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Methods of concentration. General characteristics of separation and concentration methods

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:


  1. Masking.

  2. Separation and concentration.

  3. Quantitative characteristics separation and concentration.

  4. Precipitation and coprecipitation.

  5. Adsorption, occlusion, polymorphism.

Illustrative material: presentation.

Masking, separation and concentration methods.
Often in practice chemical analysis the method used for detecting or determining 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).


  1. Masking

Masking- this is inhibition or complete suppression chemical reaction in the presence of substances that can change its direction or speed. In this case, no new phase is formed, 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.


  1. 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.

  2. 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.

  3. 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).

  4. 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.

  5. Substances with specific effects. For example, polarographic waves are suppressed in the presence of certain surface- active substances(surfactant).
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 and solutions mineral acids, organic solvents. the main objective relative concentration - replacing the matrix, which for one reason or another complicates the analysis, 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:


  1. Carry out precipitation from dilute solutions with a dilute solution of the precipitant;

  2. Add the precipitant slowly, drop by drop;

  3. Stir continuously with a glass rod;

  4. Precipitate from a hot solution (sometimes the precipitant solution is also heated);

  5. Filter off the precipitate only after the solution has cooled;

  6. 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:


  1. From hot solutions;

  2. In the presence of an electrolyte (ammonium salt, acid);

  3. 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 would 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.

Co-precipitation has 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.


  1. 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:


  1. 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;

  2. On the contrary, ions with a high charge that are in a solution of the same concentration will be preferentially adsorbed;

  3. Of the ions with the same charge, ions whose concentration in the solution is higher are preferentially adsorbed;

  4. 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


  1. the degree of adsorption decreases with increasing concentration of the substance in solution

  2. with increasing concentration of a substance in solution, the absolute amount of adsorbed substance increases

  3. 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.


  1. 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

  1. ions with a large charge are adsorbed

  2. From ions with the same charge, those ions whose concentration in the solution is higher are adsorbed

  3. 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 having similar radii can replace each other in 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 sediments are formed from colloidal solutions by coagulation, t.s. 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 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)


  1. List the factors on which the distribution coefficient depends.

  2. Give an example of masking substances used in chemical analysis.

  3. What can be classified as methods of separation and concentration.

  4. What factors determine the degree of extraction of a substance?

  5. Explain the advantages of an amorphous sediment over a crystalline one in the deposition of microcomponents.

  6. What types of interactions exist between the substance and the sorbent?

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 interference must be avoided analytical signals. In this case, the concentrations of the separated substances are usually close.

Concentration is an operation that allows you to increase the concentration of a microcomponent relative to the main components of the sample (matrix). It is used if the concentration of a microcomponent is less than the detection limit Cmin, i.e. when the analysis method is not sensitive enough. However, the concentrations of the components vary greatly. Concentration is often combined with separation.

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

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

a) classification according to the nature of the process

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

Physicochemical methods of separation and concentration are most often based on the selective distribution of a substance between two phases.

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

b) classification 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).

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

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

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

d) classification according to the type of equilibrium

Thermodynamic separation methods are based on differences in the behavior of substances in an equilibrium state. They have highest value 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 separation of both qualitative and quantitative analysis multicomponent mixtures.

Ion exchange

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

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

Chromatographic methods of analysis

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

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

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

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

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

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

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

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

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

1) the possibility of varying the selectivity of separation

2) the ability to work with analytes at various concentration levels;

3) ease of technological and hardware design;

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

5) high performance.

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

Basic terms of the extraction method:

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

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

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

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

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

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

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

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

Types of extraction systems

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

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

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

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

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

Extraction methods

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

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

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

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

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

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

Schematic illustration of a countercurrent extraction device

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

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

General information about separation and concentration

Separation is an operation that allows you to separate the components of the sample 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 WITH min, 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 - 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 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. 62.


Rice. 62.

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, V organic analysis The main method of concentration is distillation: during thermal decomposition, the matrix is ​​distilled off in the form of CO 2, H 2 O, N 2, and metals can be determined in the remaining ash.

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. 63).


Rice. 63.

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 by the number of elementary acts (stages).
  • § One-step methods- are based on a single distribution of a substance between two phases. The separation takes place under static conditions.
  • § Multi-step methods- are based on multiple distribution of a substance between two phases. There are two groups of multi-stage methods:
  • – with repetition of the single distribution process ( 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 by type of balance(Fig. 64).

Rice. 64.

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

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

Co-precipitation is used for the following purposes:

  • § 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 active carbons, Al2O3, silica, zeolites, cellulose, natural and synthetic sorbents with ionic and chelating groups are used.

Absorption of substances can occur on the surface of the phase ( Ad sorption) or in the volume of the phase ( Ab sorption). Most often used in analytical chemistry adsorption with the aim of:

  • § separation 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.