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Methods of analytical chemistry. Methods of separation and concentration Methods of separation and concentration in pharmaceutical chemistry

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1. Separation and concentration methods

General information about separation and concentration

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.

Types of concentration.

1. Absolute: the microcomponent is transferred from a large volume or large mass of the sample (Vpr or mpr) to a smaller volume or smaller mass of the concentrate (Vconc or mconc). As a result, the concentration of the microcomponent increases n times:

where n is the 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 by 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 it was 1: 10. This is usually achieved by partial removal of the matrix.

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

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 Classification of separation methods according to the nature of the process

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 Classification of separation methods according to the nature of the phases

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 Classification of separation methods by type of equilibrium

Thermodynamic separation methods are based on differences in the behavior of substances in an 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 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.

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, conditions are created such that one component completely passes into the organic phase, and the other remains in the aqueous phase. The phases are then separated using a separating funnel.

For the purpose of absolute concentration, the substance is transferred from a larger volume of aqueous solution to a smaller volume of the organic phase, as a result of which the concentration of the substance in the organic extract increases.

For the purpose of relative concentration, conditions are created so that the microcomponent passes into the organic phase, and the majority of the macrocomponent remains in the aqueous phase. As a result, in the organic extract the ratio of the concentrations of the 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 methods of subsequent determination, as a result of which a number of important hybrid methods have emerged (extraction-photometric, extraction-spectral, etc.).

Co-precipitation as a method of separation and concentration

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

Inorganic and organic poorly soluble compounds with a developed surface are used as collectors. Phase separation is carried out by filtration.

Co-precipitation is used for the following purposes:

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

§ separation of 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.

Activated carbons, Al2O3, silica, zeolites, cellulose, natural and synthetic sorbents with ionogenic and chelating groups are used as sorbents.

The absorption of substances can occur on the surface of the phase (adsorption) or in the volume of the phase (absorption). In analytical chemistry, adsorption is most often used for the purpose of:

§ separation of substances, if conditions for selective absorption are created;

§ concentration (less often).

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

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 polyelectrolytes of various structures and compositions. concentration chemical sorption gas

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.

The process of ion exchange is described by the law of mass action:

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

This equilibrium is characterized by the exchange constant (K):

where a is the activity of ions.

If K > 1, then the B ion has a greater affinity for the ion exchanger; if K< 1, то ион А обладает бульшим сродством к иониту; если же К? 1, то оба иона одинаково сорбируются ионитом.

The following factors influence the course of ion exchange:

1) the nature of the ion exchanger;

2) the 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 larger charge, and from concentrated solutions - with a smaller one;

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

Types of ion exchangers

There is a large number of different ion exchangers. They are classified according to their origin and the sign of the charge of the exchanging ions.

Depending on the origin, two groups are distinguished
ion exchangers:

1. Natural ion exchangers:

§ inorganic (clays, zeolites, apatites);

§ organic (cellulose).

2. Synthetic ion exchangers:

§ inorganic (permutites);

§ organic (high molecular weight materials).

In analytical chemistry, synthetic organic ion exchangers are most often used.

Depending on the sign of the charge of the exchanging ions, ion exchangers are called as follows:

1. Cation exchangers - exchange cations, contain acid groups:

§ -SO3H (strong acid cation exchangers, exchange occurs in a wide range of pH values);

§ -PO3H2 (medium acid cation exchangers, exchange occurs at pH > 4);

§ -COOH, -OH (weak acid cation exchangers, exchange occurs at pH > 5).

2. Anion exchangers - exchange anions, contain basic groups:

§ quaternary alkylammonium groups (highly basic anion exchangers, exchange occurs in a wide range of pH values);

§ amino and imino groups (medium and low basic anion exchangers, exchange occurs at pH< 8-9).

3. Ampholytes - exchange both cations and anions depending on conditions. They have both types of groups - acidic and basic.

Structure of synthetic organic ion exchangers. Ion exchange reactions

Synthetic organic ion exchangers have a three-dimensional chain structure. They consist of a high molecular weight (HM) matrix in which ionogenic groups are fixed.

For example, for a highly basic anion exchanger in the chloride form R-N(CH3)3Cl

Composition of the ion exchanger

stationary HM ion	mobile NM ion

	fixed ion	counterion
	ionic group

The matrix is usually a copolymer of styrene and divinylbenzene (DVB), which is a cross-linking agent: each of its molecules, like a bridge, connects 2 adjacent linear polystyrene chains.

Mobile low molecular weight (LM) ions that are part of ionogenic groups participate in ion exchange.

For example, a cation exchange reaction involving a strongly acidic cation exchanger in hydrogen form is written as follows:

and an anion exchange reaction involving a highly basic anion exchanger in chloride form

Basic physical and chemical characteristics of ion exchangers

Ionites as materials have many physical-chemical and physical-mechanical characteristics. Of these, three main physical and chemical characteristics are of greatest importance to the analytical chemist - moisture, swelling and exchange capacity.

Humidity (W, %) characterizes the ability of the ion exchanger to absorb moisture from the air. It can be calculated based on experimental data:

where mo and m are the mass of the ion exchanger before and after drying.

Typically, the humidity of ion exchangers is in the range of 10-15%.

Swelling characterizes the degree of increase in the volume of the ion exchanger upon contact with water or other solvent. The amount of swelling depends on the degree of cross-linking of the high molecular weight ion exchanger matrix (% DVB). Due to swelling, ion exchange occurs quickly. The reason for swelling is the presence of polar ionogenic groups capable of hydration or solvation. Exchange capacity (EC) is the most important quantitative characteristic of an ion exchanger. It characterizes the ability of the ion exchanger to ion exchange. The total exchange capacity (TEC) of a given ion exchanger is a constant value and is determined by the number of fixed ions in the ion exchanger matrix. It depends on the following factors: the nature of the ion exchanger;

§ solution pH value;

§ definition conditions (static or dynamic);

§ nature of the exchanged ion;

§ ion radius (sieve effect).

Mass exchange capacity shows how many millimole equivalents of an ion - n(1/z ion) - can exchange 1 gram of dry ion exchanger. It is calculated using the formula:

Volumetric exchange capacity shows how many millimole equivalents of an ion - n(1/z ion) - can exchange 1 milliliter of swollen ion exchanger. It is calculated using the formula:

Depending on the determination conditions, a distinction is made between static (SOE) and dynamic (DOE) exchange capacity, and what about SOE? DOE.

Types of dynamic exchange capacity:

§ before the breakthrough of the absorbed ion, or working (DOE), shows how many ions can be absorbed by the ion exchanger before they appear in the eluate (breakthrough);

§ total (PDOE) - shows how many ions can be absorbed by the ion exchanger until the ionogenic groups are completely saturated under given conditions.

The difference between the values of DOE and PDOE is presented in the figure:

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Rice. 4 Total dynamic exchange capacity (TDEC) and capacity before breakthrough (DEC)

Application of ion exchangers in analytical chemistry

Ion exchangers are used to solve the following problems in analytical practice.

§ Separation of substances. Ion exchange is a convenient and effective method for separating substances. For example, with its help it is possible to separate even elements with similar chemical properties, such as lanthanides.

§ Concentration of substances. First, a large volume of the dilute solution is passed through a column containing an ion exchange resin. After this, the sorbed ions are washed out of the column with a minimum amount of a suitable eluent.

§ Determination of “inconvenient” cations and anions. It is often necessary to analyze the content of so-called “inconvenient” ions. Such ions do not have chemical analytical properties that would allow them to be easily determined using chemical or instrumental methods of analysis. Of the cations, these include ions of alkali metals (Na+, K+, etc.), of anions - , etc. The determination of “inconvenient” cations is based on first passing the sample through a column with a cation exchanger in hydrogen form and subsequent titration with alkali:

The determination of “inconvenient” anions is based on preliminary passing the sample through a column with an anion exchanger in hydroxide form and subsequent titration of the released alkali with an acid:

§ Obtaining deionized water. Water is passed sequentially through a column with a cation exchange resin in the hydrogen form, then through a column with an anion exchange resin in the hydroxide form. As a result, all cations and anions are retained by ion exchangers and water is obtained that does not contain ions.

Chromatographic methods of analysis

The chromatographic method of analysis was first used by the Russian botanist M. S. Tsvet for the analysis of chlorophyll. The name of the method comes from the Greek word “chromatos” - color, although the method allows you to separate any compounds, including uncolored ones.

Currently, chromatography is one of the most promising methods of analysis. It is widely used in various industries and scientific research for the analysis of mixtures of gaseous, liquid and solid substances.

In the petrochemical and gas industry, chromatography accounts for 90% of all analyzes performed. Gas chromatography is used in biology and medicine, wood processing technology, forest chemistry and food industry and other areas. About 30% of environmental monitoring analyzes (air pollution, wastewater analysis, etc.) are performed using gas chromatographic methods.

The essence of 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.

Classification of chromatographic methods of analysis

Chromatographic methods of analysis are so diverse that there is no single classification of them. Most often, several classifications are used, which are based on the following characteristics:

§ state of aggregation of the mobile and stationary phases;

§ mechanism of interaction of the substance with the sorbent;

§ analysis technique (way of designing the process);

§ chromatography method (method of moving a substance through a column);

§ purpose of chromatography.

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.

IN dependencies from way registration process differentiate columnar And planar chromatography. IN columnar chromatography process separation lead V columns, filled sorbent. Planar chromatography includes V myself two varieties: chromatography on paper And thin-layer chromatography on records.

IN dependencies from way chromatography differentiate following kinds chromatography:

§ eluent (developing) chromatography;

§ repressive chromatography;

§ frontal chromatography.

More often In general, the development method of chromatography is used. It consists in introducing into a continuous flow of the mobile phase (eluent) a mixture of substances that are sorbed better than the eluent. As the eluent moves through the column with sorbed substances, they move along the sorbent layer at different speeds and finally leave it in separate zones separated by the eluent.

According to the purpose of the chromatographic process, they are distinguished: analytical chromatography - an independent method of separation, qualitative and quantitative analysis of substances; preparative chromatography to isolate pure substances from a mixture.

Gas chromatography

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 (gas adsorption chromatography) or liquid applied in a thin layer to the surface of an inert carrier (gas-liquid chromatography) are used as a stationary phase.

Advantages of analytical gas chromatography:

§ the ability to identify and quantify individual components of complex mixtures;

§ high clarity of separation and expressiveness;

§ the ability to study microsamples and automatically record results;

§ the ability to analyze a wide range of objects - from light gases to high-molecular organic compounds;

Main theoretical approaches

The task of the theory of chromatography is to establish the laws of motion and blurring of chromatographic zones. Most often, the following approaches are used for this:

§ theory of theoretical plates;

§ kinetic theory.

The theory of theoretical plates is based on the assumption that the column is divided into small sections - plates. These are narrow layers of the column in which equilibrium is established in the distribution of the substance between the mobile and stationary phases.

The kinetic theory relates the efficiency of separation to the processes of diffusion of the substance in the column due to the movement of the carrier gas flow. When a substance moves along the column, it is either in the mobile phase or in the stationary phase, i.e., the chromatography process is stepwise. The time a substance spends in both phases determines the speed of its movement through the column.

Chromatographic Peak Parameters

Rice. 5 Chromatogram of a mixture of three substances

1. Retention time (tR) is the time from the moment the analyzed sample is introduced until the maximum of the chromatographic peak is recorded. It depends on the nature of the substance and is a qualitative characteristic.

2. Height (h) or area (S) of the peak

S = ½ h. (4)

The height and area of the peak depend on the amount of substance and are quantitative characteristics.

The retention time consists of two components - the residence time of substances in the mobile phase (tm) and the residence time in the stationary phase (ts):

Schematic diagram of a gas chromatograph and the purpose of the main components

The sample injection device 3 allows a certain amount of the analyzed mixture in a gaseous state to be introduced into the carrier gas flow immediately before the column. It includes an evaporator and a dosing device.

The carrier gas flow introduces the analyzed sample into column 5, where the mixture is separated into its individual components.

Rice. 6 Block diagram of a gas chromatograph: 1 - cylinder with carrier gas; 2 - gas preparation unit; 3 - sample injection device; 4 - thermostat; 5 - chromatographic column; 6 - detector; 7 - amplifier; 8 - recorder

The latter, in a mixture with a carrier gas, are supplied to the detector 6, which converts the corresponding changes in the physical or physico-chemical properties of the mixture of components - carrier gas compared to pure carrier gas into an electrical signal. A detector with a corresponding power supply makes up the detection system.

The required temperature conditions of the evaporator, column and detector are achieved by placing them in the corresponding thermostats 4, controlled by a thermostat. If it is necessary to increase the column temperature during analysis, use a temperature programmer. Thermostats and a thermostat with a programmer make up a thermostating system, which also includes a device for measuring temperature.

The detector signal, converted by amplifier 7, is recorded in the form of a chromatogram by recorder 8.

Often an electronic integrator or data processing computer is included in the circuit.

Conditions for chromatographic analysis

When carrying out chromatographic analysis, it is necessary to select optimal conditions for the separation of the analyzed components. As a rule, when determining them, they are guided by literature data. Based on them, the following are experimentally selected:

§ stationary phase in gas-liquid or adsorbent in gas adsorption chromatography;

§ solid inert carrier in gas-liquid chromatography;

§ carrier gas;

§ carrier gas consumption;

§ sample volume;

§ column temperature.

Qualitative analysis

Basic methods of identifying substances:

1. Tag method

The first version of the method is based on the fact that, under the same conditions, the retention times of the reference (label) and analyte substances are experimentally determined and compared. Equality of retention parameters allows the substance to be identified.

The second version of the labeling method is that a reference component (label), the presence of which is assumed to be in the mixture, is introduced into the mixture being analyzed. An increase in the height of the corresponding peak compared to the height of the peak before the addition of the additive indicates the presence of this compound in the mixture.

2. Use of literature values of retention parameters.

Quantitative Analysis

Quantitative analysis is based on the dependence of the peak area on the amount of substance (in some cases, the peak height is used).

There are various ways to determine peak area:

§ according to the formula, as the area of a triangle;

§ using a planimeter;

§ weighing the cut out peaks (the peaks in the chromatogram are copied onto uniform paper, cut out and weighed);

§ using an electronic integrator;

§ using a computer.

The accuracy of quantitative chromatographic analysis is largely determined by the choice of the most rational method for calculating the concentration of substances. The main methods are:

§ absolute calibration method,

§ internal normalization method,

§ internal standard method.

Absolute calibration method

The essence of the method is that known quantities of a standard substance are introduced into a chromatographic column and the peak areas are determined.

Based on the data obtained, a calibration graph is constructed. Then the analyzed mixture is chromatographed and the content of this component is determined according to the graph.

To calculate these coefficients, the peak areas of at least 10 standard mixtures with different contents of a given substance i are determined. Then use the formula.

ki = shi q / (S 100),

where ki is the absolute correction factor of the i-th substance; ui is the content of the i-th component in the standard mixture (%); S - peak area;

q is the sample size (volume, cm3 - for gases, μL - for liquids, or mass, μg - for liquids and solids).

The coefficients obtained in this way are averaged. Then the test mixture is analyzed and the result is calculated using the formula

shi = ki S 100/q.

The absolute calibration method is quite simple, but the necessary conditions for its use are the accuracy and reproducibility of sample dosing, strict adherence to the constancy of the chromatographic mode parameters when calibrating the device and when determining the content of the chromatographed substance.

The absolute calibration method is especially widely used when determining one or more components of a mixture, in particular when using a chromatograph to regulate the technological process mode based on the content of one or a small number of substances in products. This method is the main one for determining trace impurities.

Relative correction factors

Due to the low accuracy of sample dosing, a number of methods have been developed in which the sample size is not used in calculations. These methods use relative correction factors. They take into account differences in the sensitivity of the detector used to the components of the analyzed sample and depend little on process parameters. They are found in advance for each component of the sample.

To determine relative correction (calibration) coefficients, a series of binary mixtures of known composition are prepared and, based on the resulting chromatograms, calculations are carried out using the formula

ki =(i /st)/(Si/Sst), (4)

You can use calibration mixtures from a larger number of substances, however, the accuracy of the determination may decrease.

Relative correction factors are used in the methods of internal normalization, internal standard, etc.

Internal normalization method

The essence of the method is that the sum of the peak areas of all components of the mixture is taken as 100%.

A necessary condition for using the method is the registration of all components (the chromatogram contains separated peaks of all components of the mixture).

The concentration of the i-th component is calculated using the formula

i = ki Si 100/ У(ki Si).

When calculating correction factors using formula (4) for this method, one of the compounds included in the mixture under study can be selected as a standard. The calibration factor for a standard substance is equal to 1.

Internal standard method

The essence of the method is that a certain amount of a standard substance (comparison substance) is introduced into the analyzed mixture.

i = ki Si 100 r/Sst..

where ki is the relative correction factor of the i-th component, calculated according to formula (4); Si and Sst. - peak areas of the i-th component and internal standard; r is the ratio of the mass of the internal standard to the mass of the analyzed mixture (without standard): r = mst./mmixture.

Requirements for a substance used as an internal standard:

§ it should not be part of the mixture being tested;

§ it must be inert with respect to the components of the mixture being analyzed and completely miscible with them;

§ The peak of the standard must be well resolved and located in close proximity to the peaks of the compounds being determined.

The internal standard is selected from among compounds that are similar in structure and physicochemical properties to the components of the mixture being analyzed. Relative correction factors for mixture components are determined in relation to the internal standard.

The method is used both when all components of the analyzed mixture are recorded on the chromatogram, and in the case of incompletely identified mixtures. The main difficulty lies in the selection and precise dosage of the standard substance.

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5. CONCLUSION

6. LIST OF REFERENCES USED

INTRODUCTION

Chemical analysis serves as a means of monitoring production and product quality in a number of sectors of the 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 a given component is 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 developing the theoretical foundations of methods, establishing the limits of their applicability, assessing metrological and other characteristics, and 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. An analysis technique is a detailed description of the 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 of analytical chemistry can be divided into methods of sampling, sample decomposition, 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 measurements of a 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 the inhibition or complete suppression of a 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 a substance from a solid state to a gaseous state and its subsequent precipitation in solid form (bypassing the liquid phase). Separation by sublimation is usually resorted to 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 a molecular weight from units to 10 6. These can be hydrogen isotopes, metal ions, synthetic polymers, proteins, etc. Using chromatography, extensive information has been obtained on the structure and properties of organic compounds of 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 of a large number of 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 of 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 of 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 of soils, and 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, "Higher School", 1999.

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

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, in 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 of colloidal solutions and solutions of high molecular weight compounds. In analytical chemistry, these methods are used less frequently.

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.

Extraction as a method of separating and concentrating substances

Gas chromatograph diagram

Classification of chromatographic methods

Chromatographic methods of analysis

In 1903 M.S. Tsvet was the first to set out the principles of chromatography (Greek “chromo” - color, “grapho” - write) and created a method for separating the pigments of green plants.

The chromatographic method allows the separation and analysis of complex mixtures. The separation of substances occurs due to the different adsorbability of the components of the mixture.

Chromatography is a dynamic process that occurs in a system of two immiscible phases, one of which is mobile and the other immobile. The mobile phase can be either a gas or a liquid, and the stationary phase can be a solid or a thin film of liquid adsorbed on a solid.

1) according to the state of aggregation of the mobile phase

Gas chromatography (GC)

Liquid chromatography (LC)

2) according to the geometry of the stationary phase layer

Speaker

Flat-layer (can be paper and thin-layer)

The chromatographic process can be represented as follows:

Column filled

solid sorbent

A stream of liquid flows through it. Substance X, dissolved in a liquid, moves with it, but at the same time tends to remain on the surface of the solid sorbent due to adsorption, ion exchange, etc. Each molecule X moves part of the time, and part of the time is held by the stationary phase.

The possibility of separating two solutes X and Y is due to the difference in their affinity for both phases, i.e. one of them is in the mobile phase most of the time, so it reaches the end of the column faster.

where k’ is the recovery factor

The ratio of the number of moles of a substance in the stationary phase to the number of moles of a substance in the mobile phase

The extraction coefficient characterizes the degree of retention of a substance.

The degree of separation of two substances can be expressed through the separation coefficient (α):

where is the extraction coefficient of the second substance,

Recovery coefficient of the first substance.

A detector placed at the outlet of the column registers, and a recorder records the detector signals.

Rice. 10. Detector signals.

Figure 10 shows a chromatogram of a four-component mixture. The area of each peak is proportional to the mass fraction of the component in the mixture.

One of the important and common methods of concentration is extraction. The method is universal: methods have now been found for extracting almost all elements and most classes of compounds. It is suitable for both the separation of microimpurities and the separation of the base substance; it is only a matter of the correct choice of the extraction system and the conditions of the separation process. Extraction usually provides high concentration efficiency. The method is characterized by speed and ease of implementation. It is used in most analytical laboratories, especially where they work with high-purity substances.

Extraction, as is known, is the process of distributing a solute between two immiscible liquid phases, as well as a method of isolation and separation. The most common case is when one phase is water, the second is an organic solvent.

The extraction method is used for two purposes:

1) for quantitative extraction of one of the dissolved substances - this is exhaustive extraction

2) to separate two solutes - this is selective extraction

In extraction there are usually two immiscible phases and one partitionable substance. This means that at constant temperature and pressure the system is monovariant. Under equilibrium conditions, the ratio of the concentrations of the distributed substance in both phases (C 0 and C b) is a constant value. This quantity is called the distribution constant (P) or distribution coefficient.

P = C 0 / C in (15)

Р= (а x) 0 / (а x) w = [X] 0 / [X] w,

where w, o – water and organic solvent

P is equal to the ratio of the activities of the component in both phases (but the ratio of concentrations is also used, since it is usually not ions, but molecules that are extracted). If polymerization occurs in the system, the distribution coefficient will depend on the concentration and the calculation will become more complex.

The Nernst-Shilov distribution law is valid when the solute is in both phases in the same form. In reality, a substance can dissociate and associate, solvate and hydrate. Thus, the law is idealized, but many extraction systems obey this law. In general, extraction systems are very diverse. The correct choice of system greatly determines the success of extraction separation and concentration. In this work, intracomplex compounds were used. This is one of the most common classes of compounds used in extraction concentration. For the first time, elements were concentrated precisely in the form of dithizonates (intracomplex compounds). Subsequently, along with dithizonate, cupferonates, dithiocarbamates, 8-hydroxynoline, oximes, etc. found wide use.

Consider the extraction of X moles of solute (Vwater – V w ml and Vorganic phase – V 0 ml).

The distribution coefficient (P) is equal to

Р = [X] 0 / [X] w = (X –Y) * V w / V 0 * Y,

where Y is the number of moles remaining in the aqueous phase after one extraction

The unextracted fraction is

Y / X = f = 1 / (1 + P * (V 0 / V w)) = V w / (V w + PV 0)

f does not depend on the initial concentration, therefore, when carrying out n sequences

f n = (1+P V 0 / V w) - n

Calculation of the limiting amount of solute remaining unextracted after n extractions tends to infinity (done by Griffin).

It is obvious that for a finite ratio V o / V w the limit is f n = 0. But such extraction is not of practical interest, because the volume of the extracted solvent should tend to infinity.

For a finite V o divided into n portions, the equation has the form

f n = (1+P V 0 / nV w) - n,

and with n tending to infinity,

f ¥ = e - V 0 P / V W

With an infinitely large number of extractions, the volume of the organic phase tends to 0. In practice, dividing the extractant into more than 4-5 portions is not very effective.

Basic terms of the extraction process

1. Distribution coefficient (or distribution constant) – see above.

2. Separation factor (S) – the ratio of the distribution coefficients of the two substances being separated, the larger to the smaller.

3.% extraction (degree of extraction) (R) – the percentage of a substance extracted under given conditions from the total amount. The % extraction distribution coefficient is related to the relation

R = 100D / (D + V in / V 0), where V in and V 0 are the equilibrium volumes of the aqueous and organic phases.

4. Extraction constant (K ext) – equilibrium constant of a heterogeneous extraction reaction

For example, for intracomplex compounds, the extraction of which proceeds according to the equation M n + + nHA o MAn (o) + nH +

the extraction constant is equal to

K ext = o * [ H + ] n / [ M n+ ] * n o

4. Extract – a separated organic phase containing a substance extracted from another phase.

5. An extractant is an organic solvent that extracts a substance from the aqueous phase.

6. Re-extraction - the process of returning the extracted substance from the extract into the aqueous phase.

7. Re-extract – a separated aqueous phase containing a substance extracted from the extract.

8. Extraction curves

Fig. 11. Extraction curves

The steeper the curves, the greater the charge of the metal ion. This means that pH 1/2 depends on the stability constant of the chelate and on the excess concentration of the reagent, but not on the concentration of the metal.

F KSMU 4/3-04/01

IP No. 6 UMS at KazSMA

dated June 14, 2007

Karaganda State Medical University

Department of Pharmaceutical Disciplines with a Chemistry Course

Topic: Methods for isolating, separating and concentrating substances in analytical chemistry.

Discipline Analytical Chemistry

Specialty 051103 “Pharmacy”

Time (duration) 50 minutes

Karaganda 2011

Approved at a chemistry course meeting

"29". 08. 2011 Protocol No. 1

Responsible for the course ______________L.M. Vlasova
Subject: Methods for isolating, separating and concentrating substances in analytical chemistry.
Target: To form ideas about the use of methods for isolating, separating and concentrating substances in analytical chemistry in order to ensure reliable analytical results, to study masking methods used to eliminate interfering components.
Plan:

Masking.
Separation and concentration.
Quantitative characteristics of separation and concentration.
Precipitation and coprecipitation.
Adsorption, occlusion, polymorphism.

Illustrative material: presentation.

Masking, separation and concentration methods.
Often in the practice of chemical analysis, the method used to detect or determine the required components does not provide reliable results without first eliminating the influence of interfering components (including the main ones that make up the “matrix” of the analyzed sample). There are two ways to eliminate interfering components. One of them is masking - transferring interfering components into a form that no longer has an interfering effect. This operation can be carried out directly in the system being analyzed, with the interfering components remaining in the same system, for example in the same solution.

Masking is not always possible, especially when analyzing multicomponent mixtures. In this case, another method is used - separation of substances (or concentration).

Masking

Masking- is the inhibition or complete suppression of a chemical reaction in the presence of substances that can change its direction or speed. In this case, no new phase is formed, which is the main advantage of masking over separation, since operations associated with separating phases from each other are eliminated. 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 detect 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. For example, the induced reaction of MnO - 4 with CI - in the presence of Fe (II) slows down in the presence of phosphate ions.

Several groups of masking substances can be distinguished.

Substances that form more stable compounds with interfering substances than with those being determined. For example, the formation of a complex of Fe (II) with the red thiocyanate ion can be prevented by introducing sodium fluoride into the solution. Fluoride ions bind iron (III) into a colorless complex FeF 3-6, more stable than Fe (SCN) n (n -3), which eliminates the interfering influence of Fe (III) when detecting Co (II) in the form of a complex blue Co (SCN) n (n -2). Triethanolamine is useful for masking Mn(II), Fe(III) and AI(III) in alkaline solutions in complexometric titrations of calcium, magnesium, nickel and zinc.
Substances that prevent acid-base reactions with the formation of poorly soluble hydroxides. For example, in the presence of tartaric acid, Fe(III) oxide hydrate is not precipitated by ammonia until pH 9-10.
Substances that change the oxidation state of an interfering ion. For example, to eliminate the interfering influence of Cr (III) during complexometric titration of aluminum and iron, it is recommended to oxidize it to Cr (VI).
Substances that precipitate interfering ions, but the precipitate does not need to be separated. For example, during complexometric titration of calcium in the presence of magnesium, which is precipitated as hydroxide but not separated.
Substances with specific effects. For example, polarographic waves are suppressed in the presence of certain surfactants.

Sometimes masking combines these techniques. For example, Cu(II) ions can be masked with cyanide and thiosulfate ions. In this case, Cu (II) is reduced to Cu (I), and then, with an excess of the masking substance, forms complexes of the composition Cu (CN) 4 3-, Cu (S 2 O 3) 2 3-.

To evaluate the effectiveness of masking, use masking index. This is the logarithm of the ratio of the total concentration of the interfering substance to its concentration remaining unbound. The masking index can be calculated by knowing the conditional equilibrium constants of the corresponding masking reactions.

The following masking substances are often used in chemical analysis: complexones; hydroxy acids (tartaric, citric, malonic, salicylic); polyphosphates capable of forming complexes with a six-membered chelate structure (sodium pyro- and tripolyphosphates); piliamines; glycerol; thiourea; halide, cyanide, thiosulfate – ion; ammonia, as well as a mixture of substances [for example, KI in a mixture with NH 3 during complexometric titration of Cu (II) in the presence of Hg (II)].

Along with masking, chemical analysis sometimes resorts to unmasking - the transformation of a masked substance into a form capable of entering into reactions usually characteristic of it. This is achieved by protonating the masking compound (if it is a weak base), irreversibly destroying or removing it (for example, by heating), changing the oxidation state, or binding into a stronger compound. For example, unmasking of metal ions from complexes with NH 3, OH -, CN -, F - can be accomplished by decreasing the pH. Complexes of cadmium and zinc with cyanide ion are destroyed by the action of formaldehyde, which reacts with cyanide ion to form glycolic acid nitrile. Peroxide complexes, for example titanium (IV), decompose by boiling in acidic solutions. Unmasking can also be achieved by oxidizing the masking compound (for example, EDTA oxidation) or changing the oxidation state of the masked substance (Fe 3- ↔ Fe 2-).

2. Separation and concentration.
The need for separation and concentration may be due to the following factors: 1) the sample contains components that interfere with the determination; 2) the concentration of the component being determined is below the detection limit of the method; 3) the components being determined are unevenly distributed in the sample; 4) there are no standard samples for calibration of instruments; 5) the sample is highly toxic, radioactive or expensive.

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

Concentration– 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.

When separated, the concentrations of the components may be close to each other, but they may also differ. Concentration is carried out under conditions where the concentrations of the components differ sharply.

When concentrating, substances present in small quantities 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). Relative concentration can be considered as separation with the difference that the initial concentrations of the components are sharply different. An example of absolute concentration is the evaporation of a matrix in the analysis of waters, solutions of mineral acids, and organic solvents. The main goal of relative concentration is to replace the matrix, which for one reason or another makes analysis difficult, with another organic or inorganic one. For example, when determining microimpurities in high-purity silver, the matrix element is extracted with O - isopropyl - N - ethyl thiocarbinate in chloroform and then, after evaporating the aqueous phase to a small volume, microcomponents are determined by any method.

Distinguish group and individual isolation and concentration: with a group method, several components are separated in one step; with an individual method, one component or several components are isolated sequentially from a sample. When using multi-element methods of determination (atomic emission, X-ray fluorescence, spark mass spectrometry, neutron activation), group separation and concentration are preferable. When determining by photometry, fluorimetry, and atomic absorption methods, on the contrary, it is more expedient to individually isolate the component.

Separation and concentration have much in common both in theoretical aspects and in technical execution. The methods for solving problems are the same, but in each specific case modifications are possible related to the relative amounts of substances, the method of obtaining and measuring the analytical signal. For example, methods of extraction, coprecipitation, chromatography, etc. are used for separation and concentration. Chromatography is used mainly for separating complex mixtures into components, coprecipitation for concentration (for example, isomorphic coprecipitation of radium with barium sulfate). You can consider the classification of methods based on the number of phases, their state of aggregation and the transfer of matter from one phase to another. Preferred methods are based on the distribution of a substance between two phases such as liquid-liquid, liquid-solid, liquid-gas and solid-gas. In this case, a homogeneous system can be transformed into a two-phase system by any auxiliary operation (precipitation and coprecipitation, crystallization, distillation, evaporation, etc.), or by introducing an auxiliary phase - liquid, solid, gaseous (these are methods of chromatography, extraction, sorption).

There are methods based on the separation of components in one phase, for example, electrodialysis, electrophoresis, diffusion and thermal diffusion methods. However, even here we can conditionally talk about the distribution of components between two “phases”, since the components, under the influence of externally applied energy, are divided into two parts, which can be isolated from each other, for example, by a semi-permeable membrane.

Each application area of chemical analysis has its own choice of separation and concentration methods. In the petrochemical industry - mainly chromatographic methods, in toxicological chemistry - extraction and chromatography, in the electronics industry - distillation and extraction.

The arsenal of separation and concentration methods is large and constantly expanding. To solve problems, almost all chemical and physical properties of substances and the processes occurring with them are used.
3. Quantitative characteristics of separation and concentration.
Most separation methods are based on the distribution of the substance between two phases (I and II). For example, for substance A the equilibrium is

A I ↔ A II (1.1)
The ratio of the total concentrations of substance A in both phases is called distribution coefficient D:

D= C II / C I (1.2)
Absolutely complete extraction, and therefore separation, is theoretically impossible. The efficiency of extracting substance A from one phase to another can be expressed recovery rate R:
R = Q II / Q II + Q I , (1.3)
where Q is the amount of substance; R is usually expressed as a percentage.

Obviously, for complete recovery of a component, the R value must be as close to 100% as possible.

In practice, recovery is considered quantitative if R A ≥ 99.9%. This means that 99.9% of substance A must go into phase II. For the interfering component B, the condition 1/R B ≥ 99.9 must be satisfied, i.e. No more than 0.1% of substance B should move into phase II.

A quantitative characteristic of the separation of substances A and B, for which equilibria are established between phases I and II, is separation factorά A/B:
ά A/B = D A / D B (1.4)

For separation, it is necessary that the value of ά A/B be high and the product D A D B be close to one. Let ά A/B = 10 4. In this case, the following combinations of values D A and D B are possible:
D A D B R A , % R B , %

10 5 10 100 90,9

10 2 10 -2 99,0 0,99

10 -1 10 -5 9,1 0,001
As can be seen, separation can be achieved with D A D B =1.

To assess the efficiency of concentration, use concentration factor S to:
S k = q/Q / q sample /Q sample, (1.5)
where q, q sample - the amount of microcomponent in the concentrate and sample; Q, Q sample - the amount of macrocomponent in the concentrate and sample.

The concentration coefficient shows how many times the ratio of the absolute amounts of micro- and macrocomponents in the concentrate changes compared to the same ratio in the original sample.
4.Precipitation and coprecipitation
Methods of separation and concentration include precipitation with the formation of crystalline and amorphous precipitates.

Conditions for the formation of crystalline deposits.

Necessary:

Carry out precipitation from dilute solutions with a dilute solution of the precipitant;
Add the precipitant slowly, drop by drop;
Stir continuously with a glass rod;
Precipitate from a hot solution (sometimes the precipitant solution is also heated);
Filter off the precipitate only after the solution has cooled;
During precipitation, add substances that increase the solubility of the precipitate.

Conditions for the formation of amorphous sediments.
Amorphous sediments arise as a result of coagulation, i.e., the sticking together of particles and their aggregation. The coagulation process can be caused by the addition of an electrolyte. You should besiege:

From hot solutions;
In the presence of an electrolyte (ammonium salt, acid);
In order to obtain a dense sediment that is easily washed and settles quickly, precipitation is carried out from concentrated solutions with concentrated solutions of the precipitant.

Contamination of a sediment with substances that should have remained in solution is called coprecipitation .

For example, if a solution containing a mixture of BaCL 2 with FeCL 3 is exposed to H 2 SO 4, then one would expect that only BaSO 4 will precipitate, because Fe 2 (SO4) 3 salt is soluble in water. In reality, this salt also partially precipitates. This can be verified if the precipitate is filtered, washed and calcined. The BaSO 4 precipitate turns out to be not pure white, but brown due to Fe 2 O 3 formed as a result of calcination of Fe 2 (SO 4) 3

Fe 2 (SO 4) 3 → Fe 2 O 3 + 3SO 3

Contamination of sediments by co-precipitation with soluble compounds occurs due to chemical precipitation, and subsequent precipitation is distinguished, in which contamination of sediments with poorly soluble substances occurs. This phenomenon occurs because near the surface of the sediment, due to adsorption forces, the concentration of precipitant ions increases and the PR is exceeded. For example, when Ca 2+ ions are precipitated by ammonium oxalate in the presence of Mg 2+, a precipitate of CaC 2 O 4 is released, magnesium oxalate remains in solution. But when the CaC 2 O 4 precipitate is kept under the mother liquor, after some time it becomes contaminated with slightly soluble MgC 2 O 4, which is slowly released from the solution.

Coprecipitation is of great importance in analytical chemistry. This is one of the sources of errors in gravimetric determination. But coprecipitation can also play a positive role. For example, when the concentration of the analyte component is so low that precipitation is practically impossible, then coprecipitation of the analyte microcomponent is carried out with some suitable collector (carrier). The technique of co-precipitation of microcomponents with a collector is very often used in the concentration method. Its importance is especially great in the chemistry of trace and rare elements.

There are several types of coprecipitation, including adsorption, occlusion, and isomorphism.

The absorption of one substance by another, occurring at the interface, is called adsorption . Pollutant – adsorbate , adsorbed by a solid surface – adsorbent .
Adsorption proceeds according to the following rules:

Advantage the precipitate (for example, BaSO 4) first adsorbs its own ions, i.e. Ba 2+ and SO 4 2-, depending on which of them are present in excess in the solution;
On the contrary, ions with a high charge that are in a solution of the same concentration will be preferentially adsorbed;
Of the ions with the same charge, ions whose concentration in the solution is higher are preferentially adsorbed;
Of the ions that are equally charged and have the same concentration, the ions that are more strongly attracted by the ions of the crystal lattice (Paneto-Faience rule) are preferentially adsorbed.

Adsorption is a reversible process; desorption occurs parallel to adsorption, i.e. transition of adsorbed ions or molecules from the surface of the adsorbent into solution. The simultaneous occurrence of these two processes leads to a state of equilibrium called adsorption equilibrium.

Adsorption equilibrium depends on the following factors:

1. Effect of the adsorbent surface area

Since substances or ions are adsorbed on the surface of an adsorbent, the amount of a substance adsorbed by a given adsorbent is directly proportional to the size of its total surface. The phenomenon of adsorption during analysis has to be taken into account most when dealing with amorphous sediments, because their particles are formed as a result of the adhesion of a large number of small primary particles to each other and therefore have a huge total surface.

For crystalline sediments, adsorption plays a lesser role.

2. Effect of concentration.

From the adsorption isotherm it is possible to establish

the degree of adsorption decreases with increasing concentration of the substance in solution
with increasing concentration of a substance in solution, the absolute amount of adsorbed substance increases
with increasing concentration of a substance in solution, the amount of adsorbed substance tends to a certain final value

adsorption

substances on

concentration of a substance in solution

3. Effect of temperature

Adsorption is an exothermic process, and its flow is facilitated by a decrease in temperature. An increase in temperature promotes desorption.

Influence of the nature of adsorbed ions.

An adsorbent adsorbs some ions more strongly than others. This is due to its selectivity. First of all, the precipitate adsorbs those ions that make up its crystal lattice. Counterions are adsorbed according to the following rules

ions with a large charge are adsorbed
From ions with the same charge, those ions whose concentration in the solution is higher are adsorbed
from ions that are equally charged and have the same concentration, ions are preferentially adsorbed that are more strongly attracted by ions of the crystal lattice (Panet-Faience rule.)

Those foreign ions that form the least soluble or low-ionized compounds with the lattice ions are more strongly attracted, for example, when AgJ is deposited in a solution of the AgNO 3 + KJ reaction containing CH 3 COO-, CH 3 COOAg will be adsorbed, and not AgNO 3, i.e. To. The first salt is less soluble in water than the second.

Occlusion. In occlusion, contaminants are contained within sediment particles. Occlusion differs from adsorption in that coprecipitated impurities are found not on the surface, but inside the sediment particle.

Causes of occlusion.

Mechanical capture of foreign impurities. This process goes faster the faster crystallization occurs.

1) there are no “ideal” crystals; they have tiny cracks and voids that are filled with the mother liquor. The smallest crystals can stick together, trapping the mother liquor.

2) Adsorption during the formation of sediment crystallization.

During the growth of a crystal, various impurities from the solution are continuously adsorbed from the smallest seed crystals on a new surface, while all the rules of adsorption are observed.

3) Formation of chemical compounds between the sediment and coprecipitated impurity.

The order in which solutions are drained is very important during occlusion. When the solution during precipitation contains an excess of anions that are part of the sediment, then the occlusion of extraneous cations occurs, and vice versa, if the solution contains an excess of cations of the same name, then the occlusion of extraneous anions occurs.

For example, when BaSO 4 (BaCL 2 + NaSO 4) is formed, Na + ions are occluded in excess SO 4 2-, and in excess Ba 2 + - CL -

To weaken the occlusions of extraneous cations, precipitation must be carried out so that the sediment crystals grow in a medium containing an excess of the sediment’s own cations. On the contrary, if you want to obtain a precipitate free from occluded foreign anions, you need to carry out precipitation in a medium containing an excess of the precipitated compound’s own anions.

The amount of occlusion is affected by the rate of infusion of the precipitant. When the precipitant is added slowly, purer sediments are usually obtained. Co-precipitation occurs only during sediment formation.

Isomorphism is the formation of mixed crystals.

Isomorphic substances are those substances that are capable of crystallizing to form a joint crystal lattice, and so-called mixed crystals are obtained.

A typical example is various alums. If you dissolve colorless crystals of aluminum - potassium alum KAl (SO 4) 2 12H 2 O with intensely violet chromium - potassium...KSr (SO 4) 2 12H 2 O, then mixed crystals are formed as a result of crystallization. The color of these crystals is more intense, the higher the concentration of KCr(SO 4) 2.

Isomorphic compounds usually form crystals of the same shape.

The essence of isomorphism is that ions with similar radii can replace each other in the crystal lattice. For example, Ra and Ba ions have close radii, therefore, when BaSO 4 is deposited, isomorphic crystals will precipitate from a solution containing small amounts of Ra 2+. In contrast to KCr(SO 4) 2 ions, which have a smaller atomic radius.

3. Co-precipitation is a major source of error in gravimetric analysis.

Co-precipitation can be reduced by choosing the right course of analysis and choosing a precipitant rationally. When precipitating with organic precipitants, much less co-precipitation of foreign substances is observed than when using inorganic precipitants. Precipitation must be carried out under conditions under which a coarse crystalline precipitate is formed. Keep the sediment under the mother solution for a long time.

To clean the sediment from adsorbed impurities, it must be thoroughly washed. To remove impurities resulting from occlusion and isomorphism, the sediment is subjected to reprecipitation.

For example, when determining Ca 2+, they are precipitated in the form of CaC 2 O 4; if Mg 2+ is present in the solution, then the sediment is heavily contaminated with MgC 2 O 4 impurities. To get rid of impurities, the precipitate is dissolved in HCL. This produces a solution in which the concentration of Mg 2+ is lower than the original solution. The resulting solution is neutralized and precipitation is repeated again. The sediment turns out to be practically free of Mg 2+.

4. Amorphous precipitates are formed from colloidal solutions by coagulation, i.e. Combinations of particles into larger aggregates, which, under the influence of gravity, will settle to the bottom of the vessel.

Colloidal solutions are stable due to the presence of the same charge, solvation or hydration shell = In order for precipitation to begin, it is necessary to neutralize the charge by adding some electrolyte. By neutralizing the charge, the electrolyte allows the particles to adhere to each other.

To remove solvation shells, a technique such as salting out is used, i.e., adding a high concentration of electrolyte, the ions of which in the solution select solvent molecules from colloidal particles and solvate themselves.

Coagulation is promoted by increased temperature. Also, the precipitation of amorphous sediments must be carried out from concentrated solutions, then the sediments are more dense, settle faster and are easier to wash off impurities.

Amorphous precipitates after precipitation are not kept under the mother liquor, but are quickly filtered and washed, since the precipitate otherwise turns out to be gelatinous.

The reverse of the coagulation process is the peptization process. When amorphous sediments are washed with water, they can again go into a colloidal state; this solution passes through the filter and part of the sediment thus passes through. gets lost. This is explained by the fact that electrolytes are washed out of the sediment, so the coagulated particles again receive a charge and begin to repel each other. As a result, large aggregates disintegrate into tiny colloidal particles, which freely pass through the filter.

To prevent peptization, the sediment is washed not with pure water, but with a dilute solution of some electrolyte.

The electrolyte must be a volatile substance and be completely removed upon ignition. Ammonium salts or volatile acids are used as such electrolytes.

Literature:
1. Kharitonov Yu.A. Analytical chemistry.book 1,2. M.; VS, 2003

2. Tsitovich I.K. Analytical chemistry course. M., 2004.

3. Vasiliev V.P. Analytical chemistry. book 1.2. M., Bustard, 2003.

4. Kellner R., Merme J.M., Otto M., Widmer G.M. Analytical chemistry. vol. 1, 2. Translation from English. language M., Mir, 2004.

5. Otto M. Modern methods of analytical chemistry vol.1,2. M., Tekhnosphere, 2003.

6. Ponomarev V.D. Analytical chemistry, parts 1, 2. M., VSh, 1982.

7. Zolotov Yu.A. Fundamentals of Analytical Chemistry, vol. 1, 2, VSh, 2000.

Security questions (feedback)

List the factors on which the distribution coefficient depends.
Give an example of masking substances used in chemical analysis.
What can be classified as methods of separation and concentration.
What factors determine the degree of extraction of a substance?
Explain the advantages of an amorphous sediment over a crystalline one in the deposition of microcomponents.
What types of interactions exist between the substance and the sorbent?

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