Physicochemical methods of separation and concentration of elements. Separation and concentration methods

Separation is an operation that allows the components of a sample to be separated 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 is not selective enough and overlap of analytical signals must be avoided. In this case, the concentrations of the separated substances are usually close.

Concentration is an operation that allows you to increase the concentration of a 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 Cmin, i.e. when the analysis method is not sensitive enough. However, the concentrations of the components vary greatly. Concentration is often combined with separation.

Separation and concentration have much in common; the same methods are used for these purposes. They are very diverse.

There are many classifications of separation and concentration methods based on different characteristics.

a) classification according to the nature of the process

Chemical methods of separation and concentration are based on the flow chemical reaction, which is accompanied by precipitation of the product and gas evolution.

Physico- chemical methods separations and concentrations are most often based on the selective distribution of a substance between two phases.

Physical methods of separation and concentration are most often based on changing the state of aggregation of a substance.

b) classification by physical nature two phases

The distribution of a substance can be carried out between phases that are in the same or different states of aggregation: gaseous (G), liquid (L), solid (S).

c) classification by the number of elementary acts (stages)

Single-stage methods are based on a single distribution of the substance between two phases. The separation takes place under static conditions.

Multistage methods are based on multiple distribution of a substance between two phases. There are two groups of multi-stage methods: with repetition of the single distribution process, methods based on the movement of one phase relative to another.

d) classification according to the type of equilibrium

Thermodynamic separation methods are based on differences in the behavior of substances in an equilibrium state. They have highest value V analytical chemistry.

Kinetic separation methods are based on differences in the behavior of substances during the process leading to an equilibrium state. For example, in biochemical research, electrophoresis is of greatest importance. Other kinetic methods are used to separate particles of 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 the separation and simultaneous qualitative and quantitative analysis of multicomponent mixtures.

Ion exchange

Ion exchange is a reversible stoichiometric process that occurs at the interface between the ionite and the electrolyte solution.

Ion exchangers are high molecular weight polyelectrolytes of various structures and composition. The main property of ion exchangers is that they absorb cations or anions from a solution, releasing into the solution an equivalent number of ions of the same charge sign.

Chromatographic methods of analysis

Chromatography is a dynamic method for the separation and determination of substances, based on the multiple distribution of components between two phases - mobile and stationary. The substance enters the sorbent layer along with the flow of the mobile phase. In this case, the substance is sorbed and then, upon contact with fresh portions of the mobile phase, desorbed. The movement of the mobile phase occurs continuously, so sorption and desorption of the substance occur continuously. In this case, part of the substance is in the stationary phase in a sorbed state, and part is in the mobile phase and moves with it. As a result, the speed of movement of the substance is less than the speed of movement of the mobile phase. The more a substance is sorbed, the slower it moves. If a mixture of substances is chromatographed, then the speed of movement of each of them is different due to different affinities for the sorbent, as a result of which the substances are separated: some components are delayed at the beginning of the journey, others move further.

Depending on the state of aggregation of the phases, a distinction is made between gas chromatography (mobile phase - gas or vapor) and liquid chromatography (mobile phase - liquid).

According to the mechanism of interaction of a substance with a sorbent, the following types of chromatography are distinguished: adsorption, distribution, ion exchange, sedimentation, redox, complexing, etc.

The gas chromatography method has become most widespread because the theory and equipment for it have been most fully developed. Gas chromatography is a hybrid method that allows simultaneous separation and determination of the components of a mixture. Gases, their mixtures or compounds that are in the gaseous or vapor state under separation conditions are used as the mobile phase (carrier gas). Solid sorbents or liquid applied in a thin layer to the surface of an inert carrier are used as a stationary phase.

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 content organic matter 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, in which the operations of separation, concentration and chemical analysis itself are combined in one device. For example, when analyzing natural objects(water, ice, soil) for the presence of heavy metals 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 analytical 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 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, with enough high values distribution coefficient, even a single extraction allows you to quantitatively extract 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 sufficient availability for chromatography large surface separation between phases and a dynamic separation method (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. This is the process of absorption of gases, vapors and dissolved substances by solids or liquid substances. Sorption is widely used for the separation and concentration of substances. In this case, good separation selectivity is usually achieved and large values concentration coefficients.

The sorption process is relatively easy to control, and the implementation of this method does not require complex instrumentation or 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 by electric current at a 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.

Exist various options 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 with the corresponding 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 very big time, therefore, they are limited to its partial selection. Concentration of the microcomponent can be achieved not only by its deposition on the electrode, but also by electrochemical dissolution of the matrix.

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

The cementation method involves reducing components (usually microcomponents) to 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, etc.

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 are certain conditions allow you to achieve high concentration coefficients.

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

There are many classifications of separation and concentration methods based on different characteristics. Let's look at the most important of them.

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

Rice. 1

Chemical methods of separation and concentration are based on the occurrence of a chemical reaction, which is accompanied by precipitation of the product and the release of gas. For example, in organic analysis, the main method of concentration is distillation: during thermal decomposition, the matrix is distilled off in the form of CO2, H2O, N2, and metals can be determined in the remaining ash.

Physicochemical methods of separation and concentration are most often based on the selective distribution of a substance between two phases. For example, in the petrochemical industry, chromatography is of greatest importance.

Physical methods of separation and concentration are most often based on changing the state of aggregation of a substance.

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 states of aggregation: gaseous (G), liquid (L), solid (S). In accordance with this, the following methods are distinguished (Fig.).

Rice. 2

In analytical chemistry, methods of separation and concentration, which are based on the distribution of a substance between the liquid and solid phases, have found the greatest importance.

3. Classification according to the number of elementary acts (stages).
§ Single-stage methods - based on a single distribution of a substance between two phases. The separation takes place under static conditions.
§ Multistage methods - based on multiple distribution of a substance between two phases. There are two groups of multi-stage methods:
– repeating the single distribution process (for example, repeated extraction). The separation takes place under static conditions;
– methods based on the movement of one phase relative to another (for example, chromatography). Separation takes place under dynamic conditions
3. Classification according to the type of equilibrium (Fig.).

Rice. 3

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

Direct instrumental methods often cannot be used in the analysis of many complex objects, either due to the inhomogeneous distribution of components in the sample, or due to calibration difficulties when there are no standard samples of known composition. This may be true for a number of industrial, geological, biological materials, objects environment, as well as high-purity substances containing some components at the level of μg/l, ng/g, ng/l. In such cases, they resort to concentration and separation of microcomponents, separation of the bulk of macrocomponents or impurity elements, followed by analysis of the resulting concentrate using various chemical and instrumental methods.

The operations of separation and concentration are based on the same processes and methods, based on the difference in the chemical and physical properties of the separated components - solubility, sorption, boiling and sublimation temperatures and differing concentrations of the separated components.

Separation is a process or operation as a result of which the components that make up the original mixture, and the concentrations of which may be comparable, are separated from each other.

Concentration is a process or operation that results in an increase in the ratio of the concentrations or amounts of microcomponents to the concentration or amount of macrocomponents.

Extraction - a method of separation and concentration based on the distribution of a solute between two immiscible phases (usually in practice one phase is water solution, and the second is an organic solvent). The main advantages of the extraction method:

1) the possibility of varying the selectivity of separation

2) the ability to work with analytes on various levels concentrations;

3) ease of technological and hardware design;

4) the possibility of implementing a continuous process, automation;

5) high performance.

Extraction methods for isolating substances have found wide application in the analysis of components of some industrial and natural objects. Extraction is performed quite quickly, while achieving high efficiency of separation and concentration, and is easily compatible with a variety of analytical methods. Many analytical extraction methods have become prototypes for important technological extraction processes, especially in nuclear energy.

Basic terms of the extraction method:

extractant- an organic solvent, containing or not containing other components and extracting the substance from the aqueous phase;

extraction component- a reagent that forms a complex or salt with the extracted component that can be extracted;

diluent- an inert (organic) solvent used to improve the physical (density, viscosity, etc.) or extraction (for example, selectivity) properties of the extractant. Inertness refers to the inability to form compounds with the extracted substance.

extract- a separated organic phase containing a substance extracted from the aqueous phase;

re-extraction- the process of reverse extraction of a substance from the extract into the aqueous phase;

re-extractant- a solution (usually aqueous or water only) used to extract the substance from the extract;

re-extract- a separated phase (usually aqueous) containing the substance extracted from the extract as a result of stripping;

salting out- improving the extraction of a substance by adding an electrolyte (salting out agent), which promotes the formation of the extracted compound in the aqueous phase.

Types of extraction systems

When performing liquid-liquid extraction, several types of extraction systems can be distinguished.

Type I extraction systems. In these extraction systems, organic solvents or mixtures thereof are used as the organic phase, and either water or aqueous solutions of salts as the aqueous phase. The wide spread of such extraction systems is explained by the low cost of water as a solvent, its limited miscibility with many organic solvents, and also by the fact that in the vast majority of cases the object that needs to be extracted is either initially in an aqueous solution or is converted into a water-soluble state during the process of sample preparation of the object .

In some cases, type I extraction systems are unsuitable for work; in this case, type II extraction systems are used.

Type II extraction systems. These extraction systems use an aliphatic hydrocarbon as the non-polar phase, and the second phase is either a polar organic solvent, an aqueous solution thereof, or a solution of zinc halide in a polar organic solvent. Typically, low-boiling hydrocarbons are most often used as the aliphatic hydrocarbon, in particular hexane, heptane, octane, cyclohexane or petroleum ether.

An important criterion for choosing solvents for an extraction system is the limited miscibility of the extraction phases.

Extraction methods

Depending on the problem being solved, simple extraction, batch extraction or countercurrent extraction are used. Batch extraction is the extraction of a substance from one phase using separate portions of fresh extractant. At residually high values of the distribution coefficient, a single extraction will allow quantitative extraction of the substance into the organic phase. The efficiency of a single extraction can be characterized by the degree of extraction -R, %, calculated by the formula: $R=org*100%/total$ where org. - the amount of substance A in the organic phase; total - the total amount of substance A in both phases.

If single extraction does not provide sufficient recovery, R can be increased by increasing the volume of the organic phase or by resorting to multiple extractions.

Batch extraction is preferably carried out in a separating funnel into which an aqueous solution containing the extracted compound and an organic solvent immiscible with the aqueous phase are introduced. Then the funnel is shaken vigorously to ensure phase contact. After shaking, the phases are separated.

A serious disadvantage of multiple extraction is the significant dilution of the extracted component, especially if the number of stages is large. Extractant consumption can be reduced if exhaustive extraction is carried out in continuous extraction machines. Continuous extraction is carried out by continuous and relative movement of two phases; one of the phases, usually aqueous, remains stationary.

Continuous extraction is particularly useful when the distribution coefficient is very small and it would be necessary to carry out very big number successive extractions. General principle continuous extraction involves distilling the extractant from the distillation flask, condensing it, and passing it through the solution to be extracted. The extractant is separated and flows back into the receiving flask, from where it is distilled off again and goes through the cycle again, while the extracted substance remains in the receiving flask. If the solvent cannot be easily distilled, portions of fresh solvent can be continuously added from the reservoir, but the consumption of extractant will be significant.

Countercurrent extraction is carried out in a Craig apparatus, which consists of a series of specially designed cells arranged in such a way that one phase (for example, organic) sequentially passes from one cell to another after each equilibrium distribution.

Schematic illustration of a countercurrent extraction device

Before extraction begins, all cells are partially filled with a heavy solvent, which is the stationary phase. The mixture to be separated in the same solvent is placed in cell 0. Then a lighter solvent (mobile phase) that is immiscible with the first is introduced into cell 0. The phases are mixed and left to separate. After phase separation upper layer from cell 0 is transferred to cell 1, and a new portion of fresh solvent is introduced into cell 0 and simultaneous extraction is carried out in both cells. Next, the upper layers from cells 0 and 1 are transferred to cells 1 and 2, respectively, a new portion of the mobile phase is again introduced into cell 0, and the extraction process is repeated. The introduction of fresh solvent into the system allows for any number of extractions.

Countercurrent extraction has high separation efficiency. With its help, it is possible to share substances with loved ones chemical properties. For example, this method has been used to separate rare earth elements. Countercurrent separation is widely used for fractionation organic compounds. A significant disadvantage of countercurrent extraction is the strong dilution of components during separation.

1) Physical methods: evaporation (evaporation), distillation

Evaporation – incomplete evaporation of the solvent (volume reduction – concentration)

Evaporation – evaporation of the solvent to dryness (followed by dissolution of the dry residue in a small volume)

Distillation – separation of volatile components

2) Chemical methods: precipitation, coprecipitation

Precipitation – separation (systematic course of analysis); concentration (precipitation of the ion to be determined from a large volume of the analyzed solution and dissolution of the precipitate in a small volume)

Co-precipitation – simultaneous precipitation from the same solution of a microcomponent soluble under given conditions with a precipitated macrocomponent.

Causes of coprecipitation: 1) surface adsorption - the co-precipitated substance is adsorbed on the surface of the collector and deposited with it; 2) occlusion - mechanical capture of part of the mother solution with the coprecipitated ion inside the collector sediment; 3) inclusion – formation of mixed crystals

Co-precipitation is used to concentrate substances present in microquantities in the analyzed solution, followed by their determination in the concentrate.

3) Physico-chemical methods: extraction, chromatography

Extraction – a method of extracting a substance from a solution or dry mixture using a suitable solvent. To extract from a solution, solvents are used that are immiscible with this solution, but in which the substance dissolves better than in the first solvent. Extraction is used in the chemical, oil refining, food, metallurgical, and pharmaceutical industries.

Chromatography – dynamic sorption method for separating and analyzing mixtures of substances, as well as studying physical and chemical properties substances. It is based on the distribution of substances between two phases - stationary (solid phase or liquid bound on an inert carrier) and mobile (gas or liquid phase).

88. Methods of qualitative chemical analysis

Microcrystalloscopic analysis

Reactions that produce compounds with characteristic crystal shapes can be used to detect cations and anions. The shape and rate of crystal formation are affected by the reaction conditions. A significant role in microcrystalloscopic reactions is played by the rapid evaporation of the solvent, which leads to concentration of the solution and, consequently, an increase in the sensitivity of ion determination.

Pyrochemical analysis

When heating substances in a burner flame, one can observe various characteristic phenomena: evaporation, melting, color change, flame coloring. All these phenomena are used in qualitative analysis for preliminary testing of the substance. Sometimes, with the help of pyrochemical reactions, it is possible to increase selectivity and sensitivity of determination. Pyrochemical reactions used for mineral analysis in the field.

Flame coloring

When a metal salt solution is introduced into a flame, a number of complex processes occur: evaporation, formation of solid aerosols, dissociation, ionization, interaction with oxygen, excitation of atoms, ions and molecules. The end result of these processes is the analytically used effect - flame glow.

89. Determination methods quantitative composition connections

90. Basic physical quantities

Physical quantity – physical property material object, physical phenomenon, a process that can be characterized quantitatively.

Physical quantity value – a number characterizing this physical quantity, indicating the unit of measurement on the basis of which they were obtained.

System physical units – a set of units of measurement of physical quantities, in which there is a certain number of so-called basic units of measurement, and the remaining units of measurement can be expressed through these basic units. SI (System International) – international system of units. SI is the most widely used system of units in the world, as in Everyday life, and in science and technology.

In the SI system, each basic quantity has a corresponding unit: unit of length– meter (m); unit of time– second (s); unit of mass– kilogram (kg); units initial strength electric current – ampere (A); temperature unit– kelvin (K); unit of quantity of substance– mole (mol); unit of luminous intensity– candela (cd)

At practical use units International system are often either too large or too small, so decimal multiples and submultiples can be formed using special prefixes.

soundboard	Yes	10 1	deci	d	10 -1
hecto	G	10 2	centi	With	10 -2
kilo	To	10 3	Milli	m	10 -3
mega	M	10 6	micro	mk	10 -6
giga	G	10 9	nano	n	10 -9
tera	T	10 12	pico	P	10 -12
peta	P	10 15	femto	F	10 -15
exa	E	10 18	atto	A	10 -18

91. The concept of physical methods and their classification

92. Use of physical methods in expert research

93. The concept of the physical quantity “density”. Methods for determining density

Density – physical quantity, equal to the ratio body mass to its volume ( ρ = m/V). Based on the definition of density, its dimension kg/m 3 in the SI system.

The density of a substance depends on the mass of the atoms of which it consists, and on the packing density of atoms and molecules in the substance. The greater the mass of atoms and the closer they are located to each other, the greater the density.

Density meters used to measure the density of liquids, gases and solids.

Density of inhomogeneous matter - the ratio of mass and volume when the latter contracts to the point at which density is measured. The ratio of the densities of two substances at certain standard physical conditions called relative density; for liquid and solid substances it is measured at temperature t, usually relative to the density of distilled water at 4°C, for gases– in relation to the density of dry air or hydrogen at normal conditions (T= 273K, p = 1.01 10 5 Pa).

For bulk and porous solids, densities are distinguished true (the mass of a unit volume of a dense material that does not contain pores), apparent (the mass of a unit volume of porous material made from grains or granules) and bulk (the mass of a unit volume of a layer of material).

94. The concept of the physical quantity “mass”. Methods for determining mass

Weight – a scalar physical quantity, one of the main characteristics of matter, determining its inertial and gravitational properties. There are inertial mass and gravitational mass.

The concept of mass was introduced into mechanics I. Newton. In classical mechanics Newton mass is included in the definition of momentum (quantity of movement) of a body: impulse R proportional to the speed of the body V , p=mv (1). Proportionality factor – a constant value for a given body m– and there is body mass. The equivalent definition of mass is obtained from the equation of motion classical mechanics F=ma(2). Here mass is the coefficient of proportionality between the force acting on the body F and the acceleration of the body caused by it a. The mass determined by relations (1) and (2) is called inertial (inertial) mass ; it characterizes the dynamic properties of the body, is a measure of the inertia of the body: with a constant force, the greater the mass of the body, the less acceleration it acquires, i.e. the slower the state of its motion changes.

In the theory of gravity Newton mass acts as a source of the gravitational field. Each body creates a gravitational field proportional to the mass of the body (and is affected by the gravitational field created by other bodies, the strength of which is also proportional to the mass of the bodies). This field causes the attraction of any other body to this body with a force determined by Newton’s law of gravity: F = G* (m 1 *m 2 / R 2) - (3), where R– distance between bodies, G is the universal gravitational constant, a m 1 And m 2– masses of attracting bodies.

From formula (3) it is easy to obtain the formula for weight R body mass m in the Earth's gravitational field: P = mg(4). Here g = G*M/r 2- acceleration of gravity in the Earth's gravitational field. The mass determined by relations (3) and (4) is called gravitational mass of the body .

Scales - a device for determining the mass of bodies (weighing) by the weight acting on them, approximately considering it equal to the force of gravity. Let us consider as an example the measurement of body weight, which we measure using ordinary equal-arm scales. Under the influence of gravity, forces are created. The mass of the body, together with these forces, presses on one cup, and the mass of the weights on the other. By selecting weights, we achieve balance, i.e. equality of these forces. This gives us the right to say that the mass of the body being weighed is equal to the mass of the weights, assuming that the force of gravity at the distance between the cups remains the same. As you can see, to measure mass we had to convert the masses of the body and weights into forces, and to compare forces with each other, convert their action into mechanical movement of the levers of the scales.