Concentration methods include: Methods for isolating, separating and concentrating substances in analytical chemistry

In the practice of chemical analysis, situations often arise when the reliable and accurate determination of a component is hampered by other components present in the analyzed sample, including the main ones that make up the sample matrix. There are two ways to eliminate the influence of throwing components. The first method, the so-called masking, consists of converting the interfering component into an analytically inactive form. This operation can be carried out directly in the analytical system, and the interfering components remain in the same system.

This technique is not always possible to implement, especially when analyzing multicomponent mixtures. In this case, the second method is used - separation of components and (or) concentration of the component being determined. Concentration of the component being determined is also used if its content in the analyzed system is below the detection limit of the selected analytical method. Separation and concentration operations are often combined.

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

Concentration - an operation (process) that results in an increase in the ratio of the concentration or amount of a microcomponent to the concentration or amount of a macrocomponent.

When concentrated, microcomponents are either collected in a smaller volume or mass ( absolute concentration), or are separated from the macrocomponent in such a way that the ratio of the concentration of the microcomponent to the concentration of the macrocomponent increases ( relative concentration). An example of absolute concentration is the evaporation of a sample when analyzing natural waters.

Distinguish group And individual separation and concentration. With a group treatment, several components are released at one time, with an individual treatment - one.

Many separation and concentration methods are based on differences in the distribution of substances between two phases. In this case, the process includes two stages: the first is phase contact and the establishment of equilibrium between them, and the second is phase separation.

Separation methods are classified:

• a) by the nature of separation processes;
• b) the state of aggregation of the contacting phases;
• c) the nature of separation processes.

The most general classification is based on the nature of separation processes: physicochemical (precipitation and coprecipitation, extraction, sorption, electrochemical methods, etc.) and physical (evaporation, zone melting, directional crystallization, etc.). Moreover, each field of science or technology in which chemical analysis is used is characterized by its own set of separation and concentration methods. For example, when analyzing waste and natural waters for the content of organic substances, if necessary, sorption methods, evaporation and freezing, separation of volatile organic substances by evaporation, extraction and chromatographic separation methods are usually used.

During separation, the following combinations of contacting phases are possible: gas - liquid, gas - solid, liquid - liquid, liquid - solid. Separation can be carried out by static (single-stage), dynamic or chromatographic (multi-stage) methods.

When describing separation and concentration, the following quantitative characteristics are used:

Distribution coefficient between contacting phases

where C, and C„ are the concentration of the component in the first and second phases, respectively;

Extraction rate

Split factor

Concentration factor

where Q 0 and Q° m- the amount of the determined component and matrix in the sample before the process of separation and (or) concentration; Q And Q m- the amount of the determined component and matrix in the analyzed system after the process of separation and (or) concentration.

Currently, so-called hybrid and combined methods are becoming increasingly widespread, in which the operations of separation, concentration and chemical analysis themselves are combined in one device. For example, when analyzing natural objects (water, ice, soil) for the presence of heavy metals in them, the method of stripping voltammetry is quite widely used. In this method, at the first stage, electrochemical separation and concentration of trace impurities of heavy metals occurs on the surface of the electrode, and at the second stage, voltammetric analysis of the resulting concentrate occurs.

Masking. Masking is achieved by introducing a substance into the analyzed system, which converts the component interfering with the analysis into an analytically inactive form. In this case, no new phase is formed, as occurs during separation, and therefore phase separation operations before analysis are eliminated.

There are two types of masking - thermodynamic (equilibrium) and kinetic (nonequilibrium). During thermodynamic masking, conditions are created under which the concentration of the interfering component in the analytically active form is below the detection limit of the analytical method used. With kinetic masking, a significant difference is achieved in the rates of reaction of the detected and interfering components with the reagent used for their detection.

To carry out the masking operation, the following groups of masking substances are used.

• 1. Substances that convert an interfering component into a stable one complex compound. For example, iron(III) forms a blood-red complex 3.
• 2. Substances that change the oxidation state of the interfering ion. For example, to eliminate the interfering influence of chromium(III), it is usually oxidized to chromium(U1).
• 3. Substances that precipitate interfering ions, but the precipitate may not be separated.
• 4. Substances with specific effects. For example, in the stripping voltammetry method, formic acid can be added to the analyzed system, which, decomposing into radicals under the influence of ultraviolet radiation, binds dissolved oxygen and destroys organic surfactants.

To assess the effectiveness of masking, the so-called masking index is used. 1 t:

Where From 0- total concentration of the interfering component; S a - concentration of a component in an analytically active form. The masking index can be calculated from the equilibrium constants of the corresponding masking reactions.

Extraction. Extraction are the physicochemical process of distributing a substance between two phases, most often between two immiscible liquids (usually between water and organic solvents), and the corresponding method of isolating, separating and concentrating substances.

During extraction, several processes can occur simultaneously: the formation of extracted compounds, the distribution of these compounds between two phases, reactions in the organic phase (dissociation, association, polymerization). The component responsible for the formation of the extractable compound is called extractant. Inert organic solvents in which the extractant is dissolved and which help improve the physical and extraction properties of the extractant are called thinners. The diluent must have a density significantly greater or less than the density of water and low solubility in water, in order to make it easier to separate the aqueous and organic phases, as well as low toxicity. The phase containing the extracted compound is called extract. The reverse transfer of the extracted substance from the organic phase to the aqueous phase is called re-extraction, and the solution used for this is re-extractor.

Extraction only takes place if the compound being extracted is more soluble in the organic phase than in water. This is possible if the compound is hydrophobic. Hydrophobicity is ensured by the transfer of the extracted substance into an intracomplex compound (chelate complex) containing large hydrophobic organic ligands, neutralization of its charge due to the formation of neutral complexes or ionic associates, and solvation of the extracted compound by extractant molecules. Extraction of ionic associates improves with increasing ion sizes and decreasing their charge.

Extraction can be carried out using batch or continuous methods. Batch extraction is the extraction of a substance with separate portions of fresh extractant. In this case, at sufficiently high values ​​of the distribution coefficient, even a single extraction allows quantitative extraction of the substance. Continuous extraction occurs with continuous contact and relative movement of the two phases. In this case, one of the phases remains stationary, and the second is passed through the volume of the first in the form of separate drops.

Extraction methods are suitable for separation, concentration, extraction of micro- or macrocomponents, individual and group extraction of components in the analysis of various natural objects. The method is simple and fast, provides high separation and concentration efficiency, and is compatible with a variety of analytical methods. The selectivity of separation can be improved by optimizing the process conditions, for example, choosing the appropriate pH, diluent, extractant concentration, or introducing a masking agent.

Chromatography. In cases where the distribution coefficients of the mixture components between two phases differ slightly, they can only be separated using dynamic chromatographic methods. Chromatography is a method of separating substances based on the difference in their distribution coefficients between two phases, one of which is stationary, and the second moves directionally relative to the first. Necessary conditions for carrying out chromatography are the presence of a sufficiently large interface between the phases and a dynamic method of separation (one phase moves relative to the second). The combination of these two conditions ensures high efficiency of chromatography, which makes it possible to separate substances that are very similar in their properties, such as, for example, isotopes of elements or optical isomers.

There are several ways to classify chromatographic methods.

• 1. Based on the state of aggregation of the mobile phase, liquid and gas chromatography are distinguished. Depending on the state of aggregation of the stationary phase, liquid chromatography is divided into solid-liquid-phase and liquid-liquid-phase chromatography. The latter is often called partition chromatography. Gas chromatography, depending on the state of aggregation of the stationary phase, is divided into gas adsorption (solid stationary phase) and gas-liquid or gas distribution.
• 2. Depending on the mechanism of distribution of components, chromatography is divided into molecular and chemisorption. In molecular chromatography, the interaction between the stationary phase and the components of the mixture being separated is carried out due to intermolecular forces such as van der Waals forces. Chemisorption chromatography includes ion exchange, sedimentation, ligand exchange (complexation), and redox. In this case, the separation of the mixture components occurs as a result of appropriate chemical reactions.
• 3. According to the method of implementation, chromatography is classified into frontal, developing (eluent) and displacement. IN analytical chemistry Development chromatography is most often used.
• 4. Based on the technique used, a distinction is made between column chromatography (the stationary phase is in a column) and planar chromatography - paper or thin-layer (the stationary phase is a sheet of paper or a thin layer of sorbent on a glass or metal plate).

The essence of the chromatographic method is as follows. A small volume of the mixture to be separated (many times smaller than the volume of the stationary phase) is added to the top of the column, onto a thin layer of sorbent or onto a strip of paper. The components of the mixture are sorbed in the upper layers of the sorbent in the column or at the point of application of the sample in the case of plane chromatography, and weakly sorbed components move along the column or along the radius of the spot somewhat further than strongly sorbed components. A so-called primary chromatogram is formed, in which complete separation of the components, as a rule, does not occur.

To achieve complete separation, the primary chromatogram is developed by washing the column (treating a thin layer of sorbent, paper) with a suitable solvent (mobile phase). The speed of movement of the separated components in the direction of movement of the mobile phase is determined by the value of their distribution coefficient between the mobile and stationary phases. The higher the distribution coefficient, the faster the component moves. If the process conditions (the nature of the stationary and mobile phases, the length of the column, the speed of movement of the mobile phase) are selected correctly, then the components are completely separated, and they leave the column one after another. Thus, it becomes possible to select fractions containing individual components of the mixture and analyze them using suitable analytical methods.

In modern gas and liquid chromatographs, a detector is placed at the outlet of the column, which makes it possible to record the fact that any component passes through the column. By the time of passage of a component, you can determine its nature, and by the magnitude of the detector signal, its quantity. Non-selective analyzers such as conductometers, refractometers, etc. are used as detectors. Thus, in chromatographs, separation occurs with simultaneous qualitative and quantitative analysis of the components.

Sorption. It is the process of absorption of gases, vapors and dissolved substances by solid or liquid substances. Sorption is widely used for the separation and concentration of substances. In this case, good separation selectivity and large concentration coefficients are usually achieved.

The sorption process is relatively easy to control, and the implementation of this method does not require complex instrumentation and extreme conditions. It is easily combined with various analytical methods for the subsequent determination of components. Therefore, the sorption method is convenient for carrying out work in the field.

The classification of sorption methods is based on differences in the mechanism of interaction of a substance with sorbents. Distinguish adsorption(physical sorption and chemisorption on the solid phase), distribution of substances between two immiscible phases (liquid phase on the sorbent) and capillary condensation - the formation of a liquid phase in the pores and capillaries of a solid sorbent when absorbing vapors of a substance. These mechanisms are usually not observed in their pure form.

The sorption process can be carried out by two methods: statistical and dynamic. The latter forms the basis of chromatographic separation methods. In analytical practice, a variety of sorbents are used: activated carbons, ion-exchange and chelating resins, conventional and chemically modified silicas and cellulose, oxides, hydroxides, aluminosilicates, heteropolyacids and their salts, etc.

Electrochemical methods of separation and concentration. Electrochemical separation and concentration methods include controlled potential electrolysis, cementation method (internal electrolysis) and electrophoresis.

Electrolysis. The method is based on the deposition of an element or some compound of this element on an electrode electric shock at controlled potential. The most common option is cathodic deposition of metals; anodic deposition, for example in the form of oxides, is rarely used. The electrode material can be mercury, including in the form of a thin-film mercury electrode, carbon (graphite, glassy carbon), platinum and its alloys, silver, copper, tungsten. The composition of the deposit formed on the electrode depends on the process conditions (primarily the value of the electrode potential), the composition of the electrolyte and the material of the electrodes.

There are various variations of the method. In one case, by selecting the appropriate electrolyte composition and potential value, it is possible to selectively isolate a certain component, in the second (by varying the potential within wide limits) - a group of components, and then determine each of them using appropriate selective methods. Complete separation can be achieved when the component being determined is separated from electrochemically inactive substances. For example, when isolated from an aqueous solution at the cathode, such substances will include salts of active metals and organic compounds.

When concentrating microcomponents, it is more convenient to isolate them on the electrode rather than the matrix components, since in this case the losses of the microcomponent, possible due to its mechanical capture by the depositing matrix, the formation of intermetallic compounds and solid solutions, are reduced. In most cases, complete isolation of a microcomponent requires a very long time, so partial isolation is limited. Concentration of the microcomponent can be achieved not only by its deposition on the electrode, but also by electrochemical dissolution of the matrix.

Electrolytic evolution is in most cases an integral part of inversion electrochemical methods, of which stripping voltammetry is the most common.

The cementation method involves the reduction of components (usually microcomponents) on active metals (aluminum, zinc, magnesium) or amalgams of these metals. During cementation, two processes occur simultaneously: cathodic (release of a component) and anodic (dissolution of the cementing metal). For example, this method is used to isolate relevant trace elements (mainly heavy metals) from natural waters and then determine them by atomic emission spectroscopy.

Electrophoresis. The method is based on the dependence of the speed of movement of charged particles in an electric field on the magnitude of their charge, shape and size. This dependence for spherical particles is described by the equation

where z is the effective charge of the particle, which in solutions is less than the charge of the ion due to the influence of the ionic atmosphere; E - tension electric field; G - effective particle radius, taking into account the thickness of the solvation shell; G- viscosity of the medium. The speed of particle movement is strongly influenced by the composition of the medium, in particular pH, which is used to increase the selectivity of separation.

There are two options for electrophoresis: frontal and zone (on a carrier). In the first case, a small volume of the test solution is placed in a capillary with an electrolyte. In the second case, the movement of ions occurs in a reagent medium with which the paper is specially treated. In this case, the particles are retained on the paper after the field is turned off. The main area of ​​application of classical electrophoresis is biochemical analysis: separation of proteins, enzymes, nucleic acids and so on.

Capillary electrophoresis has been intensively developed since the early 1980s. This was due to a significant decrease in the diameter of the capillary

(up to 50-100 microns) and transition to direct spectrophotometric determination of components directly in the capillary. The main advantages of the method include its high efficiency and simplicity of hardware design. Capillary electrophoresis has been used for the analysis of waste and natural waters for the content of inorganic components (cations and anions).

Other methods of separation and concentration. There are a number of other separation and concentration methods that have been used for analytical purposes with varying degrees of success. These include precipitation and coprecipitation, evaporation methods (distillation, distillation, sublimation), and freezing. All these methods, under certain conditions, make it possible to achieve high concentration coefficients.

Filtration, sedimentation, and ultracentrifugation are widely used to separate heterogeneous systems.

4.3. CHEMICAL METHODS

4.8. THERMAL METHODS

5. CONCLUSION

6. LIST OF REFERENCES USED

INTRODUCTION

Chemical analysis serves as a means of monitoring production and product quality in a number of industries National economy. Mineral exploration is based to varying degrees on the results of analysis. Analysis is the main means of monitoring environmental pollution. Determining the chemical composition of soils, 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 and the laboratory’s equipment 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. Analytical chemistry methods make it possible to answer questions about what a substance consists of and what components are included in its composition. These methods often make it possible to find out in what form this component present in a substance, for example, to determine the oxidation state of an element. It is sometimes 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 theoretical foundations methods, establishing the limits of their applicability, assessing metrological and other characteristics, creating methods for analyzing 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 method of analysis 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 a method of analysis, they mean the underlying principle, a quantitative expression of the relationship between the composition and any measured property; selected implementation techniques, including identification and elimination of interference; devices for practical implementation and methods for processing measurement results. The analysis technique is detailed description analysis of a given object using the selected method.

Three functions of analytical chemistry as a field of knowledge can be distinguished:

1. solving general questions of analysis,

2. development of analytical methods,

3. solving specific analysis problems.

You can also highlight qualitative And quantitative tests. The first solves the question of which components the analyzed object includes, the second provides 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, sample digestion, separation of components, detection (identification) and determination. There are hybrid methods that combine separation and determination. Detection and definition methods have much in common.

Determination methods are of greatest importance. They can be classified according to the nature of the property being measured or the method of recording the corresponding signal. Determination methods are divided into chemical, physical And biological. Chemical methods are based on chemical (including electrochemical) reactions. This also 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: accuracy 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, you need 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 sampling and preparation of the sample, the stage of chemical analysis begins, at which the component is detected or its quantity is determined. For this purpose, they measure analytical signal. In most methods, the analytical signal is the average of the measurements physical quantity at the final stage of analysis, functionally related to the content of the component being determined.

If it is necessary to detect any component, it is usually fixed appearance analytical signal - the appearance of a precipitate, color, line 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 that can change its direction or speed. In this case, no new phase is formed. There are two types of masking: thermodynamic (equilibrium) and kinetic (nonequilibrium). With 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 record the analytical signal. Kinetic masking is based on increasing the difference between the rates of reaction of the masked and analyte substances 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 component being determined is below the detection limit of the method; the components being determined are unevenly distributed in the sample; there are no standard samples for calibration of instruments; the sample is highly toxic, radioactive and expensive.

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

Concentration is an operation (process) that results in an increase in the ratio of the concentration or amount of microcomponents to the concentration or amount of macrocomponents.

Precipitation and coprecipitation.

Precipitation is typically used to separate inorganic substances. Precipitation of microcomponents with organic reagents, and especially their coprecipitation, provides a high concentration coefficient. These methods are used in combination with determination methods that are designed to obtain an analytical signal from solid samples.

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

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

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 a variety of industrial and natural objects. The method is simple and fast to perform, provides high separation and concentration efficiency, and is compatible with various determination methods. Extraction allows you to study the state of substances in solution under various conditions and determine physicochemical characteristics.

Sorption.

Sorption is well used for separating and concentrating substances. Sorption methods usually provide good separation selectivity and high 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 is electrolysis, in which the separated or concentrated substance is isolated on solid electrodes in an elemental state or in the form of some kind of compound. Electrolytic separation (electrolysis) based on the deposition of a substance by electric current at a controlled potential. The most common option is 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 radius of the particles. There are two options for electrophoresis: frontal (simple) and zone (on a carrier). In the first case, a small volume of solution containing the components to be separated is placed in a tube with an electrolyte solution. In the second case, movement occurs in a stabilizing environment, which holds the particles in place after the electric field is turned off.

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

Evaporation methods.

Methods distillation based on different volatility of substances. A substance changes from a liquid to a gaseous state and then condenses to form a liquid or sometimes a solid phase again.

Simple distillation (evaporation)– single-step separation and concentration process. Evaporation removes substances that are in the form of ready-made volatile compounds. These can be macrocomponents and microcomponents; distillation of the latter is used less frequently.

Sublimation (sublimation)- transfer of substance from solid state into gaseous and subsequent precipitation in solid form (bypassing the liquid phase). Separation by sublimation is resorted to, as a rule, if the components being separated are difficult to melt or difficult to dissolve.

Controlled crystallization.

When a solution, melt or gas is cooled, the formation of nuclei of the solid phase occurs - crystallization, which can be uncontrolled (volumetric) and controlled. With uncontrolled crystallization, crystals arise spontaneously throughout the entire volume. With controlled crystallization, the process is set by external conditions (temperature, direction of phase movement, etc.).

There are two types of controlled crystallization: directional crystallization(in a given direction) and zone melting(movement of a liquid zone in a solid in a certain direction).

With directional crystallization, one interface appears between a solid and a liquid—the crystallization front. In zone melting there are two boundaries: the crystallization front and the melting front.

4.2. CHROMATOGRAPHIC METHODS

Chromatography is the most commonly used analytical method. The latest chromatographic methods can determine gaseous, liquid and solid substances with molecular weight from units to 10 6. These can be hydrogen isotopes, metal ions, synthetic polymers, proteins, etc. Using chromatography, extensive information about the structure and properties of organic compounds many classes.

Chromatography is a physicochemical method for the separation of substances, based on the distribution of components between two phases - stationary and mobile. The stationary phase is usually a solid substance (often called a sorbent) or a liquid film deposited on a solid substance. The mobile phase is a liquid or gas flowing through the stationary phase.

The method allows you to separate a multicomponent mixture, identify components and determine its quantitative composition.

Chromatographic methods are classified according to the following criteria:

a) according to the aggregate state of the mixture, in which it is separated into components - gas, liquid and gas-liquid chromatography;

b) according to the separation mechanism - adsorption, distribution, ion exchange, sedimentation, redox, adsorption - complexing chromatography;

c) according to the form of the chromatographic process - column, capillary, planar (paper, thin-layer and membrane).

4.3. CHEMICAL METHODS

Chemical detection and determination methods are based on three types of chemical reactions: acid-base, redox, and complexation. Sometimes they are accompanied by a change in the state of aggregation of the components. The most important among chemical methods are gravimetric and titrimetric. These analytical methods are called classical. The criteria for the suitability of a chemical reaction as the basis of an analytical method in most cases are completeness and high speed.

Gravimetric methods.

Gravimetric analysis involves isolating a substance in its pure form and weighing it. Most often, such isolation is carried out by precipitation. Less commonly, the component being determined is isolated in the form of a volatile compound (distillation methods). In some cases, gravimetry is the best way to solve an analytical problem. This is the absolute (reference) method.

The disadvantage of gravimetric methods is the duration of determination, especially in serial analyzes large number samples, as well as non-selectivity - precipitating reagents, with a few exceptions, are rarely specific. Therefore, preliminary separations are often necessary.

The analytical signal in gravimetry is mass.

Titrimetric methods.

The titrimetric method of quantitative chemical analysis is a method based on measuring the amount of reagent B spent on the reaction with the determined component A. In practice, it is most convenient to add the reagent in the form of a solution of a precisely known concentration. In this embodiment, titration is the process of continuously adding a controlled amount of a reagent solution of precisely known concentration (titran) to a solution of the component being determined.

In titrimetry, three titration methods are used: direct, reverse, and substituent titration.

Direct titration- this is the titration of a solution of the analyte A directly with a titran solution B. It is used if the reaction between A and B proceeds quickly.

Back titration consists of adding to the analyte A an excess of a precisely known amount of standard solution B and, after completing the reaction between them, titrating the remaining amount of B with titran solution B’. This method is used in cases where the reaction between A and B does not proceed quickly enough, or there is no suitable indicator to fix the equivalence point of the reaction.

Titration by substituent consists of titrating with titrant B not a determined amount of substance A, but an equivalent amount of substituent A’ resulting from a previously carried out reaction between the determined substance A and some reagent. This titration method is usually used in cases where direct titration is not possible.

Kinetic methods.

Kinetic methods are based on the use of the dependence of the rate of a chemical reaction on the concentration of reactants, and in the case of catalytic reactions, on the concentration of the catalyst. The analytical signal in kinetic methods is the rate of the process or a value proportional to it.

The reaction underlying the kinetic method is called indicator. A substance, by the change in concentration of which the speed of the indicator process is judged, is an indicator.

Biochemical methods.

Among modern methods In chemical analysis, biochemical methods occupy an important place. Biochemical methods include methods based on the use of processes occurring with the participation of biological components (enzymes, antibodies, etc.). In this case, the analytical signal is most often either the initial rate of the process or the final concentration of one of the reaction products, determined by any instrumental method.

Enzymatic methods are based on the use of reactions catalyzed by enzymes - biological catalysts characterized by high activity and selectivity of action.

Immunochemical methods analyzes are based on the specific binding of the detected compound - antigen - by the corresponding antibodies. The immunochemical reaction in solution between antibodies and antigens is a complex process that occurs in several stages.

4.4. ELECTROCHEMICAL METHODS

Electrochemical methods of analysis and research are based on the study and use of processes occurring on the surface of the electrode or in the near-electrode space. Any electrical parameter (potential, current, resistance, etc.), functionally related to the concentration of the analyzed solution and amenable to correct measurement, can serve as an analytical signal.

There are direct and indirect electrochemical methods. Direct methods use the dependence of the current strength (potential, etc.) on the concentration of the component being determined. In indirect methods, the current strength (potential, etc.) is measured in order to find the end point of titration of the analyte with a suitable titrant, i.e. The dependence of the measured parameter on the titrant volume is used.

For any kind of electrochemical measurements, an electrochemical circuit or electrochemical cell is required, of which the analyzed solution is an integral part.

There are different ways to classify electrochemical methods, from very simple to very complex, involving consideration of the details of the electrode processes.

4.5. SPECTROSCOPIC METHODS

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

4.6. MASS SPECTROMETRIC METHODS

The mass spectrometric method of analysis is based on the ionization of atoms and molecules of the emitted substance and the subsequent separation of the resulting ions in space or time.

The most important application of mass spectrometry is to identify and determine the structure of organic compounds. It is advisable to carry out molecular analysis of complex mixtures of organic compounds after their chromatographic separation.

4.7. ANALYSIS METHODS BASED ON RADIOACTIVITY

Analysis methods based on radioactivity arose during the era of the development of nuclear physics, radiochemistry, and nuclear technology and are successfully used today in conducting various analyzes, including in industry and the geological service. These methods are very numerous and varied. Four main groups can be distinguished: radioactive analysis; isotope dilution and other radiotracer methods; methods based on absorption and scattering of radiation; purely radiometric methods. The most widespread radioactivation method. This method appeared after the discovery artificial radioactivity and is based on the formation of radioactive isotopes of the element being determined by irradiating a sample with nuclear or g-particles and recording the artificial radioactivity obtained during activation.

4.8. THERMAL METHODS

Thermal analysis methods are based on the interaction of a substance with thermal energy. The greatest application in analytical chemistry is thermal effects, which are the cause or consequence of chemical reactions. To a lesser extent, methods based on the release or absorption of heat as a result of physical processes are used. These are processes associated with the transition of a substance from one modification to another, with a change in the state of aggregation and other changes in intermolecular interaction, for example, occurring during dissolution or dilution. The table shows the most common thermal analysis methods.

Thermal methods are successfully used for the analysis of metallurgical materials, minerals, silicates, as well as polymers, for phase analysis soils, determination of moisture content in samples.

4.9. BIOLOGICAL ANALYSIS METHODS

Biological methods of analysis are based on the fact that for life activity - growth, reproduction and generally normal functioning of living beings, an environment of a strictly defined chemical composition is necessary. When this composition changes, for example, when any component is excluded from the environment or an additional (detectable) compound is introduced, the body sends an appropriate response signal after some time, sometimes almost immediately. Establishing a connection between the nature or intensity of the body's response signal and the amount of a component introduced into the environment or excluded from the environment serves to detect and determine it.

Analytical indicators in biological methods are various living organisms, their organs and tissues, physiological functions, etc. Microorganisms, invertebrates, vertebrates, and plants can act as indicator organisms.

5. CONCLUSION

The importance of analytical chemistry is determined by the need of society for analytical results, to establish the qualitative and quantitative composition of substances, the level of development of society, the social need for the results of analysis, as well as the level of development of analytical chemistry itself.

Quote from the textbook on analytical chemistry by N.A. Menshutkin, published in 1897: “Having presented the entire course of classes in analytical chemistry in the form of problems, the solution of which is provided to the student, we must point out that for such a solution of problems, analytical chemistry will provide a strictly defined path. This certainty (systematic solution of analytical chemistry problems) is of great pedagogical importance. The student learns to apply the properties of compounds to solve problems, derive reaction conditions, and combine them. This entire series of mental processes can be expressed this way: analytical chemistry teaches you to think chemically. Achieving the latter seems to be the most important for practical studies in analytical chemistry.”

LIST OF REFERENCES USED

1. K.M. Olshanova, S.K. Piskareva, K.M. Barashkov “Analytical chemistry”, Moscow, “Chemistry”, 1980

2. "Analytical chemistry. Chemical methods of analysis", Moscow, "Chemistry", 1993.

3. “Fundamentals of analytical chemistry. Book 1", Moscow, " graduate School", 1999

4. “Fundamentals of analytical chemistry. Book 2", Moscow, "Higher School", 1999.

Methods of separation and concentration occupy an important place among the techniques of modern analytical chemistry. In its most general form, the process of chemical analysis includes sample selection and its preparation for determination, the actual determination and processing of the results. Modern development Analytical instrumentation and computer technology ensures reliable performance of the second and third stages of analysis, which is often carried out automatically. On the contrary, the sample preparation stage, an integral and integral part of which are the operations of separation and concentration, still remains the most labor-intensive and least accurate operation of chemical analysis. If the duration of measurement and processing of results is on the order of minutes and less often than seconds, then the duration of sample preparation is on the order of hours and less often than minutes. According to experts working in the field of environmental analytical control, only 10% of the total analysis error occurs at the signal measurement stage, 30% in sample preparation and 60% in sampling. Errors made during sample collection and preparation can completely distort the results of a chemical analysis.

Interest in methods of separation and concentration does not wane for a number of reasons, of which several can be highlighted: increasing requirements for the sensitivity of the analysis and its accuracy, depending on the possibility of eliminating the matrix effect, as well as the requirement to ensure an acceptable cost of the analysis. Many modern instruments allow analysis without prior separation and concentration, but they themselves and their operation are very expensive. Therefore, it is often more profitable to use so-called combined and hybrid methods, which provide the optimal combination of preliminary separation and concentration of components with their subsequent determination using relatively inexpensive analytical instruments.

Basic concepts and terms. Types of concentration

Let us clarify the semantic concept of the basic terms that are used when describing separation and concentration methods

Under division imply an operation (process) as a result of which several fractions of its components are obtained from an initial mixture of substances, that is, the components that make up the initial mixture are separated from one another. When separated, the concentrations of the components may be close to each other, but they may also differ.

Concentration - an operation (process) that results in an increase in the ratio of the concentration or quantity of microcomponents relative to the matrix or matrix components. Microcomponents mean components contained in industrial, geological, biological and other materials, as well as in objects environment, in concentrations less than 100 µg/g. In this case, the matrix means the environment in which the microcomponents are located. Often the matrix is ​​water or an aqueous solution of acids or salts. In the case of solid samples, concentration is carried out after transferring the sample into solution; in this case, the solution contains matrix components along with microcomponents. Concentration is carried out under conditions where the concentrations of the components are sharply different.

Concentration of microcomponents during their determination is resorted to, first of all, in cases where the sensitivity of the methods direct definition these components are insufficient. The main advantage of concentration is the reduction of relative and sometimes absolute limits of detection of microcomponents due to the elimination or sharp reduction of the influence of macrocomponents on the determination results. Concentration is also necessary if the component is distributed inhomogeneously in the analyzed sample; it allows you to work with representative samples. In addition, concentration makes it possible to do without a large number of reference samples, including standard samples, since as a result of concentration it is possible to obtain concentrates on a single basis, for example, on coal powder in the case of atomic emission analysis. During the concentration process, it is also convenient to introduce so-called internal standards, if they are needed. However, concentration also has disadvantages: it lengthens and complicates the analysis, in some cases losses and contamination increase, and sometimes the number of components being determined decreases.

Separation and concentration have much in common, both in the theoretical aspect and in the technique of execution. Obviously, concentration is a special case of separation. The classification of “concentration” as an independent concept of analytical chemistry is justified due to the practical importance of this operation in chemical analysis and the differences in its purpose compared to separation. Thanks to the use of separation, it is possible to simplify the analysis and eliminate the influence of interfering components, while the main purpose of concentration is to increase the sensitivity of the determination.

(Question 1). Distinguish absolute And relative concentration. At absolute concentration microcomponents are transferred from a large sample mass to a small concentrate mass; at the same time, the concentration of microcomponents increases. An example of absolute concentration is the evaporation of a matrix in the analysis of waters and solutions mineral acids, organic solvents. Say, when evaporating 20 ml of a lead solution to 1 ml, we increase the ratio of the mass of the component being determined to the total mass of the sample by 20 times (provided that the component being determined remains completely in the solution). In other words, we concentrated 20 times.

As a result of relative concentration the matrix, which for one reason or another complicates the analysis, is replaced with another organic or inorganic matrix and the ratio between the micro and the main interfering macrocomponents increases. In this case, the ratio of the initial and final samples of great importance does not have. Let us assume that the same 20 ml of lead solution also contained zinc, and there was 100 times more of it than lead. We concentrated lead, for example by extraction, while the amount of zinc was reduced by 20 times, now it is only 5 times more than lead. We can obtain a concentrate of the same volume of 20 ml, while the concentration of lead has not changed, but the concentration of zinc has changed, and the amounts of zinc that remain in the solution will no longer interfere with the subsequent determination of lead. In practice, absolute and relative concentration are often combined; they replace the matrix elements with another organic or inorganic matrix and “compress” the trace element concentrate to the required mass by additional action, for example, simple evaporation.

The practice of chemical analysis requires both individual, so group concentration. Individual concentration is an operation as a result of which one microcomponent or several microcomponents are isolated sequentially from the analyzed object, whereas when group concentration several microcomponents are isolated at once. Both methods are used in practice. The choice of method depends on the nature of the analyzed object and the concentration method used. Group concentration is usually combined with subsequent determination by chromatography, chromatography-mass spectrometry, stripping voltammetry, and individual concentration with single-element analysis methods such as spectrophotometry, atomic absorption spectrometry, and fluorimetry.

Concentration can be done in two ways: removal of the matrix and isolation of microelements. The choice of method depends on the nature of the analyzed object. If the matrix is ​​simple (one or two elements), it is easier to delete the matrix. If the base is multi-element (complex minerals and alloys, soils), microelements will be released. The choice also depends on the concentration method used. For example, co-precipitation is used only to isolate trace elements, and evaporation is used to separate a matrix of relatively simple objects: natural waters, acids and organic solvents.

Separation and concentration methods

General information about separation and concentration

Separation is an operation that allows separate sample components from each other.

It is used if some components of the sample interfere with the determination or detection of others, i.e. when the analytical method not selective enough and the overlap of analytical signals must be avoided. In this case, the concentrations of separated substances are usually close.

Concentration is an operation that allows increase concentration microcomponent relative to the main components of the sample (matrix).

It is used if the concentration of a microcomponent is less than the detection limit WITH min , i.e. when the analysis method not sensitive enough. At the same time, the concentrations of components vary greatly. Concentration is often combined with separation.

Types of concentration.

1. Absolute: microcomponent is transferred from big volume or large sample mass ( V pr or m pr) in less volume or less mass of concentrate ( V conc or m conc). As a result, the concentration of the microcomponent increases n times:

Where n – degree of concentration.

The smaller the volume of concentrate, the greater the degree of concentration. For example, 50 mg of cation resin absorbed germanium from 20 L of tap water, then germanium was desorbed with 5 ml of acid. Consequently, the degree of concentration of germanium was:

2. Relative (enrichment): The microcomponent is separated from the macrocomponent so that the ratio of their concentrations increases. For example, in the initial sample the ratio of concentrations of micro- and macrocomponents was 1: 1000, and after enrichment - 1: 10. This is usually achieved by partial matrix removal.

Separation and concentration have many general, are used for these purposes the same methods. They are very diverse. Next, the methods of separation and concentration that are of greatest importance in analytical chemistry will be considered.

Classification of separation and concentration methods

Exists a bunch of classifications of separation and concentration methods based on different signs. Let's look at the most important of them.

1. Classification according to the nature of the process is given in Fig. 62.

Rice. 62. Classification of separation methods according to the nature of the process

Chemical separation and concentration methods are based on the flow chemical reaction, which is accompanied by precipitation of the product and gas evolution. For example, V organic analysis The main method of concentration is distillation: during thermal decomposition, the matrix is ​​distilled off in the form of CO 2, H 2 O, N 2, and metals can be determined in the remaining ash.

Physico-chemical selective distribution substances between two phases. For example, in the petrochemical industry, chromatography is of greatest importance.

Physical separation and concentration methods are most often based on change in state of aggregation substances.

2. Classification according to the physical nature of the two phases. The distribution of a substance can be carried out between phases that are in the same or different aggregate state: gaseous (G), liquid (L), solid (S). In accordance with this, the following methods are distinguished (Fig. 63).

Rice. 63. Classification of separation methods by the nature of phases

In analytical chemistry highest value found methods of separation and concentration that are based on the distribution of the substance between liquid and solid phase.

3. Classification by the number of elementary acts (stages).

§ One-step methods- based on one-time distribution of matter between two phases. The division takes place in static conditions.

§ Multi-step methods- based on multiple distribution of matter between two phases. There are two groups multi-stage methods:

– with repetition of the single distribution process ( For example, repeated extraction). The division takes place in static conditions;

– methods based on the movement of one phase relative to another ( For example, chromatography). The division takes place in dynamic conditions

3. Classification by type of balance(Fig. 64).

Rice. 64. Classification of separation methods by type of equilibrium

Thermodynamic separation methods are based on differences in the behavior of substances in equilibrium state. They are of greatest importance in analytical chemistry.

Kinetic separation methods are based on differences in the behavior of substances during the process, leading to equilibrium state. For example, in biochemical research, electrophoresis is of greatest importance. Other kinetic methods are used to separate particles colloidal solutions and solutions of high molecular weight compounds. In analytical chemistry, these methods are used less frequently.

Chromatographic methods are based on both thermodynamic and kinetic equilibrium. They are of great importance in analytical chemistry, since they allow separation of both qualitative and quantitative analysis multicomponent mixtures.

Extraction as a method of separation and concentration

Extraction is a method of separation and concentration based on the distribution of a substance between two immiscible liquid phases(most often aqueous and organic).

For the purpose of extraction separation create conditions such that one component completely passes into the organic phase, while the other remains in the aqueous phase. The phases are then separated using separatory funnel.

With the aim of absolute concentration the substance is transferred from more volume of aqueous solution in less the volume of the organic phase, as a result of which the concentration of the substance in the organic extract increases.

With the aim of relative concentration create conditions so that microcomponent passed into the organic phase, and most of macro component would have stayed in the water. As a result, in an organic extract the ratio of concentrations of micro- and macrocomponents increases in favor of the microcomponent.

Advantages of extraction:

§ high selectivity;

§ ease of implementation (only a separating funnel is needed);

§ low labor intensity;

§ speed (3–5 min);

§ extraction combines very well with subsequent determination methods, resulting in a number of important hybrid methods(extraction-photometric, extraction-spectral, etc.).

Co-precipitation as a method of separation and concentration

Co-precipitation– this is the capture of a microcomponent reservoir sediment during its formation, and the microcomponent passes into sediment from unsaturated solution (PS< ПР).

As collectors use inorganic and organic poorly soluble compounds with developed surface. Phase separation is carried out by filtering.

Co-precipitation is used with the aim of:

§ concentration impurities as a very effective and one of the most important methods, which allows you to increase the concentration by 10–20 thousand times;

§ departments impurities (less often).

Sorption as a method of separation and concentration

Sorption– is the absorption of gases or dissolved substances by solid or liquid sorbents.

As sorbents They use activated carbons, Al 2 O 3, silica, zeolites, cellulose, natural and synthetic sorbents with ionic and chelating groups.

Absorption of substances can occur at surfaces phases ( A d sorption) or in volume phases ( A b sorption). Most often used in analytical chemistry adsorption with the aim of:

§ separation substances, if you create conditions for selective absorption;

§ concentration(less often).

Besides, sorption under dynamic conditions forms the basis for the most important method of separation and analysis - chromatography.

Ion exchange- This reversiblestoichiometric process that occurs at the interface ionite– solution electro
lita.

Ionites- This high molecular weight polyelectrolytes different structure and composition.

Main property ion exchangers is what they absorb from solution cations or anions, releasing into solution equivalent number of ions same sign of charge.

The ion exchange process is described law of mass action:

where A and B are ions in solution, and are ions in the ion exchanger phase.

This equilibrium is characterized exchange constant (TO):

Where A– ion activity.

If TO> 1, then ion B has a larger affinity for ion exchanger; If TO < 1, то ион А обладает бóльшим сродством к иониту; если же TO≈ 1, then both ions are equally sorbed by the ion exchanger.

The course of ion exchange is influenced by the following: factors:

1) nature of the ion exchanger;

2) nature of the ion: the greater the ratio of the ion charge to the radius of the hydrated ion (z/r), the greater the affinity for the ion exchanger;

3) properties of the solution:

§ pH value(see the following sections);

§ ion concentration: from dilute solutions, the ion exchanger sorbs ions with a higher charge, and from concentrated solutions – with a smaller one;

§ ionic strength of solution: the smaller μ, the better the ions are sorbed.

Course work:

Methods of separation and concentration in elemental analysis

Introduction

general characteristics separation methods

Extraction as a separation method

General characteristics of concentration methods

Co-precipitation as a concentration method

Conclusion

Bibliography

Introduction

The development of analytical chemistry proceeds in two main ways: the development of the most selective methods for the determination of individual substances and the optimal combination of separation and concentration methods with non-selective methods of determination in combined methods of analysis. In this case, the selectivity of the method means the ability to register an analytical signal corresponding to the substance being determined against the background created by accompanying impurities and the matrix. The concept of combined is in full accordance with the semantic content of this word: connected together to achieve a common goal. Accordingly, there may be combined separation methods, which aim to improve the separation, and combined analytical methods, which provide an optimal combination of preliminary separation with the final determination. The widespread use of combined methods of analysis, primarily chromatographic, cannot be considered only as a consequence of the limited selectivity of the known methods for the direct determination of substances in the object of analysis.

In addition to the important role of separation and concentration methods for combined methods of analysis, separation methods have their own value in solving preparative problems. Analysts constantly need high-purity substances: solvents, primarily water, reagents, and finally all the substances that they analyze. The tasks of preparing standard samples are as varied as the objects of analysis. And it is not always possible to use ready-made samples and their components of the required degree of purity. In its preparative interests, analytical chemistry comes into close contact with chemical technology. Methods of separation and concentration developed by analysts are often implemented in technological processes without undergoing fundamental changes. In this case, we can talk about large-scale extraction technologies for obtaining rare metals, and about processes in the pharmaceutical industry, in biochemical production, where the line between the scale of laboratory experiment and industrial production is practically absent.

General characteristics of separation methods

By separation methods we mean a set of chemical and physical processes characteristic of them and methods for their implementation. The process itself, for example, of separating substances between two liquid phases, is not yet a separation method. In combination with a method of implementation that ensures the transition of substances from one phase to another as a result of their equilibrium distribution between phases, such a process will become extraction, and in combination with a chromatographic method - liquid-liquid chromatography.

Difficulties in any attempt to systematize separation methods are introduced by combined methods of analysis. The names “gas” and “liquid” chromatography hide both methods for the chromatographic separation of substances in the gas and liquid phases, and corresponding combined methods.

There is still no generally accepted classification of separation and concentration honeys. When considering various methods together, you most often encounter a simple listing of them. Or with unification into groups on some formal basis outside general classification. When systematizing separation methods, in the simplest case, the starting point is that the method belongs to one or another field of science that gave birth to it: chemical, physicochemical, physical methods. Let us classify separation methods in the tables below.

Table 1. Separation methods based on the formation of a new phase by the isolated substance

Aggregate state of the phase in which the initial mixture of substances is located Aggregate state of the isolated phase Solid body Gas Liquid Liquid Precipitation, electrodeposition, freezing, crystallization Distillation, distillation, rectification - Gas - Freezing Solid - High temperature distillation when interacting with a gaseous reagent, sublimation Selective dissolution

Phase separation methods comprise four groups and are based on:

2.differences in the distribution of substances between phases;

.differences in mass transfer, manifested during the induced transition of a substance from one phase to another through the third phase separating them;

.mechanisms of intraphase separation.

For the first group of methods, the characteristic features are the aggregative states of the initial mixture of substances and the isolated phase (Table 1). Methods of the second group are based on the general patterns of distribution of substances between phases and can be characterized by their state of aggregation and the method of carrying out the process of interphase distribution. For the third and fourth groups, in addition to the state of aggregation of the phases, a characteristic feature is the nature of the driving forces of the process.

The significance for analytical chemistry of the separation methods included in the first group is far from ambiguous. The processes of freezing out both liquid and gas phases and selective dissolution of the solid phase are used relatively rarely. Freezing is used in gas analysis to separate moisture and to cryogenically concentrate higher-boiling impurities. Selective dissolution is used in two variants: partial or complete dissolution of the matrix and selective dissolution of phases. An example of complete dissolution of the matrix is ​​the dissolution of steels and alloys when determining non-metallic inclusions: oxides, carbides, nitrides.

Of utmost importance for analytical chemistry are separation methods based on differences in the distribution of substances between phases: extraction, sorption, various chromatographic methods. A characteristic feature of the separation methods of this group, in addition to the phase system, is the method of carrying out the process of interphase separation (Table 2)

Table 2. Separation methods based on differences in the distribution of substances between phases

Phase system Method of carrying out the process of interphase separation Single equilibrium distribution Multiple repetitions of the distribution process ion exchange Multiple recrystallizationIon exchange, adsorption, gel permeation, affinity chromatographyLiquid-gasGas extractionBubblingGas-liquid and liquid-gas chromatographyCritical substance-solid (liquidity)Supercritical fluid extractionMultistage fluid extractionSupercritical fluid chromatography

Extraction as a separation method

Of the separation methods based on a single equilibrium separation of substances, extraction has the greatest practical importance. Extraction is one of the most reliable, very effective and widespread methods for concentrating and separating substances. The research and application of extraction is a leading, rapidly developing direction in modern chemistry.

Extraction refers to both the process of distributing substances between two phases and the separation method based on this process. In the most general case, we can consider phase equilibria in systems liquid-liquids, liquid - gas. A wide variety of liquid phases are possible: water and aqueous solutions, organic solvents and solutions of other organic compounds in them, molten salts and metals, molten solids. normal conditions organic compounds. The gas extraction method (liquid-gas system and, less commonly, solid-gas system) has a narrower purpose - for the analysis of gaseous and volatile compounds in condensed phases, and differs from conventional extraction only in that gas is used as an extractant, which does not interfere with the analytical determination of gaseous compounds. impurities.

Many methods close to extraction, such as paper and column partition chromatography, are based on the distribution of substances between two liquid phases. In partition chromatography, one of the phases, organic or aqueous, is fixed on an inert carrier, while the other is moving. This achieves multiple exchanges between phases. Of particular importance is the extraction of various metal compounds from aqueous solutions into organic solvents that are immiscible with them.

Extraction is carried out by 1) bringing solutions (metal ions and extractant) into contact (mixing); 2) mechanical phase separation; 3) regeneration of the extractant.

The areas of application of extraction are very diverse. Extraction makes it possible to separate even small amounts of impurities from the base, which is especially important when obtaining and analyzing high-purity materials, separating radioisotopes, purifying biological materials, etc.

Extraction is used less frequently to separate the base from traces and, as a rule, only in cases where impurities cannot be isolated. Macrocomponents are usually extracted in the form of complex metal halide acids (for example, iron from HCI solutions diethyl ether) or coordination-solvation salts. Microimpurities are often extracted in the form of intracomplex compounds, less often - in the form of complex metal acids.

Extraction is also effective in separating components with similar properties, including high-boiling substances and azeotropic mixtures.

Extraction is widely used to increase the sensitivity of determinations by many chemical and physicochemical methods of analysis. Extraction plays a significant role in the study of equilibria in solutions, complex formation processes, and in general in the study of the state of substances in solutions.

Such advantages of extraction as versatility, rapidity, ease of implementation, speed, low operating temperatures, accessibility, lack of complex equipment, relatively small (or even absence) co-extraction, and others, make extraction very effective method concentration of microimpurities and separation of substances. To date, methods have been developed for the extraction of almost all elements and many classes of compounds both for preparative purposes and in technology, especially nuclear technology.

Extraction can be used for both absolute and relative concentration. Relative extraction concentration, at which enrichment is achieved, i.e., the ratio between macro- and microcomponents decreases, is more important for analysis.

In the practice of chemical analysis, extraction is used either only as a separation method; the isolated element in this case (if necessary, the extract is pre-mineralized) is determined by any conventional method, or in combination with subsequent determination (extraction-photometric, extraction-polarographic and other, so-called combined methods). Determination of the element of interest can be done in both aqueous and organic environments

Phase system Driving force of the process Chemical potential gradient Electrical potential gradient Pressure gradient Liquid-liquid-liquid Dialysis through liquid membranesElectrodialysis through liquid membranes - Liquid-solid-liquidDialysis Electrodialysis, electroosmosisUltrafiltration, reverse osmosis, piezodialysis Liquid-solid-gasEvaporation through a membrane - - Gas-solid-gas gas diffusion separation - -

Separation methods based on multiple equilibrium distributions always have fundamental limitations on the purity of the isolated fractions. Obviously, good separation in this case can be achieved only if there is a significant difference in the properties of the substances being separated. The process in the field of these methods is entirely determined by the creation of new selective sorbents or extractants. But the emergence of fundamentally new classes of such compounds, such as organophosphorus extractants crown ethers or chiral sorbents, is a relatively rare phenomenon. As a rule, the search leads to the need to obtain increasingly complex and expensive compounds, and the positive effect of their use does not justify the search costs.

Based on the data given in the table. 2 classification of methods based on differences in the distribution of substances between phases, one can see another way to increase the efficiency of methods of this group, which consists in improving the methods of carrying out interphase distribution processes, in the transition from single processes to multi-stage and chromatographic ones.

It may seem somewhat strange that the chromatographic method is classified as a method for carrying out the process of interfacial distribution of substances. Typically, chromatographic methods are considered in isolation from other separation methods based on differences in the distribution of substances between phases. Numerous definitions of chromatography, proposed over the years, emphasize that chromatography is a separation method or a process leading to the separation of substances. There is no contradiction here. Historically, chromatography arose as a separation method, the essence of which was the separation of zones of individual substances when a solution containing their mixture passes through a layer of solid sorbent. When considering the physicochemical phenomena that cause this “dissolution”, it is natural to speak of chromatography as a process occurring in a two-phase system. When it comes to the totality methodological techniques, applied to almost any two-phase system, which makes it possible to greatly increase the separation coefficients achieved with a single equilibrium distribution of substances between phases, we can say that chromatography is a way to carry out the process of interphase separation.

Chromatography as a method of carrying out the interphase process consists in the relative movement of phases in a limited space under conditions where one of them is constantly in a dispersed state or in the form of a film on the surface of the walls delimiting this space. This movement occurs in a column, in a capillary, in a thin layer. One of the phases may be stationary or both will be in motion. In any case, the chromatographic method provides multiple sequential redistribution of substances between mutually moving phases, leading to differences in the speeds of movement of the zones of individual substances in the separation space, and in the case of simultaneous movement of both phases in different directions, to differences in the direction of movement of the zones. And the actual separation method will be the application of one of the possible manifestations of the chromatographic method of implementing the process of interphase distribution to a specific two-phase system.

The proposed interpretation of the concept of “chromatography” seems essential for understanding the generality of chromatographic separation methods that underlie the most common chromatographic methods of analysis today. Interest in chromatographic methods is determined primarily by the fact that they theoretically have no restrictions on the separation coefficients of substances, no matter how small the differences in distribution coefficients may be for them. This is the qualitative leap that comes from the transition from methods based on a single equilibrium distribution to chromatographic ones.

In methods for separating substances based on differences in interfacial distribution, there are always restrictions on mass transfer. More substances cannot move from one phase to another than follows from the distribution coefficient, the value of which, as a rule, decreases with increasing amount of substance in the phase system. The transition to multi-stage and chromatographic methods allows for improved separation, but introduces even more stringent restrictions on the amount of substances to be separated. So, for chromatographic methods prerequisite the coefficient KD becomes independent of concentration, i.e., the requirement for the linearity of the interphase distribution isotherm. Hence, for solving problems that require an increase in mass transfer without increasing the volume of the separating phase, the most promising group of methods is based on induced interphase transfer of matter. We are talking about processes in which separation is carried out under the influence of a constantly acting driving force. In the general case, the implementation scheme of such processes involves the transfer of a substance from one phase to another through a third phase separating them, which is a partition, a membrane. Hence the name of this group of separation methods - membrane methods. In a particular case, there are known attempts to implement induced transfer within a two-phase system - the process of electrical extraction. But since the method is not widely used, and its mechanism is described within one of the stages of the general scheme of the extraction-membrane process, it does not deserve special consideration.

The classification by membrane type is a tribute to the history of the development of membrane methods, because their appearance in most cases was initiated by the creation of selectively permeable materials. Only in last years There has been a trend toward a targeted search for membranes that meet the requirements of a specific membrane separation method. Based on the definition of membrane separation methods as processes of indicated transfer of a substance from one phase to another through a third phase separating them, their main classification features are considered to be the throne system of phases and the driving force of the process (Table 3). Since the main advantage of the membrane scheme for carrying out the separation process is the increase in mass transfer of the substance through the separating phase, membrane methods naturally fall primarily into the sphere of interest of chemical technology. However, a number of interesting areas of application of membrane methods in chemical analysis have already been found, but a mutually enriching exchange of ideas between chemical technology and analytical chemistry in the future cannot be ruled out.

Finally, there remains the possibility of separating substances due to differences in the properties of their ions, atoms or molecules, manifested within one homogeneous system under the influence of electric, magnetic, gravitational, thermal fields or centrifugal forces. At the same time, the possibility of phase transformations is not excluded when transferring the initial mixture of substances to the state of aggregation in which separation occurs, or when separating fractions of its individual components. The separation effect is achieved due to different spatial movements of substances within the phase in which their separation occurs. Differences in the speed of spatial movement of ions, atoms or molecules will appear depending on their mass, size, charge, energy of interaction of particles with ions and molecules forming the medium in which separation occurs. The relative role of certain factors in achieving the final separation effect, in turn, depends on the nature of the forces acting on them. The most obvious case is electrophoretic or, as it is sometimes called, electromigration separation of ions in solutions due to the different speeds of their movement in the electric will. Here the most important factors the size and charge of the ion are determined. Differences in mass and charge are most pronounced when ionized particles are exposed to an accelerating electric field and a deflecting magnetic field. This method of influencing the system is the basis of the mass separation method. When separating under the influence of centrifugal forces—ultracentrifugation—the determining factor is the mass of the molecules.

Table 4. Intraphase separation methods

Aggregate state of the phases in which separation occurs Type of forces causing spatial movement of ions, atoms or molecules Electric field Electric and magnetic field Centrifugal force or gravitational field Liquid Electrophoresis (electromigration) - ultracentrifugation Gas Electrophoresis Mass separation ultracentrifugation

Consequently, any of the known methods of intraphase separation can be characterized by the aggregative state of the phase within which the separation occurs and the type of forces causing the spatial movement of ions, atoms or molecules.

Intraphase separation methods in general are characterized by complex hardware solutions, and the feasibility of their use in analytical chemistry is justified in proportion to the capabilities that other methods do not have. The simplest technical design is the method of electrophoretic (electromigration) separation of ions in solution, which retains certain areas of application in analytical chemistry. Mass separation as a separation method is interesting primarily because it is the basis of one of the widely used methods of chemical analysis - mass spectrometry. Here there was an even closer merging of the separation method and methods final definition than in the case of chromatographic methods of analysis. When describing a mass spectrometric method, it is usually not even mentioned that it is one of the combined methods of analysis. The complexity of the equipment and high energy consumption in the mass separation method are compensated by its versatility and practically unlimited separation capacity.

General characteristics of concentration methods

The determination of microimpurities is an urgent task due to increased requirements for the purity of materials and the need for analytical monitoring of the environment. To determine trace amounts, only methods that allow the detection of impurities weighing 10 -7-10-8g, and sometimes up to 10 -14d. The most important are physical methods of analysis: atomic adsorption, neutron activation, X-ray fluorescence and some others.

The main tasks when determining macrocomponents:

1.The use of very small portions or sample volumes with significant contents of the components being determined;

2.Analysis of samples of large mass or volume to determine the content of trace amounts of substances.

To solve the first problem, in addition to the indicated physical methods of analysis, methods of ultramicroanalysis, including ultramicrochemical analysis, are suitable. It is a set of techniques for using special equipment for working with ultra-small volumes of solutions. To solve the second problem, concentration is used as a preliminary operation. It is necessary in cases where it is necessary to increase the concentrations of microcomponents for subsequent analysis or to separate trace amounts of analyte components from the main or other microcomponents. With absolute concentration, microcomponents are transferred from a larger volume to a smaller one.

As follows from the definition, concentration is always associated with the separation and redistribution of substances into different phases, therefore all methods suitable for separation are used for concentration. The most common methods are listed in table. 5. when choosing a concentration method, they are guided by the nature of the object and its chemical composition, the subsequent method of analysis, the duration of all operations, the provision of all necessary equipment, etc.

Table 5. Concentration methods for trace analysis

Method Characteristics and advantages Disadvantages Extraction Allows you to concentrate both impurities of a group of substances and individual substances. The method is universal, simple to design; Use of expensive reagents; Co-precipitation Allows concentration of substances; Long-lasting, less universal, low selectivity; Ion exchange chromatography; Used to exchange the main component of a mixture for H +(OH -) or for concentrating microcomponents from large volumes of solutions. Complete separation is achieved at low values ​​of distribution coefficients. Selectivity is low, losses and contamination are possible due to sorption processes, the process is labor-intensive Distillation It is used to concentrate highly volatile impurities with their condensation on a small surface. Does not require additional reagents and solvents. Application limited to certain classes of substances.

Co-precipitation as a concentration method

IN Lately Co-precipitation, one of the most effective and long-known methods for concentrating trace amounts of various elements, is of particular importance for analytical purposes.

Co-precipitation is a type of distribution, i.e. the distribution of a microcomponent caused by the release of the reservoir into the solid phase. In other words, it represents the simultaneous transition of micro- to macrocomponents in the forming solid phase of the reservoir. Co-precipitation involves the capture of impurities during the growth of collector particles (introduced into the system as newly formed ones). During Ostwald ripening of sediment, as well as during structural and morphological improvement of solid phase particles. Metal hydroxides (iron, aluminum, etc.), sulfides (CdS, HgS), phosphates (Ca3(PO4)2, etc.), sulfates (BaSO4, etc.) and other inorganic compounds are used as collectors.

There are two broad classes of pollution:

Co-precipitation, when the main substance and the impurity are deposited together. The fact that two substances precipitate simultaneously does not indicate coprecipitation. Thus, if, for example, traces of beryllium hydroxide are quantitatively precipitated with a large amount of aluminum hydroxide under conditions where both are insoluble, that is, from their saturated solutions, then one should speak of co-precipitation, and not of coprecipitation (conjugate precipitation);

Post-deposition is the transition of impurities into sediment not during its formation, but after. First, a pure main precipitate is released, and then an impurity. Typically, post-precipitation occurs in a supersaturated solution. Thus, traces of zinc (II), indium turn into precipitate upon long-term contact of their solutions with the precipitate of metal sulfides.

When deposition involving one solid phase, the following cases of coprecipitation are distinguished:

Formation of a chemical compound. The gross composition of the solid phase is different from the composition of each of its ingredients, but the local composition is the same.

Formation of a solid solution. A change in the gross composition of the solid phase with a change in the concentration of components in the initial mixture indicates the formation of a solid solution. The solid phase resulting from this type of coprecipitation is sometimes called a compound of variable composition. The formation of solid solutions occurs as a result of molecular processes that can be considered as quasi-chemical exchange or addition reactions. The generality of the molecular mechanism of formation is an important argument in favor of combining all cases of coprecipitation with the formation of solid solutions in one subclass. This subclass can be divided into two types depending on whether the solid solution is crystalline or amorphous. Co-precipitation with the formation of a crystalline solid phase is usually called co-crystallization.

Formation of a solid phase with an impurity segregated on defects. The gross composition of the solid phase in this case depends on the composition of the initial mixture, and the local composition is not the same. There are two types of segregation:

A) During their growth, only the macrocomponents of the initial mixture or the products of their interaction pass into the volume of sediment particles during their growth, and the microcomponent is pushed aside by the growing particles, accumulating near the phase interface (episegregation). This type is associated with the capture of impurities by the surface of growing sediment particles;

b) the microcomponent is localized in the volume of the solid phase (endosegregation), near dislocations (dislocation endosegregation), at intercrystalline boundaries (intercrystalline endosegregation) or is located within isolated inclusions of the uterine medium (occlusive endosegregation).

In coprecipitation with the participation of several solid phases, a distinction is made between coprecipitation with the participation of separable and inseparable solid phases.

If the coprecipitation process occurs either only on the surface of the solid phase, or also inside it, then two types of coprecipitation are distinguished:

adsorption - deposition of impurities on the surface of particles;

occlusion - deposition of an impurity within the primary particles through any possible mechanism.

The phenomenon of coprecipitation is widely used in analytical chemistry as a simple and effective method extracting traces of elements from highly dilute solutions in which the solubility product of the precipitate is not achieved.

Conclusion

Methods of separation and concentration are accustomed to be viewed as something that complements analytical chemistry, expanding its capabilities, but not as its fundamental core part. On the other hand, as already noted in the introduction, one of the two main directions in the development of analytical chemistry is a priori focused on a limited combination of methods for the separation and determination of substances in objects of analysis.

Bibliography

1.Moskvin L.N., Tsaritsina L.G. Methods of separation and concentration in analytical chemistry. - L., “Chemistry”, 1991. - 256 p.

2.Skorokhod O.R. Chemical analysis: Fundamentals of methods for concentrating and separating substances. - Mn., "BSU Publishing House", 1980. - 272 p.

.Posypaiko V.I. and others. Chemical methods of analysis: Textbook. manual for chemical technologists. universities / Posypaiko V.I., Kozyreva N.A., Logacheva Yu.P. - M., “Higher. school", 1989. - 448 p.

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