General characteristics of separation and concentration methods. Ion exchange Methods for separating substances in analytical chemistry
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, 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 concentration ratio is also used, since it is usually molecules, not ions, 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 (К 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.
Methods of separation and concentration occupy an important place among the techniques of modern analytical chemistry. In its most general form, the process of chemical analysis includes sample selection and its preparation for determination, the actual determination and processing of the results. Modern development of analytical instrumentation and computer technology ensures reliable performance of the second and third stages of analysis, which is often carried out automatically. On the contrary, the sample preparation stage, an integral and integral part of which are the operations of separation and concentration, still remains the most labor-intensive and least accurate operation of chemical analysis. If the duration of measurement and processing of results is on the order of minutes and less often than seconds, then the duration of sample preparation is on the order of hours and less often than minutes. According to experts working in the field of environmental analytical control, only 10% of the total analysis error occurs at the signal measurement stage, 30% in sample preparation and 60% in sampling. Errors made during sample collection and preparation can completely distort the results of a chemical analysis.
Interest in methods of separation and concentration does not wane for a number of reasons, of which several can be highlighted: increasing requirements for the sensitivity of the analysis and its accuracy, depending on the possibility of eliminating the matrix effect, as well as the requirement to ensure an acceptable cost of the analysis. Many modern instruments allow analysis without prior separation and concentration, but they themselves and their operation are very expensive. Therefore, it is often more profitable to use so-called combined and hybrid methods, which provide the optimal combination of preliminary separation and concentration of components with their subsequent determination using relatively inexpensive analytical instruments.
Basic concepts and terms. Types of concentration
Let us clarify the semantic concept of the basic terms that are used when describing separation and concentration methods
Under division imply an operation (process) as a result of which several fractions of its components are obtained from an initial mixture of substances, that is, the components that make up the initial mixture are separated from one another. When separated, the concentrations of the components may be close to each other, but they may also differ.
Concentration - an operation (process) that results in an increase in the ratio of the concentration or quantity of microcomponents relative to the matrix or matrix components. Microcomponents mean components contained in industrial, geological, biological and other materials, as well as in environmental objects, in concentrations less than 100 μg/g. In this case, the matrix means the environment in which the microcomponents are located. Often the matrix is water or an aqueous solution of acids or salts. In the case of solid samples, concentration is carried out after transferring the sample into solution; in this case, the solution contains matrix components along with microcomponents. Concentration is carried out under conditions where the concentrations of the components are sharply different.
Concentration of microcomponents during their determination is resorted to, first of all, in cases where the sensitivity of methods for direct determination of these components is insufficient. The main advantage of concentration is the reduction of relative and sometimes absolute limits of detection of microcomponents due to the elimination or sharp reduction of the influence of macrocomponents on the determination results. Concentration is also necessary if the component is distributed inhomogeneously in the analyzed sample; it allows you to work with representative samples. In addition, concentration makes it possible to do without a large number of reference samples, including standard samples, since as a result of concentration it is possible to obtain concentrates on a single basis, for example, on coal powder in the case of atomic emission analysis. During the concentration process, it is also convenient to introduce so-called internal standards, if they are needed. However, concentration also has disadvantages: it lengthens and complicates the analysis, in some cases losses and contamination increase, and sometimes the number of components being determined decreases.
Separation and concentration have much in common, both in the theoretical aspect and in the technique of execution. Obviously, concentration is a special case of separation. The classification of “concentration” as an independent concept of analytical chemistry is justified due to the practical importance of this operation in chemical analysis and the differences in its purpose compared to separation. Thanks to the use of separation, it is possible to simplify the analysis and eliminate the influence of interfering components, while the main purpose of concentration is to increase the sensitivity of the determination.
(Question 1). Distinguish absolute And relative concentration. At absolute concentration microcomponents are transferred from a large sample mass to a small concentrate mass; at the same time, the concentration of microcomponents increases. An example of absolute concentration is the evaporation of a matrix in the analysis of waters, solutions of mineral acids, and organic solvents. Say, when evaporating 20 ml of a lead solution to 1 ml, we increase the ratio of the mass of the component being determined to the total mass of the sample by 20 times (provided that the component being determined remains completely in the solution). In other words, we concentrated 20 times.
As a result of relative concentration the matrix, which for one reason or another complicates the analysis, is replaced with another organic or inorganic matrix and the ratio between the micro and the main interfering macrocomponents increases. In this case, the ratio of the initial and final samples does not matter much. Let us assume that the same 20 ml of lead solution also contained zinc, and there was 100 times more of it than lead. We concentrated lead, for example by extraction, while the amount of zinc was reduced by 20 times, now it is only 5 times more than lead. We can obtain a concentrate of the same volume of 20 ml, while the concentration of lead has not changed, but the concentration of zinc has changed, and the amounts of zinc that remain in the solution will no longer interfere with the subsequent determination of lead. In practice, absolute and relative concentration are often combined; they replace the matrix elements with another organic or inorganic matrix and “compress” the trace element concentrate to the required mass by additional action, for example, simple evaporation.
The practice of chemical analysis requires both individual, so group concentration. Individual concentration is an operation as a result of which one microcomponent or several microcomponents are isolated sequentially from the analyzed object, whereas when group concentration several microcomponents are isolated at once. Both methods are used in practice. The choice of method depends on the nature of the analyzed object and the concentration method used. Group concentration is usually combined with subsequent determination by chromatography, chromatography-mass spectrometry, stripping voltammetry, and individual concentration with single-element analysis methods such as spectrophotometry, atomic absorption spectrometry, and fluorimetry.
Concentration can be done in two ways: removal of the matrix and isolation of microelements. The choice of method depends on the nature of the analyzed object. If the matrix is simple (one or two elements), it is easier to delete the matrix. If the base is multi-element (complex minerals and alloys, soils), microelements will be released. The choice also depends on the concentration method used. For example, co-precipitation is used only to isolate trace elements, and evaporation is used to separate a matrix of relatively simple objects: natural waters, acids and organic solvents.
F KSMU 4/3-04/01IP No. 6 UMS at KazSMA
dated June 14, 2007Karaganda 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.
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 calibrating 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 co-precipitation, 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 co-precipitation 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 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
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.
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.)
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 extraneous 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 coprecipitation 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 precipitate 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, solvate 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 turn 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?
There are many classifications of separation and concentration methods based on different characteristics. Let's look at the most important of them.
1. Classification according to the nature of the process is given in Fig.
Rice. 1
Chemical methods of separation and concentration are based on the occurrence of a chemical reaction, which is accompanied by precipitation of the product and the release of gas. For example, in organic analysis, the main method of concentration is distillation: during thermal decomposition, the matrix is distilled off in the form of CO2, H2O, N2, and metals can be determined in the remaining ash.
Physicochemical methods of separation and concentration are most often based on the selective distribution of a substance between two phases. For example, in the petrochemical industry, chromatography is of greatest importance.
Physical methods of separation and concentration are most often based on changing the state of aggregation of a substance.
2. Classification according to the physical nature of the two phases. The distribution of a substance can be carried out between phases that are in the same or different states of aggregation: gaseous (G), liquid (L), solid (S). In accordance with this, the following methods are distinguished (Fig.).
Rice. 2
In analytical chemistry, methods of separation and concentration, which are based on the distribution of a substance between the liquid and solid phases, have found the greatest importance.
- 3. Classification according to the number of elementary acts (stages).
- § Single-stage methods - based on a single distribution of a substance between two phases. The separation takes place under static conditions.
- § Multistage methods - based on multiple distribution of a substance between two phases. There are two groups of multi-stage methods:
- – repeating the single distribution process (for example, repeated extraction). The separation takes place under static conditions;
- – methods based on the movement of one phase relative to another (for example, chromatography). Separation takes place under dynamic conditions
- 3. Classification according to the type of equilibrium (Fig.).
Rice. 3
Thermodynamic separation methods are based on differences in the behavior of substances in an equilibrium state. They are of greatest importance in analytical chemistry.
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.
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, that is, when the analytical method is not selective enough and overlap of analytical signals must be avoided. In this case, the concentrations of the separated substances are usually close.
Concentration is an operation that allows you to increase the concentration of a microcomponent relative to the main components of the sample (matrix). It is used if the concentration of a microcomponent is less than the detection limit Cmin, i.e. when the analysis method is not sensitive enough. However, the concentrations of the components vary greatly. Concentration is often combined with separation.
Separation and concentration have much in common; the same methods are used for these purposes. They are very diverse.
There are many classifications of separation and concentration methods based on different characteristics.
a) classification according to the nature of the process
Chemical methods of separation and concentration are based on the 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 a 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 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.
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 its theory and equipment 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.
