, gas-liquid chromatography, etc.); radiometric (activation analysis, etc.); mass spectrometric. The listed methods, inferior to chemical ones in accuracy, significantly exceed them in sensitivity, selectivity, speed of execution. Accuracy of chemical methods K. a. is usually in the range of 0.005-0.1%; errors in the determination by instrumental methods are 5-10%, and sometimes much more. Sensitivity of some methods To. and. is given below (%):

Volume................................................. ......10 -1

Gravimetric .......................................... 10 -2

Emission Spectral..............................10 -4

Absorption X-ray spectral ...... 10 -4

Mass spectrometric ............................... 10 -4

Coulometric .............................................. 10 -5

Fluorescent ............................................... 10 -6 -10 -5

Photometric colorimetric ......... 10 -7 -10 -4

Polarographic .........................................10 -8 -10 -6

Activation .............................................................10 -9 -10 -8

When using physical and physico-chemical methods To. and. as a rule, microquantities of substances are required. Analysis can in some cases be performed without destroying the sample; sometimes continuous and automatic recording of results is also possible. These methods are used to analyze high purity substances, evaluate product yields, study the properties and structure of substances, etc. See also Electrochemical methods of analysis, Spectral analysis, Chromatography, Kinetic methods of analysis, Nephelometry, Colorimetry, Activation analysis.

1) chemical methods of analysis:

Gravimetric- based on the determination of the mass of a substance isolated in pure form or in the form of a compound of known composition.

the positive side "+" - gives a result of high strength,

the negative side of the "-" is a very time-consuming job.

Titrimetric -(volumetric) - based on an accurate measurement of the reagent spent on the reaction with a certain component. The reagent is taken in the form of a solution of a certain concentration (titrated solution).

High speed of analysis;

Less accurate result compared to gravimetry.

Depending on the type of reactions occurring during the titration, the following are distinguished: methods:

Acid-base titration methods,

Reductive titration method,

precipitation method,

Complex formation.

2) Physico-chemical method- based on the measurement of absorption, transmission, scattering of light by the determined solution.

For most photometric methods, the color intensity of the solution is estimated visually or using appropriate instruments.

It is used for a specific component that is part of the analyte in very small quantities;

The accuracy of the method is lower than in gravimetry and titrimetry.

Electrochemical methods- electrogravimetric analysis, conductometry, potentiometry and polarography.

Chromatographic Method- based on the use of the phenomenon of selective adsorption of a solution of a substance and ions by various substances or adsorbents: Al 2 O 3 , silica gel, starch, talc,

permutide, synthetic resins and other substances.

Application: both in quantitative analysis and in qualitative analysis, especially widely used for the determination of substances and ions.

There are many types of analysis. They can be classified according to different criteria:

- by the nature of the information received. Distinguish qualitative analysis(in this case, they find out what this substance consists of, which components are included in its composition) and quantitative analysis(determine the content of certain components, for example, in% by weight, or the ratio of different components). The line between qualitative and quantitative analysis is very conditional, especially in the study of microimpurities. So, if in the course of a qualitative analysis a certain component was not detected, then it is necessary to indicate what the minimum amount of this component could be detected using this method. Perhaps the negative result of a qualitative analysis is not due to the absence of a component, but to the insufficient sensitivity of the method used! On the other hand, quantitative analysis is always performed taking into account the previously found qualitative composition of the material under study.

- classification by objects of analysis: technical, clinical, forensic and etc.

- classification by objects of definition.

Do not confuse terms - analyze and determine. Objects definitions name the components whose content needs to be established or reliably detected. Taking into account the nature of the component being determined, various types of analysis are distinguished (Table 1.1).

Table 1-1. Classification of types of analysis (by objects of definition or detection)

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

The classification "by objects of definition" is very important, as it helps to choose the appropriate way to carry out the analysis (analytical method). Yes, for elemental analysis often used spectral methods based on the registration of radiation of atoms at different wavelengths. Most spectral methods involve complete destruction (atomization) of the analyte. If it is necessary to establish the nature and quantitative content of various molecules that make up the composition of the organic substance under study ( molecular analysis), then one of the most suitable methods will be chromatographic, which does not involve the destruction of molecules.

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

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

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

When it comes to determining the total content of all molecules that have some common structural features, the same functional groups, and hence similar chemical properties, use the term structural-group(or functional) analysis. For example, the sum of alcohols (organic compounds having an OH group) is determined by conducting a reaction common to all alcohols with metallic sodium, and then measuring the volume of hydrogen released. The amount of unsaturated hydrocarbons (having double or triple bonds) is determined by oxidizing them with iodine. The total content of the same type of components is sometimes also established in inorganic analysis - for example, the total content of rare earth elements.

A specific type of analysis is phase analysis. So, carbon in cast irons and steels can dissolve in iron, can form chemical compounds with iron (carbides), or can form a separate phase (graphite). The physical properties of the product (strength, hardness, etc.) depend not only on the total carbon content, but also on the distribution of carbon between these forms. Therefore, metallurgists are interested not only in the total carbon content in cast iron or steel, but also in the presence of a separate phase of graphite (free carbon) in these materials, as well as the quantitative content of this phase.

The main focus of the basic course in analytical chemistry is elemental and molecular analysis. In other types of analysis, very specific methods are used, and isotope, phase and structural group analyzes are not included in the basic course program.

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

The direct opposite of express analysis - arbitration analysis. The main requirement for it is to ensure the greatest possible accuracy of the results. Arbitration analyzes are carried out quite rarely (for example, to resolve a conflict between a manufacturer and a consumer of industrial products). To perform such analyzes, the most qualified performers are involved, the most reliable and repeatedly proven methods are used. The time spent on performing such an analysis, as well as its cost, are of no fundamental importance.

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

There are other ways of classification, other types of analysis. For example, take into account the mass of the material under study, directly used in the course of the analysis. Within the framework of the corresponding classification, there are macroanalysis(kilograms, liters), semi-microanalysis(fractions of a gram, milliliters) and microanalysis. In the latter case, weighings of the order of a milligram or less are used, the volumes of solutions are measured in microliters, and the result of the reaction sometimes has to be observed under a microscope. Microanalysis is rarely used in analytical laboratories.

1.3. Analysis Methods

The concept of "method of analysis" is the most important for analytical chemistry. This term is used when they want to reveal the essence of this or that analysis, its main principle. The method of analysis is a fairly universal and theoretically justified way of conducting an analysis, regardless of which component is determined and what exactly is analyzed. There are three main groups of methods (Fig. 1-1). Some of them are aimed primarily at separating the components of the mixture under study (subsequent analysis without this operation turns out to be inaccurate or even impossible). In the course of separation, the concentration of the components to be determined usually also occurs (see Chapter 8). An example would be extraction methods or ion exchange methods. Other methods are used in the course of qualitative analysis, they serve for reliable identification (identification) of the components of interest to us. The third, the most numerous, are intended for the quantitative determination of components. The respective groups are called methods of separation and concentration, methods of identification and methods of determination. The methods of the first two groups, as a rule , play a supporting role; they will be discussed later. The most important for practice are determination methods.

In addition to the three main groups, there are hybrid methods. Figure 1.1 does not show these methods. In hybrid methods, separation, identification and determination of components are organically combined in one instrument (or in a single set of instruments). The most important of these methods is chromatographic analysis. In a special device (chromatograph), the components of the test sample (mixture) are separated, since they move at different speeds through a column filled with solid powder (sorbent). By the time of release of the component from the column, its nature is judged and thus all components of the sample are identified. The components leaving the column in turn fall into another part of the device, where a special device - a detector - measures and records the signals of all components. Often, the automatic calculation of the contents of all components is immediately carried out. It is clear that chromatographic analysis cannot be considered only as a method of separation of components, or only as a method of quantitative determination, it is precisely a hybrid method.

Each method of determination combines many specific methods in which the same physical quantity is measured. For example, to carry out a quantitative analysis, one can measure the potential of an electrode immersed in the test solution, and then, using the found potential value, calculate the content of a certain component of the solution. All methods, where the main operation is to measure the potential of the electrode, are considered special cases. potentiometric method. When attributing a technique to one or another analytical method, it does not matter which object is being studied, which substances and with what accuracy are determined, which device is used and how the calculations are carried out - it is only important what we are measuring. The physical quantity measured during the analysis, which depends on the concentration of the analyte, is usually called analytical signal.

In a similar way, one can single out the method spectral analysis. In this case, the main operation is the measurement of the intensity of light emitted by the sample at a certain wavelength. Method titrimetric (volumetric) analysis is based on measuring the volume of the solution spent on the chemical reaction with the determined component of the sample. The word "method" is often omitted, they simply say "potentiometry", "spectral analysis", "titrimetry", etc. AT refractometric analysis the signal is the refractive index of the test solution, in spectrophotometry- absorption of light (at a certain wavelength). The list of methods and their corresponding analytical signals can be continued; in total, several dozen independent methods are known.

Each method of determination has its own theoretical basis and is associated with the use of specific equipment. The areas of application of different methods differ significantly. Some methods are mainly used for the analysis of petroleum products, others - for the analysis of drugs, others - for the study of metals and alloys, etc. Similarly, methods for elemental analysis, methods of isotopic analysis, etc. can be distinguished. There are also universal methods used in the analysis of a wide variety of materials and suitable for determining the most diverse components in them. For example, the spectrophotometric method can be used for elemental, molecular, and structural group analysis.

Accuracy, sensitivity, and other characteristics of individual methods related to the same analytical method differ, but not as much as the characteristics of different methods. Any analytical problem can always be solved by several different methods (for example, chromium in alloyed steel can be determined by the spectral method, and titrimetric, and potentiometric). The analyst chooses a method, taking into account the known capabilities of each of them and the specific requirements for this analysis. It is impossible to choose the “best” and “worst” methods once and for all, everything depends on the problem being solved, on the requirements for the analysis results. Thus, gravimetric analysis, as a rule, gives more accurate results than spectral analysis, but it requires a lot of labor and time. Therefore, gravimetric analysis is good for arbitration analysis, but not suitable for express analysis.

The determination methods are divided into three groups: chemical, physical and physico-chemical. Often, physical and physico-chemical methods are combined under the common name “instrumental methods”, since in both cases instruments are used, and the same ones. In general, the boundaries between groups of methods are very arbitrary.

Chemical Methods are based on carrying out a chemical reaction between the determined component and a specially added reagent. The reaction proceeds according to the scheme:

Hereinafter, the symbol X denotes the component being determined (molecule, ion, atom, etc.), R is the added reagent, Y is the totality of reaction products. The group of chemical methods includes classical (long-known and well-studied) methods of determination, primarily gravimetry and titrimetry. The number of chemical methods is relatively small, they all have the same theoretical foundations (the theory of chemical equilibrium, the laws of chemical kinetics, etc.). As an analytical signal in chemical methods, the mass or volume of a substance is usually measured. Complex physical instruments, with the exception of analytical balances, and special standards of chemical composition are not used in chemical methods. These methods have much in common in terms of their capabilities. They will be discussed in chapter 4.

Physical Methods not associated with chemical reactions and the use of reagents. Their main principle is the comparison of the same type of analytical signals of the X component in the material under study and in a certain reference (sample with a precisely known concentration of X). Having built a calibration graph in advance (the dependence of the signal on the concentration or mass X) and measuring the signal value for a sample of the material under study, the X concentration in this material is calculated. There are other ways to calculate concentrations (see Chapter 6). Physical methods are usually more sensitive than chemical ones; therefore, the determination of microimpurities is carried out mainly by physical methods. These methods are easy to automate and require less time for analysis. However, physical methods require special standards, rather complex, expensive and highly specialized equipment. In addition, they are usually less accurate than chemical ones.

An intermediate place between chemical and physical methods in terms of their principles and capabilities is occupied by physical and chemical analysis methods. In this case, the analyst conducts a chemical reaction, but its course or its result is followed not visually, but with the use of physical instruments. For example, it gradually adds to the test solution another - with a known concentration of the dissolved reagent, and at the same time controls the potential of the electrode dipped into the titrated solution (potentiometric titration), The analyst judges the completion of the reaction by the jump in potential, measures the volume of titrant spent on it, and calculates the result of the analysis. Such methods are generally as accurate as chemical methods and almost as sensitive as physical methods.

Instrumental methods are often divided according to another, more clearly expressed feature - the nature of the measured signal. In this case, subgroups of optical, electrochemical, resonant, activation and other methods are distinguished. There are also few and as yet underdeveloped methods biological and biochemical methods.

1. Sampling:

A laboratory sample consists of 10-50 g of material, which is taken so that its average composition corresponds to the average composition of the entire lot of the analyte.

2. Decomposition of the sample and its transfer to the solution;

3. Carrying out a chemical reaction:

X is the component to be determined;

P is the reaction product;

R is a reagent.

4. Measurement of any physical parameter of the reaction product, reagent or analyte.

Classification of chemical methods of analysis

I By reaction components

1. Measure the amount of reaction product P formed (gravimetric method). Create conditions under which the analyte is completely converted into a reaction product; further, it is necessary that the reagent R does not give minor reaction products with foreign substances, the physical properties of which would be similar to the physical properties of the product.

2. Based on the measurement of the amount of the reagent consumed in the reaction with the analyte X:

– the action between X and R must be stoichiometric;

- the reaction must proceed quickly;

– the reagent must not react with foreign substances;

– a way to establish the equivalence point is needed, i.e. the moment of titration when the reagent is added in an equivalent amount (indicator, color change, potential island, electrical conductivity).

3. Records the changes that occur with the analyte X itself in the process of interaction with the reagent R (gas analysis).

II Types of chemical reactions

1. Acid-base.

2. Formation of complex compounds.

Acid-base reactions: used mainly for the direct quantitative determination of strong and weak acids and bases, and their salts.

Reactions for the formation of complex compounds: determined substances are converted into complex ions and compounds by the action of reagents.

The following separation and determination methods are based on complex formation reactions:

1) Separation by means of precipitation;

2) Extraction method (water-insoluble complex compounds often dissolve well in organic solvents - benzene, chloroform - the process of transferring complex compounds from aqueous phases to dispersed ones is called extraction);

3) Photometric (Co with nitrous salt) - measure the optimal density of solutions of complex compounds;

4) Titrimetric analysis method

5) Gravimetric method of analysis.

1) cementation method - reduction of metal Me ions in solution;

2) electrolysis with a mercury cathode - during the electrolysis of a solution with a mercury cathode, ions of many elements are reduced by electric current to Me, which dissolve in mercury, forming an amalgam. The ions of other Me remain in solution;

3) identification method;

4) titrimetric methods;

5) electrogravimetric - an el is passed through the test solution. a current of a certain voltage, while the Me ions are restored to the Me state, the released is weighed;

6) coulometric method - the amount of a substance is determined by the amount of electricity that must be spent for the electrochemical transformation of the analyzed substance. Analysis reagents are found according to Faraday's law:

M is the amount of the element being determined;

F is the Faraday number (98500 C);

A is the atomic mass of the element;

n is the number of electrons involved in the electrochemical transformation of a given element;

Q is the amount of electricity (Q = I ∙ τ).

7) catalytic method of analysis;

8) polarographic;

III Classification of separation methods based on the use of various types of phase transformations:

The following types of equilibria between phases are known:

Equilibrium L-G or T-G is used in the analysis when substances are released into the gas phase (CO 2 , H 2 O, etc.).

Equilibrium W 1 - W 2 is observed in the extraction method and in electrolysis with a mercury cathode.

Zh-T is typical for the processes of deposition and the processes of precipitation on the surface of the solid phase.

Analysis methods include:

1. gravimetric;

2. titrimetric;

3 optical;

4. electrochemical;

5. catalytic.

Separation methods include:

1. precipitation;

2. extraction;

3. chromatography;

4. ion exchange.

Concentration methods include:

1. precipitation;

2. extraction;

3. grouting;

4. stripping.

Physical methods of analysis

A characteristic feature is that they directly measure any physical parameters of the system associated with the amount of the element being determined without prior chemical reaction.

Physical methods include three main groups of methods:

I Methods based on the interaction of radiation with a substance or on the measurement of the radiation of a substance.

II Methods based on measuring the parameters of el. or magnetic properties of matter.

IIIMethods based on the measurement of density or other parameters of the mechanical or molecular properties of substances.

Methods based on the energy transition of the outer valence electrons of atoms: include atomic emission and atomic absorption methods of analysis.

Atomic emission analysis:

1) Flame photometry - the analyzed solution is sprayed into the flame of a gas burner. Under the influence of high temperature, the atoms go into an excited state. The outer valence electrons move to higher energy levels. The reverse transition of electrons to the main energy level is accompanied by radiation, the wavelength of which depends on the atoms of which element were in the flame. The intensity of radiation under certain conditions is proportional to the number of atoms of the element in the flame, and the wavelength of radiation characterizes the qualitative composition of the sample.

2) Emission method of analysis - spectral. The sample is introduced into the flame of an arc or a condensed spark, under high temperature the atoms pass into an excited state, while the electrons pass not only to the closest to the main, but also to more distant energy levels.

Radiation is a complex mixture of light vibrations of different wavelengths. The emission spectrum is decomposed into the main parts of the special. instruments, spectrometers, and photographing. Comparison of the position of the intensity of individual lines of the spectrum with the lines of the corresponding standard, allows you to determine the qualitative and quantitative analysis of the sample.

Atomic absorption methods of analysis:

The method is based on measuring the absorption of light of a certain wavelength by unexcited atoms of the element being determined. A special radiation source produces resonant radiation, i.e. radiation corresponding to the transition of an electron to the lowest orbital with the lowest energy, from the orbital closest to it with a higher energy level. The decrease in the intensity of light when it passes through the flame due to the transfer of the electrons of the atoms of the element being determined into an excited state is proportional to the number of unexcited atoms in it. In atomic absorption, combustible mixtures with temperatures up to 3100 ° C are used, which increases the number of elements to be determined, in comparison with flame photometry.

X-ray fluorescent and X-ray emission

X-ray fluorescent: the sample is exposed to x-rays. top electrons. The orbitals closest to the nucleus of the atom are knocked out of the atoms. Their place is taken by electrons from more distant orbitals. The transition of these electrons is accompanied by the appearance of secondary X-ray radiation, the wavelength of which is functionally related to the atomic number of the element. Wavelength - qualitative composition of the sample; intensity - the quantitative composition of the sample.

Methods based on nuclear reactions - radioactive. The material is exposed to neutron radiation, nuclear reactions occur and radioactive isotopes of elements are formed. Next, the sample is transferred into a solution and the elements are separated by chemical methods. After that, the intensity of radioactive radiation of each element of the sample is measured, and the reference sample is analyzed in parallel. The intensity of radioactive radiation of individual fractions of the reference sample and the analyzed material is compared and conclusions are drawn about the quantitative content of elements. Limit of detection 10 -8 - 10 -10%.

1. Conductometric - based on measuring the electrical conductivity of solutions or gases.

2. Potentiometric - there is a method of direct and potentiometric titration.

3. Thermoelectric - based on the occurrence of thermoelectromotive force, which arose when heating the place of contact of steel, etc. Me.

4. Mass spectral - is used with the help of strong elements and magnetic fields, gas mixtures are separated into components in accordance with the atoms or molecular weights of the components. It is used in the study of a mixture of isotopes. inert gases, mixtures of organic substances.

Densitometry - based on the measurement of density (determination of the concentration of substances in solutions). To determine the composition, viscosity, surface tension, sound speed, electrical conductivity, etc. are measured.

To determine the purity of substances, the boiling point or melting point is measured.

Prediction and calculation of physical and chemical properties

Theoretical foundations for predicting the physicochemical properties of substances

Approximate prediction calculation

Prediction implies an assessment of physicochemical properties based on a minimum number of readily available initial data, and may also assume the complete absence of experimental information about the properties of the substance under study (“absolute” prediction relies only on information about the stoichiometric formula of the compound).

CHEMICAL ANALYSIS

Analytical chemistry. Tasks and stages of chemical analysis. Analytical signal. Classification of methods of analysisbehind. Identification of substances. Fractional analysis. Systematic analysis.

Main tasks of analytical chemistry

One of the tasks in carrying out environmental protection measures is the knowledge of the patterns of cause-and-effect relationships between various types of human activity and changes occurring in the natural environment. Analysis It is the main means of controlling environmental pollution. The scientific basis of chemical analysis is analytical chemistry. Analytical chemistry - the science of methods and means for determining the chemical composition of substances and materials. Method- this is a fairly universal and theoretically justified way to determine the composition.

Basic requirements for methods and techniques of analytical chemistry:

1) correctness and good reproducibility;

2) low detection limit- this is the lowest content at which the presence of the determined component with a given confidence probability can be detected using this method;

3) selectivity (selectivity)- characterizes the interfering influence of various factors;

4) range of measured contents(concentrations) using this method according to this method;

5) expressiveness;

6) simplicity in analysis, the possibility of automation, cost-effectiveness of determination.

Chemical analysis is a complex multi-stage about cess, which is a collection of ready-made techniques and related services.

Analysis tasks

1. Identification of the object, i.e. establishing the nature of the object (checking the presence of certain main components, impurities).

2. Quantitative determination of the content of one or another component in the analyzed object.

Stages of analysis of any object

1. Statement of the problem and choice of method and scheme of analysis.

2. Sampling (competent sampling of a part of the sample allows you to draw the correct conclusion about the composition of the entire sample). Try- this is a part of the analyzed material, representative of the a chewing its chemical composition. In some cases, the entire analytical material is used as a sample. Sample storage time should be kept to a minimum. eh nym. Storage conditions and methods should exclude uncontrolled loss of volatile compounds and any other physical and chemical changes in the composition of the analyzed sample.

3. Preparation of samples for analysis: transferring the sample to the desired state (solution, steam); separation of components or separation of interfering; concentration of components;

4. Obtaining an analytical signal. Analytical signal- this is a change in any physical or physico-chemical property of the determined component, functionally related to its content (formula, table, graph).

5. Analytical signal processing, i.e. separation of signal and noise. Noises- side signals arising in measuring instruments, amplifiers and other devices.

6. Application of the results of the analysis. Depending on the property of the substance underlying the definition, the methods of analysis are divided into:

On the chemical methods analysis based on a chemical analytical reaction, which is accompanied by a pronounced effect. These include gravimetric and titrimetric methods;

- physical and chemical methods, based on the measurement of any physical parameters of a chemical system that depend on the nature of the components of the system and change during a chemical reaction (for example, photometry is based on a change in the optical density of a solution as a result of a reaction);

- physical methods analysis not involving the use of chemical reactions. The composition of substances is established by measuring the characteristic physical properties of the object (for example, density, viscosity).

Depending on the measured value, all methods are divided into the following types.

Methods for measuring physical quantities

Measured physical quantity	Method name
	Gravimetry
	Titrimetry
Equilibrium potential of the electrode	Potentiometry
Polarization resistance of the electrode	polarography
The amount of electricity	Coulometry
Solution conductivity	Conductometry
Photon absorption	Photometry
Emission of photons	Emission spectral analysis

Substance identification is based on methods of qualitative recognition of elementary objects (atom, molecules, ions, etc.) that make up substances and materials.

Very often, the analyzed sample of a substance is converted into a form convenient for analysis by dissolving in a suitable solvent (usually water or aqueous acid solutions) or fusing with some chemical compound, followed by dissolution.

Chemical methods of qualitative analysis are based on using reactions of identifiable ions with certain substances - analytical reagents. Such reactions should be accompanied by precipitation or dissolution of the precipitate; the appearance, change or disappearance of the color of the solution; release of gas with a characteristic odor; the formation of crystals of a certain shape.

Reactions that take place in solutions by way of execution are classified into test-tube, microcrystalloscopic and drip. Microcrystalloscopic reactions are carried out on a glass slide. Observe the formation of crystals of a characteristic shape. Drop reactions are performed on filter paper.

Analytical reactions used in qualitative analysis, by area of ​​application share:

1.) on group reactions- these are reactions for the precipitation of a whole group of ions (one reagent is used, which is called group);

2;) characteristic reactions:

a) selective (selective)- give the same or similar analytical reactions with a limited number of ions (2~5 pcs.);

b) specific (highly selective)- selective towards alone component.

There are few selective and specific reactions, so they are used in combination with group reactions and with special techniques to eliminate the interfering influence of the components present in the system along with the substance being determined.

Simple mixtures of ions are analyzed fractional method, without prior separation of interfering ions, individual ions are determined by means of characteristic reactions. M destroying ion- this is an ion that, under the conditions of detection of the desired one, gives a similar analytical effect with the same reagent or an analytical effect that masks the desired reaction. The detection of different ions in fractional analysis is carried out in separate portions of the solution. If it is necessary to eliminate interfering ions, use the following methods of separation and camouflage.

1. Conversion of interfering ions to precipitate. The basis is the difference in the magnitude of the solubility product of the resulting precipitates. In this case, the PR of the connection of the ion being determined with the reagent should be greater than the PR of the connection of the interfering ion.

2. Binding of interfering ions into a strong complex compound. The resulting complex must have the necessary stability in order to complete the binding of the interfering ion, and the desired ion must not react at all with the introduced reagent, or its complex must be unstable.

3. Change in the oxidation state of interfering ions.

4. The use of extraction. The method is based on the extraction of interfering ions from aqueous solutions with organic solvents and the separation of the system into its component parts (phases) so that the interfering and determined components are in different phases.

Advantages of fractional analysis:

Speed ​​of execution, as the time for long-term operations of sequential separation of some ions from others is reduced;

Fractional reactions are easily reproducible; they can be repeated several times. However, if it is difficult to select selective (specific) reactions for detecting ions, masking reagents, calculating the completeness

removal of ions and other causes (complexity of the mixture) resort to performing a systematic analysis.

Systematic analysis- this is a complete (detailed) analysis of the object under study, which is carried out by dividing all the components in the sample into several groups in a certain sequence. The division into groups is based on the similarity (within the group) and differences (between groups) of the analytical properties of the components. In a dedicated analysis group, a series of successive separation reactions are used until only components that give characteristic reactions with selective reagents remain in one phase (Fig. 23.1).

Several analytical classifications have been developed ka thions and anions into analytical groups, which are based on the use of group reagents (i.e., reagents for isolating a whole group of ions under specific conditions). Group reagents in the analysis of cations serve both for detection and for separation, and in the analysis of anions - only for detection (Fig. 23.2).

Analysis of mixtures of cations

Group reagents in the qualitative analysis of cations are acids, strong bases, ammonia, carbonates, phosphates, alkali metal sulfates, oxidizing and reducing agents. The combination of substances into analytical groups is based on the use of similarities and differences in their chemical properties. The most important analytical properties include the ability of an element to form various types of ions, the color and solubility of compounds, the ability to enter in certain reactions.

The group reagents are selected from the general reagents because the group reagent is required to release a relatively large number of ions. The main method of separation is precipitation, i.e. the division into groups is based on the different solubility of cationic precipitates in certain media. When considering the action of group reagents, the following groups can be distinguished (Table 23.2).

In addition, three cations remain (Na + , K + , NH4) that do not form precipitates with the indicated group reagents. They can also be separated into a separate group.

Cation groups

In addition to the indicated general approach, when choosing group reagents, one proceeds from the values ​​of precipitation solubility products, since, by varying the precipitation conditions, it is possible to separate substances from a group by the action of the same reagent.

The most widespread is the acid-base classification of cations. Advantages of the acid-base method of systematic analysis:

a) the basic properties of the elements are used - their relationship to acids, alkalis;

b) analytical groups of cations to a greater extent co correspond to the groups of the periodic system of elements D.I. Mendeleev;

c) the analysis time is significantly reduced in comparison with the hydrogen sulfide method. The study begins with preliminary tests, in which the pH of the solution is set by a universal indicator and NH 4, Fe 3+, Fe 2+ ions are detected by specific and selective reactions.

Division into groups. The general scheme of division into groups given in table. 23.3. In the analyzed solution, first of all, cations of groups I and II are separated. To do this, 10-15 drops of the solution are placed in a test tube and a mixture of 2M HCl and 1M H 2 S0 4 is added dropwise. The precipitate is left for 10 min, then it is centrifuged and washed with water acidified with HCl. A mixture of chlorides and sulfates Ag + , Pb 2+ , Ba 2+ , Ca 2+ remains in the precipitate. The presence of basic antimony salts is possible. In solution - cations III-VI groups.

Group III is separated from the solution by adding a few drops of 3% H 2 0 2 and an excess of NaOH while heating and stirring. Excess hydrogen peroxide is removed by boiling. In the sediment - hydroxides of cations of groups IV-V, in solution - cations of groups III and VI and partially Ca 2+, which may not completely precipitate in the form of CaS0 4 when separating groups I and II.

Group V cations are separated from the precipitate. The precipitate is treated with 2N Na 2 CO 3 and then with excess NH 3 while heating. Group V cations pass into solution in the form of ammonia, in the precipitate - carbonates and basic salts of group IV cations.

The virtue of systematic analysis- Obtaining sufficiently complete information about the composition of the object. Flaw- bulkiness, duration, laboriousness. Complete schemes of systematic qualitative analysis are rarely carried out. Usually they are used partially if there is information about the origin, approximate composition of the sample, a So in the course of analytical chemistry.

Magnesium hydroxide dissolves in a mixture of NH 3 + NH 4 C1. Thus, after dividing the cations into groups, four test tubes were obtained containing a) a precipitate of chlorides and sulfates of cations of I-II groups; b) a solution of a mixture of cations III and VI groups; c) a solution of ammoniates of group V cations; d) sediment of carbonates and basic salts of group IV cations. Each of these objects is analyzed separately.

Analysis of anion mixtures

General characteristics of the studied anions. Anions are formed mainly by elements of groups IV, V, VI and VII of the periodic system. One and the same element can form several anions that differ in their properties. For example, sulfur forms anions S 2 -, S0 3 2 ~, S0 4 2 ~, S 2 0 3 2 ~, etc.

All anions are constituents of acids and corresponding branching salts. Depending on the composition of which substance the anion is included, its properties change significantly. For example, for the ion SO 4 2 "in the composition of concentrated sulfuric acid, oxidation-reduction reactions are characteristic, and in the composition of salts - precipitation reactions.

The state of anions in a solution depends on the medium of the solution. Some anions decompose under the action of concentrated acids with the release of the corresponding gases: CO 2 (anion CO 2-3), H 2 S (anion S 2 "), N0 2 (anion N0 3), etc. Under the action of dilute acids, anions MoO 4 2 -, W0 4 2 ~, SiO 3 2 "form water-insoluble acids (H 2 Mo0 4, H 2 W0 4 * H 2 0, H 2 SiO 3 ). Anions of weak acids (C0 3 2 ~, P0 4 ", Si0 3 2 ~, S 2") in aqueous solutions are partially or completely hydrolyzed, for example:

S 2 "+ H 2 0 →HS" + OH _.

Most of the elements that form anions have a variable valence and, under the action of oxidizing or reducing agents, change the oxidation state, while changing the composition of the anion. Chloride ion, for example, can be oxidized to C1 2, ClO", ClO 3, ClO 4. Iodide ions, for example, are oxidized to I 2, IO 4; sulfide ion S 2 ~ - to S0 2, SO 4 2- ; anions N0 3 can be reduced to N0 2, NO, N 2, NH 3.

Reducing anions (S 2 ~, I - , CI -) reduce Mn0 4 - ions in an acidic environment, causing their discoloration. oxidizing ions (NO3 , CrO 4 2 ", V0 3 -, Mn0 4 ~) oxidize iodide ions to acid oh medium to a free ion, color diphenylamine blue. These properties are used for qualitative analysis, the redox properties of chromate, nitrate, iodide, vanadate, molybdate, tungstate ions underlie them typical reactions.

Group reactions of anions. According to their action on anions, the reagents are divided into the following groups:

1) reagents that decompose substances with the release of gases. These reagents include dilute mineral acids (HC1, H 2 S0 4);

2) reagents that release anions from solutions in the form of slightly dissolved precipitates (Table 23.4):

a) ВаС1 2 in a neutral medium or in the presence of Ba (OH) 2 precipitates: SO 2-, SO, 2 ", S 2 0 3 2 ~, CO 3 2", PO 4 2 ", B 4 0 7 2 ~, As0 3 4 ", SiO 3 2";

b) AgNO 3 in 2n HNO 3 precipitates: SG, Br - , I - , S 2- (SO 4 2 only in concentrated solutions);

3) reducing agents (KI) (Table 23.5);

4) oxidizing reagents (KMn0 4, solution of I 2 in KI, HNO 3 (conc), H 2 S0 4).

Anions in the analysis basically do not interfere with the detection of each other, therefore, group reactions are used not for separation, but for preliminary verification of the presence or absence of a particular group of anions.

Systematic methods for the analysis of a mixture of anions, based nye on dividing them into groups, are rarely used, mainly zom for the study of simple mixtures. The more complex the mixture of anions, the more cumbersome the analysis schemes become.

Fractional analysis makes it possible to detect anions that do not interfere with each other in separate portions of the solution.

In semi-systematic methods, the separation of anions into groups using group reagents and the subsequent fractional detection of anions takes place. This leads to a reduction in the number of required sequential analytical operations and ultimately simplifies the scheme for analyzing an anion mixture.

The current state of qualitative analysis is not limited to the classical scheme. In the analysis of both inorganic, So and organic substances, instrumental methods are often used, such as luminescence, absorption spectroscopic, various electrochemical methods, which are options for chromatography, etc. However, in a number of cases (field, factory express laboratories, etc.), classical analysis has not lost its significance due to its simplicity, accessibility, and low cost.