The analysis of a substance can be carried out in order to establish its qualitative or quantitative composition. Accordingly, a distinction is made between qualitative and quantitative analysis.

Qualitative analysis allows you to establish what chemical elements the analyzed substance consists of and what ions, groups of atoms or molecules are included in its composition. When studying the composition of an unknown substance, a qualitative analysis always precedes a quantitative one, since the choice of a method for the quantitative determination of the constituent parts of the analyzed substance depends on the data obtained during its qualitative analysis.

Qualitative chemical analysis is mostly based on the transformation of the analyte into some new compound with characteristic properties: color, a certain physical state, crystalline or amorphous structure, a specific smell, etc. The chemical transformation that occurs in this case is called a qualitative analytical reaction , and the substances that cause this transformation are called reagents (reagents).

For example, to open a solution of -ions, the analyzed solution is first acidified with hydrochloric acid, and then a solution of potassium hexacyanoferrate (II) is added. In the presence of a blue precipitate of iron hexacyanoferrate (II) (Prussian blue):

Another example of a qualitative chemical analysis is the detection of ammonium salts by heating the analyte with an aqueous solution of sodium hydroxide. Ammonium ions in the presence of -ions form ammonia, which is recognized by the smell or by the blue color of wet red litmus paper:

In the examples given, solutions of potassium hexacyanoferrate (II) and sodium hydroxide are, respectively, reagents for and -ions.

When analyzing a mixture of several substances with similar chemical properties, they are first separated and only then characteristic reactions are carried out for individual substances (or ions), therefore, qualitative analysis covers not only individual reactions for detecting ions, but also methods for their separation.

Quantitative analysis allows you to establish the quantitative ratio of the constituent parts of a given compound or mixture of substances. Unlike qualitative analysis, quantitative analysis makes it possible to determine the content of individual components of the analyte or the total content of the analyte in the test product.

Methods of qualitative and quantitative analysis that allow determining the content of individual elements in the analyzed substance are called elemental analysis; functional groups - functional analysis; individual chemical compounds characterized by a certain molecular weight - molecular analysis.

A set of various chemical, physical and physicochemical methods for separating and determining individual structural (phase) components of heterogeneous! systems that differ in properties and physical structure and are limited from each other by interfaces are called phase analysis.

Subject and tasks of analytical chemistry.

Analytical Chemistry called the science of methods for the qualitative and quantitative study of the composition of substances (or their mixtures). The task of analytical chemistry is the development of the theory of chemical and physico-chemical methods of analysis and operations in scientific research.

Analytical chemistry consists of two main branches: qualitative analysis consists in “opening”, i.e. detection of individual elements (or ions) that make up the analyte. Quantitative Analysis consists in determining the quantitative content of individual components of a complex substance.

The practical importance of analytical chemistry is great. Using the methods of chem. analysis, laws were discovered: the constancy of composition, multiple ratios, the atomic masses of elements, chemical equivalents were determined, the formulas of many compounds were established.

Analytical chemistry contributes to the development of natural sciences - geochemistry, geology, mineralogy, physics, biology, technological disciplines, medicine. Chemical analysis is the basis of modern chemical-technological control of all industries in which the analysis of raw materials, products and production waste is carried out. Based on the results of the analysis, the course of the technological process and the quality of the products are judged. Chemical and physico-chemical methods of analysis underlie the establishment of state standards for all manufactured products.

The role of analytical chemistry in the organization of environmental monitoring is great. This is monitoring of pollution of surface waters, soils with heavy metals, pesticides, oil products, radionuclides. One of the objectives of monitoring is to create criteria that set the limits of possible environmental damage. For example MPC - maximum permissible concentration- this is such a concentration, under the influence of which on the human body, periodically or throughout life, directly or indirectly through ecological systems, there are no diseases or changes in the state of health that are detected by modern methods immediately or in the long term of life. For each chem. substances have their own MPC value.

Classification of methods of qualitative analysis.

When examining a new compound, first of all, it is determined what elements (or ions) it consists of, and then the quantitative relationships in which they are found. Therefore, qualitative analysis usually precedes quantitative analysis.

All analytical methods are based on obtaining and measuring analytical signal, those. any manifestation of the chemical or physical properties of a substance that can be used to establish the qualitative composition of the analyzed object or to quantify the components contained in it. The analyzed object can be an individual connection in any state of aggregation. mixture of compounds, natural object (soil, ore, mineral, air, water), industrial products and foodstuffs. Before analysis, sampling, grinding, sifting, averaging, etc. are carried out. The object prepared for analysis is called sample or test.

Choose a method depending on the task at hand. Analytical methods of qualitative analysis according to the method of execution are divided into: 1) “dry” analysis and 2) “wet” analysis.

Dry analysis carried out with solids. It is divided into pyrochemical and rubbing method.

pyrochemical (Greek - fire) type of analysis is carried out by heating the test sample in the flame of a gas or alcohol burner, it is performed in two ways: obtaining colored “pearls” or coloring the burner flame.

1. “Pearls”(French - pearls) are formed by dissolving NaNH 4 PO 4 ∙ 4 H 2 O, Na 2 B 4 O 7 ∙ 10 H 2 O salts in a melt - borax) or metal oxides. Observing the color of the obtained pearls of “glasses”, the presence of certain elements in the sample is established. So, for example, chromium compounds make pearl green, cobalt - blue, manganese - violet-amethyst, etc.

2. Flame coloring- volatile salts of many metals, when they are introduced into the non-luminous part of the flame, color it in different colors, for example, sodium - intense yellow, potassium - purple, barium - green, calcium - red, etc. These types of analyzes are used in preliminary tests and as a “quick” method.

Rubbing analysis. (1898 Flavitsky). The test sample is ground in a porcelain mortar with an equal amount of solid reagent. The presence of the ion to be determined is judged by the color of the obtained compound. The method is used in preliminary tests and carrying out "express" analysis in the field for the analysis of ores and minerals.

2. Analysis by “wet” way is the analysis of a sample dissolved in a solvent. The most commonly used solvent is water, acids or alkalis.

According to the method of carrying out, the methods of qualitative analysis are divided into fractional and systematic. Fractional analysis method- this is the definition of ions using specific reactions in any sequence. It is used in agrochemical, factory and food laboratories, when the composition of the test sample is known and it is only required to check the absence of impurities or in preliminary tests. Systematic analysis - this is an analysis in a strictly defined sequence, in which each ion is detected only after the interfering ions are detected and removed.

Depending on the amount of substance taken for analysis, as well as on the technique of performing operations, the methods are divided into:

- macroanalysis - carried out in relatively large quantities of the substance (1-10 g). The analysis is performed in aqueous solutions and in test tubes.

- microanalysis - examines very small amounts of a substance (0.05 - 0.5 g). It is performed either on a strip of paper, a watch glass with a drop of solution (drop analysis) or on a glass slide in a drop of solution, crystals are obtained, in the form of which a substance is determined under a microscope (microcrystalloscopic).

Basic concepts of analytical chemistry.

Analytical reactions - these are reactions accompanied by a well-marked external effect:

1) precipitation or dissolution of the precipitate;

2) change in the color of the solution;

3) gas evolution.

In addition, two more requirements are imposed on analytical reactions: irreversibility and sufficient reaction rate.

Substances that cause analytical reactions to take place are called reagents or reagents. All chem. reagents are divided into groups:

1) by chemical composition (carbonates, hydroxides, sulfides, etc.)

2) according to the degree of purification of the main component.

Conditions for performing chem. analysis:

1. Reaction environment

2. Temperature

3. Concentration of the determined ion.

Wednesday. Acid, alkaline, neutral.

Temperature. Most chem. reactions are performed at room conditions “in the cold”, or sometimes require cooling under a tap. Many reactions take place when heated.

Concentration- this is the amount of a substance contained in a certain weight or volume of a solution. A reaction and a reagent capable of causing to a noticeable extent its inherent external effect even at a negligible concentration of the analyte are called sensitive.

The sensitivity of analytical reactions is characterized by:

1) limiting dilution;

2) limiting concentration;

3) the minimum volume of the extremely dilute solution;

4) detection limit (discoverable minimum);

5) an indicator of sensitivity.

Limiting dilution Vlim - the maximum volume of a solution in which one gram of a given substance can be detected (in more than 50 experiments out of 100 experiments) using a given analytical reaction. The limiting dilution is expressed in ml/g.

For example, in the reaction of copper ions with ammonia in an aqueous solution

Cu 2+ + 4NH 3 = 2+ ¯bright blue complex

The limiting dilution of the copper ion is (Vlim = 2.5 10 5 mg/l), i.e. copper ions can be discovered using this reaction in a solution containing 1 g of copper in 250,000 ml of water. In a solution containing less than 1 g of copper (II) in 250,000 ml of water, these cations cannot be detected by the above reaction.

Limiting concentration Сlim (Cmin) – the lowest concentration at which an analyte can be detected in solution by a given analytical reaction. Expressed in g/ml.

The limiting concentration and limiting dilution are related by the relationship: Сlim = 1 / V lim

For example, potassium ions in an aqueous solution are opened with sodium hexanitrocobaltate (III)

2K + + Na 3 [ Co(NO 2) 6 ] ® NaK 2 [ Co(NO 2) 6 ] ¯ + 2Na +

The limiting concentration of K + ions in this analytical reaction is C lim = 10 -5 g/ml, i.e. the potassium ion cannot be opened by this reaction if its content is less than 10 -5 g in 1 ml of the analyzed solution.

Minimum volume of extremely dilute solution Vmin is the smallest volume of the analyzed solution required to detect the substance to be discovered by a given analytical reaction. Expressed in ml.

Limit of detection (opening minimum) m is the smallest mass of the analyte that can be unambiguously discovered by a given an. reaction in the minimum volume of an extremely dilute solution. Expressed in µg (1 µg = 10 -6 g).

m = C lim V min × 10 6 = V min × 10 6 / V lim

Sensitivity index analytical reaction is determined

pС lim = - lg C lim = - lg(1/Vlim) = lg V lim

An. the reaction is the more sensitive, the smaller its opening minimum, the minimum volume of the maximum dilute solution, and the greater the maximum dilution.

The value of the detection limit depends on:

1. Concentrations of test solution and reagent.

2. The duration of the course an. reactions.

3. Method of observing the external effect (visually or using an instrument)

4. Compliance with the conditions for the implementation of an. Reactions (t, pH, amount of reagent, its purity)

5. Presence and removal of impurities, foreign ions

6. Individual features of an analytical chemist (accuracy, visual acuity, ability to distinguish colors).

Types of analytical reactions (reagents):

Specific- reactions that allow the determination of a given ion or substances in the presence of any other ions or substances.

For example: NH4 + + OH - = NH 3 (smell) + H 2 O

Fe 3+ + CNS - = Fe(CNS) 3 ¯

blood red

selective- reactions allow you to selectively open several ions at once with the same external effect. The fewer ions a given reagent opens, the higher its selectivity.

For example:

NH 4 + + Na 3 \u003d NH 4 Na

K + + Na 3 \u003d NaK 2

Group reactions (reagents) allow you to detect a whole group of ions or some compounds.

For example: group II cations - group reagent (NH4)2CO3

СaCI 2 + (NH 4) 2 CO 3 \u003d CaCO 3 + 2 NH 4 CI

BaCI 2 + (NH 4) 2 CO 3 \u003d BaCO 3 + 2 NH 4 CI

SrCI 2 + (NH 4) 2 CO 3 \u003d SrCO 3 + 2 NH 4 CI

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Since ancient times, people have tested the properties and suitability of food (meat, vegetables, fruits, etc.), using organoleptic properties - color, smell, taste, etc. Nowadays, a variety of chemical, physical and physico-chemical methods of analysis are widely used. So far, the Pharmacopoeia provides organoleptic properties for most drugs. However, in verifying the authenticity and suitability of a drug, preference is given to the use of a variety of chemical reactions used in analytical chemistry. Analytical chemistry is divided into two parts: a) qualitative analysis b) quantitative analysis.

Qualitative analysis allows you to establish what chemical elements the test sample consists of, what ions, functional groups or molecules are included in its composition. When examining unknown substances, qualitative analysis always precedes quantitative analysis.

Depending on the composition of the object under study, there are:

Analysis of inorganic substances, which includes the detection of cations and anions;

Analysis of organic substances, which includes:

a) elemental analysis - detection and determination of chemical elements;

b) functional analysis - determination of functional groups consisting of several chemical elements and having certain properties;

c) molecular analysis - detection of individual chemical compounds. Thus, the main task of qualitative analysis is the detection of the corresponding cations, anions, functional groups, molecules, etc. in the test sample. The main task of quantitative analysis is to determine the amount of one or another component contained in the analyzed sample. The tasks and methods of quantitative analysis are discussed in detail in the "Methodological Manual on Quantitative Analysis for Students of the Faculty of Pharmacy".

PApplication of Qualitative Analysis in Pharmacy

Various methods of qualitative analysis are widely used to check and evaluate the quality of drugs. Qualitative chemical reactions in pharmaceutical analysis use

to determine the authenticity of the medicinal substance;

to test for purity and the presence of impurities;

for the identification of individual ingredients in medicinal products consisting of several substances.

Oidentification and purity testing of pharmaceuticals

To determine the authenticity of the study drug, analytical chemical reactions are carried out, and, if necessary, the corresponding physico-chemical constants (boiling point, melting point, etc.) are measured.

The analysis of substances that are electrolytes in aqueous solutions is reduced to the determination of cations and anions.

The identification of most organic medicinal substances is carried out using specific reactions, which are based on the chemical properties of the functional groups that make up their composition. The main requirement for these reactions is sufficient sensitivity with respect to the ions or functional groups being determined and a high rate of their occurrence.

Purity Tests and Impurity Limits

The criterion for the purity of a medicinal substance is the absence of some impurities and a limited amount of others. Impurities can be conditionally divided into two groups: 1) impurities that adversely affect the pharmacological action of the drug; 2) impurities that do not affect the pharmacological action, but reduce the content of the active ingredient in the preparation. For the first group of impurities that adversely affect the pharmacological action of the drug, the test should be negative. The second group of impurities does not affect the pharmacological effect and may be present in the preparation in small quantities. The list of indicators and norms for the content of these impurities is presented in the relevant literature.

Mqualitative analysis methods

In chemical methods of qualitative analysis, qualitative analytical reactions are used. With the help of such reactions, the desired chemical element or functional group is converted into a compound that has a number of characteristic properties: color, smell, state of aggregation. The substance that is used to carry out a qualitative analytical reaction is called a reagent or a reagent. Chemical methods are characterized by high selectivity, ease of implementation, reliability, but their sensitivity is not very high: 10-5 - 10-6 mol/l. In cases where higher sensitivity is needed, physicochemical or physical methods of analysis are used. Physical methods are based on the measurement of a certain physical parameter of the system, which depends on the content of the component. For example, in qualitative spectral analysis, emission spectra are used, since each chemical element has a characteristic emission spectrum for it. In the emission spectrum, the inert chemical element helium was first discovered in the sun, and then discovered on earth. Qualitative luminescent analysis uses luminescent emission spectra that are characteristic of an individual substance. In physicochemical methods of analysis, the corresponding chemical reaction is first performed, and then some physical method is used to study the resulting reaction product.

With the help of physical and physico-chemical methods of analysis, both qualitative analysis and quantitative analysis are often carried out. The use of these methods often requires the use of expensive equipment. Therefore, in qualitative analysis, physical and physicochemical methods of analysis are not used as often as chemical methods. When performing a qualitative chemical analysis, a certain amount of a substance is needed. Depending on the amount of substance taken for analysis, the methods of analysis are divided into macromethods, semimicromethods, micromethods and ultramicromethods of analysis. In macroanalysis, 0.5 - 1.0 g of a substance or 20 - 50 ml of a solution are used. The analysis is carried out in conventional test tubes, beakers, flasks, the precipitates are separated by filtration through filters, such as paper. In microanalysis, as a rule, from 0.01 to 0.001 g of a substance or from 0.05 to 0.5 ml of a solution are used, the reactions are carried out by the drop or microcrystalloscopic method. Semi-microanalysis occupies an intermediate position between macromethods and micromethods. For analysis, usually from 0.01 to 0.1 g of dry matter or 0.5 - 5.0 ml of solution is used. Analytical reactions are usually carried out in conical test tubes, the dosing of the solution is carried out using a dropper. The separation of the solid and liquid phases is carried out using a centrifuge.

FROMways to perform analytical reactions

Analytical reactions are carried out by "dry" and "wet" methods. In the first case, the analyzed sample and the analytical reagent are taken in the solid state and, as a rule, heated to a high temperature. These reactions include:

1. Flame color reaction. Volatile salts of certain metals on a platinum wire are introduced into that part of the burner flame that does not glow, and the color of the flame is observed in a characteristic color.

Fig. 2. The reaction of the formation of “pearls” of borax Na2B4O7 or ammonium and sodium hydrophosphate NaNH4HPO4. A small amount of one of these salts is fused in the eye of a platinum wire to form a vitreous mass that resembles a pearl (pearl). Then, several grains of the analyte are applied to the hot pearl and again brought into the burner flame. By changing the color of the pearls, a conclusion is made about the presence of the corresponding chemical elements.

3. Fusion reactions with dry substances: (Na2CO3; KClO3; KNO3, etc.) to obtain specifically colored products.

The reactions that are performed by the "dry" method are of an auxiliary nature and are used for preliminary tests. Reactions performed by the "wet" method (in solution) are the main ones in qualitative analysis.

Reactions that are carried out in a “wet” way must be accompanied by an “external” effect:

changing the color of the solution

the formation or dissolution of a precipitate,

gas release, etc.

Hsensitivity and specificity of analytical reactions

In a qualitative analysis, chemical reactions are characterized by the following parameters: a) specificity and selectivity. b) sensitivity. A specific reaction is one that can be used to determine the presence of a certain ion in the presence of other ions. An example of a specific reaction is the opening of ions by the action of a strong alkali solution when heated:

If there are ammonium ions in the analyzed sample, then gaseous ammonia is released during heating, which can be easily determined by the smell or by the change in color of red litmus paper. This reaction is specific; no other ions interfere with it.

Few specific reactions are known in qualitative analysis, therefore, reactions are used that can be carried out only when there are no ions in the analyzed solution that interfere with the desired reaction. A selective reaction is called a reaction, for which you must first remove from the solution those ions that interfere with the desired qualitative reaction. For example, a pharmacopoeial qualitative reaction to K + ions is the action of a solution of acidic sodium tartrate:

If there are potassium ions in the analyzed sample, then a white precipitate of acidic potassium tartrate is formed. But exactly the same effect is given by ions:

Therefore, ammonium ions interfere with the determination of potassium ions. Therefore, before determining potassium ions, ammonium ions must be removed. Efficient performance of selective reactions is possible if ions that interfere with the determination of a given ion or substance are removed from the solution. Most often, for this, the system is separated (into a precipitate and a solution) so that the ion that is determined and the ion that interferes with this would be in different parts of the system.

The sensitivity of a reaction (reagent) is a measure of the ability of a reagent to produce a reliably detectable analytical effect with the ion being determined. The smaller the amount of a substance that can be detected using a certain reaction, the more sensitive it is. Therefore, when choosing reactions for the detection of various ions, it is necessary to know the quantitative characteristic of the reaction sensitivity. The quantitative characteristics of the sensitivity of the reaction are the opening minimum (the minimum that is detected), the detection limit and the limiting dilution.

The smallest amount of a substance or ions that can be detected using a particular reaction under certain conditions is called the discoverable minimum. This value is very small, it is expressed in micrograms, that is, in millionths of a gram, and is denoted by the Greek letter g (gamma); 1g = 0.000001g = 10-6g.

At the suggestion of the terminological commission IUPAC (International Union of Pure and Applied Chemistry), to characterize the lowest content that can be determined by this method, I recommend using the term - the definition boundary. Thus, the detection limit is the smallest content of the component, at which the presence of the component being determined is determined using this technique with a given confidence probability of 0.9. For example, Сmin 0.9= 0.01 µg means that 0.01 µg of a substance is determined by this method with a confidence level of 0.9. The confidence probability is denoted by "p", then in general terms the definition boundary should be denoted as follows: Cmin p.

It should be remembered that the sensitivity of a reaction cannot be characterized only by the absolute amount of a substance. The concentration of ions or substances in solution is also important. The lowest concentration of ions or substances at which they can be detected using a given reaction is called the limiting concentration. In analytical practice, the reciprocal of the limiting concentration is used, which is called the "limiting dilution". Quantitative limit dilution (h) is expressed by the ratio:

where V (r-ra) is the volume of the most dilute solution (in ml) containing 1 g of the substance or ions to be opened. For example, for a reaction to iron ions using potassium thiocyanate, the limiting dilution is 1:10,000. This means that when diluting a solution that contains 1 g of iron ions in a volume of 10000 ml (10 l), the detection of Fe3 + ions using this reaction is still possible.

The sensitivity of the reactions largely depends on the conditions of their implementation (pH of the solution, heating or cooling, the use of non-aqueous solvents, etc.). The sensitivity of reactions is also affected by foreign ions, which in most cases are present in the analyzed solution.

Qualitative analysis of the test sample is usually carried out by the following two methods:

a) fractional analysis;

b) systematic analysis.

Fractional analysis is used to identify the desired ions in the presence of other ions. Since there are few specific reactions that allow the detection of a particular ion in the presence of any other ions, in fractional analysis, many qualitative reactions are carried out after pretreatment of the analyzed sample with reagents that precipitate or mask ions that interfere with the analysis. A significant contribution to the theory and practice of fractional analysis was made by N.A. Tananaev. Analytical reactions used in fractional analysis are called fractional reactions.

When choosing and carrying out fractional reactions, it is necessary:

choose the most specific reaction to detect the analyzed ion;

find out from the literature or experimentally which cations, anions or other compounds interfere with the selected reaction;

establish the presence in the analyzed sample of ions that interfere with the selected reaction;

select, guided by reference data, a reagent that removes or masks such ions and does not react with the analyzed ions.

As an example, consider a fractional Ca2+ detection reaction using the most commonly used Ca2+ detection reaction - the reaction with ammonium oxalate (NH4)2C2O4:

Ca2++ C2O42? = CaС2O4v. The sample contains Fe2+ and Ba2+ ions, which also form water-insoluble oxalates. It is known from the literature data that many ions of d-elements, as well as s2-elements (Sr2+, Ba2+), interfere with the reaction with oxalates. Iron (II) can be removed by the action of ammonia in the form of Fe (OH) 2 (PR = 7.9 10-16). Under these conditions, Ca2+ ions will not precipitate, since Ca(OH)2 is a strong base, quite soluble in water. In the presence of oxalates, Fe2+ will almost completely precipitate Fe(OH)2, and Ca2+ will react with C2O42?. To remove Ba2+, it is advisable to use the action of sulfates, given that CaSO4 is somewhat soluble in water. The technique for performing a fractional reaction for determining Ca2+ ions is as follows. Ammonia solution (up to pH 8-9) and (NH4)2SO4 solution are added to the test solution. The resulting Fe(OH)3 and BaSO4 precipitates are filtered off. Add (NH4)2C2O4 to the filtrate. The appearance of a white precipitate of CaC2O4 indicates the presence of Ca2+ ions in the analyzed sample. Systematic analysis is the analysis of the studied mixture of ions by dividing them into several analytical groups. Ions of a certain analytical group are separated from the solution by the action of a group reagent. The group reagent should quantitatively precipitate the ions of the corresponding analytical group, and the excess of the group reagent should not interfere with the determination of the ions remaining in solution. The resulting precipitate must be soluble in acids or other reagents in order to be able to identify the ions that were in the precipitate.

Xchemical reagents and work with them

Chemical reagents are substances that are used for chemical reactions. According to the degree of purity and purpose, the following categories of reagents are distinguished:

1) special purity (ultra-high purification), (special purity)

2) chemically pure (“chemically pure”),

3) pure for analysis (“analytical grade”),

4) clean (“h.”),

5) technical products packaged in small containers (“technical”).

High purity reagents are prepared for special purposes; their purity can be extremely high.

The purity of reagents of different categories is regulated by GOST and technical conditions (TU), the numbers of which are indicated on the labels. These labels also indicate the content of the main impurities.

Reagents are also divided depending on their composition and purpose. By composition, the reagents are divided into the following groups:

a) inorganic reagents,

b) organic reagents,

c) reagents labeled with radioactive isotopes, etc.

Organic analytical reagents, complexones, fixans, pH indicators, primary standards, solvents for spectroscopy, etc. are isolated according to their intended use. The purpose of the reagents is often indicated on the labels, which sometimes also indicate a number of other information, especially in the case of organic substances. The full rational name, name in several languages, formula, molar mass, melting point or other characteristics are indicated, as well as the batch number and date of issue. When working with chemical reagents, their toxicity must be taken into account and safety regulations must be observed.

All work with concentrated solutions of acids, alkalis, ammonia, hydrogen sulfide, as well as organic solvents is carried out in a fume hood.

When working with acids and alkalis, you must remember the rules for careful handling of them. If it comes into contact with human skin, they can cause burns, and if it comes into contact with clothing, it can damage it.

When diluting concentrated sulfuric acid, carefully pour the acid into the water, and not vice versa.

Wash your hands thoroughly after working in the laboratory.

To Qualitative analysis of inorganic substances

Qualitative analysis of inorganic substances makes it possible to establish the qualitative composition of both individual substances and mixtures, as well as to determine the authenticity (authenticity) of a pharmaceutical product and the presence of impurities in it. Qualitative analysis of inorganic substances is divided into cation analysis and anion analysis.

To Qualitative analysis of cations

There are several methods for the systematic analysis of cations, depending on the use of group reagents:

a) sulfide (hydrogen sulfide) method, group reagents in which are hydrogen sulfide and ammonium sulfide (table 1);

b) ammonia-phosphate method, group reagent - a mixture of (NH4)2HPO4 + NH3 (table 2);

c) acid-base method, group reagents - acids (HCl, H2SO4), bases (NaOH, KOH, NH3 H2O) (table 3).

Table 1 Classification by sulfide method

group number	Group Reagent
		Li+; Na+; K+; NH4+
	(NH4)2CO3 + NH3 + NH4Cl Carbonates do not dissolve in water	(Mg2+); Ca2+; Sr2+; Ba2+
	(NH4)2S + NH3 + NH4Cl Sulfides do not dissolve in water, ammonia, but dissolve in HCl.	Ni2+; Co2+; Fe2+; Fe3+; Al3+; Cr3+; Mn2+; Zn2+
	H2S + HCl Sulfides do not dissolve in HCl.	Cu2+; CD2+; Bi3+; Hg2+; As3+; As5+; Sb3+; Sb5+; Sn2+; Sn4+
	HCl Chlorides are insoluble in water and acids	Ag+; Pb2+; Hg22+

Table 2 Ammonium - phosphate classification of cations

group number	Group Reagent

	(NH4)2HPO4 + NH3. Phosphates are insoluble in water and ammonia	Mg2+; Ca2+; Sr2+; Ba2+;Mn2+; Fe2+; Fe3+; Al3+; Cr3+;Bi3+; Li+
	Phosphates dissolve in ammonia to form ammonia	Cu2+; CD2+; Hg2+; Co2+; Ni2+; Zn2+
	HNO3. Cations are oxidized to higher oxidation states	As3+; As5+; Sb3+; Sb5+; Sn2+; Sn4+
	HCl. Chlorides are insoluble in water and acids	Ag+; Pb2+; Hg22+

Table 3 Acid - basic classification of cations

group number	Group Reagent
	No. Chlorides, sulfates and hydroxides are soluble in water
	HCl Chlorides are insoluble in water and acids.	Ag+; Pb2+; Hg22+
	H2SO4 Sulphates are insoluble in water, acids and alkalis.	Ca2+; Sr2+; Ba2+
	NaOH Hydroxides are insoluble in water, soluble in both acids and alkalis.	Zn2+; Al3+; Cr3+; Sn2+; Sn(IV); As(III); As(V);
	NaOH Hydroxides are insoluble in water, ammonia and alkalis.	Mn2+; Mg2+; Fe2+; Fe3+; Bi3+; Sb(III); Sb(V)
	NH3 Hydroxides do not dissolve in water, excess alkali, dissolve in ammonia, form ammonia.	Cu2+; CD2+; Ni2+; Co2+; Hg2+

In pharmaceutical practice, the acid-base method is more often used, based on the different solubility of hydroxides and some salts formed by these cations (chlorides, sulfates) (table 3).

Systematic analysis begins with preliminary tests, which are most often carried out by the dry route (see page 3). Then the sample is dissolved and individual cations (NH4+, Fe2+, Fe3+, etc.) are determined, for which specific qualitative reactions are known. After that, cations of groups 2–6 are precipitated in the form of hydroxides and basic salts, acting on separate portions of the K2CO3 or Na2CO3 solution, and Na + ions are found in the filtrate (if K2CO3 acted) and K + (if Na2CO3 acted). Then, in a separate portion of the solution, the second analytical group is precipitated, acting with a solution of hydrochloric (hydrochloric) acid. Cations of III analytical group in the form of sulfates are precipitated with a 1M solution of sulfuric acid in the presence of ethanol, and cations of I, III, VI analytical groups remain in solution. By adding an excess of NaOH, the mixture under study is separated in the following way: cations of groups I and IV are in solution, and cations of groups V and VI are in the precipitate in the form of hydroxides. Further separation of the cations of groups V and VI is carried out by the action of an excess of ammonia. In this case, the hydroxides of cations of the VI analytical group form soluble ammonia, and the hydroxides of the V analytical group remain in the precipitate.

Thus, the main task of a group analytical reagent is:

a) determination of cations of the corresponding analytical group in the analyzed solution;

b) separation of cations of a certain group from cations of other analytical groups.

Analytical properties of cations . To cations of the first analytical group

The first analytical group of cations includes the alkali metal cations K+, Na+, as well as the complex cation NH4+. These cations have a low polarization capacity due to their large ionic radii. The ionic radii of K+ and NH4+ are close, because these ions have almost the same analytical properties. Most compounds of cations of the 1st analytical group are soluble in water. Therefore, the 1st analytical group of cations does not have a group reagent.

In solution, hydrated K+, Na+, and NH4+ ions are colorless. The color of some sodium, potassium or ammonium compounds is due to the color of the anion, for example: Na2CrO4 is yellow, and KMnO4 is red-violet.

Reactions of potassium ions K+

The action of a mixture of tartaric acid and sodium acetate (pharmacopoeial reaction).

Potassium ions form a white crystalline precipitate of potassium hydrotartrate:

KCl + H2C4H4O6 + CH3COONa = KHC4H4O6v + NaCl + CH3COOH

K+ + H2C4H4O6 + CH3COO? = KHC4H4O6v + CH3COOH

The same effect is achieved by the action of the acid salt of tartaric acid (sodium hydrotartrate) NaHC4H4O6:

KCl + NaHC4H4O6 = KHC4H4O6v + NaCl

K+ + HC4H4O6? = KHC4H4O6v

The KHC4H4O6 precipitate dissolves in mineral acids and alkalis:

KHC4H4O6 + H+ = K+ + H2C4H4O6

KHC4H4O6 + OH? = K+ + C4H4O62? + H2O

Therefore, the analysis of potassium ions is carried out in a neutral environment. The solubility of the KHC4H4O6 precipitate increases with increasing temperature. Therefore, to form this precipitate, the solution is cooled with cold water.

2. Action of sodium hexanitrocobaltate (III) Na3. Potassium ions with this reagent form a yellow crystalline precipitate of sodium potassium hexanitrocobaltate (ІІІ):

2KCl + Na3 = K2Na v + 2NaCl

2K+ + Na+ + 3? = K2Nav

The precipitate can dissolve in mineral acids to form an unstable acid H3 at pH<4.

K2Na + 3H+ = 2K+ + Na+ + H3

Alkalis decompose the reagent with the formation of a brown precipitate of Co (OH) 3:

K2Na + 3KOH = Co(OH)3v + 5KNO2 + NaNO2

K2Na + 3OH? = Co(OH)3v + 2K+ + Na+ + 6NO2?

Ammonium ions interfere with the determination of potassium ions, because they react similarly to potassium ions.

3. Flame color reaction (pharmacopoeial reaction). Potassium salts color a colorless burner flame purple. In the presence of sodium ions in the solution, which color the flame yellow and mask the violet color of potassium ions, the flame should be observed through cobalt blue glass. In this case, the yellow radiation from sodium is absorbed by the blue glass. Potassium radiation will be observed as purplish red.

Reactions of sodium ions Na+

1. The action of potassium hexahydroxoantibiate K. Concentrated solutions of sodium salts, when interacting with this reagent, form a white crystalline precipitate:

NaCl + K = Nav + KCl

Na + + ? = Nav

Na is a small crystalline precipitate that quickly settles to the bottom of the test tube, partially adheres to the walls. The precipitate is clearly visible if the test tube is tilted or the solution is poured out of it. If the precipitate does not immediately fall out (supersaturated solution), rub the walls of the test tube with a glass rod and cool the solution.

Features of the reaction conditions.

1. The test solution must contain a neutral or slightly alkaline medium. In an acidic environment, reagent K decomposes, resulting in the formation of a white amorphous precipitate of metaantimony acid НSbO3:

K + HCl = KCl + Hv = HSbO3v + 3H2O

This precipitate is taken as a Na precipitate and an erroneous conclusion is made about the presence of sodium ions in the solution. Therefore, acidic solutions are first neutralized with KOH alkali.

2. The Na salt is noticeably soluble in water and is capable of forming supersaturated solutions; therefore, a precipitate does not precipitate from dilute solutions or precipitate after a long time. The concentration of the sodium salt in the solution must be quite high, dilute solutions are first concentrated by evaporation.

3. The reaction must be carried out in the cold, since the solubility of Na increases with increasing temperature.

4. Ammonium salts interfere with the reaction. As a result of hydrolysis, aqueous solutions of ammonium salts are acidic, therefore reagent K decomposes in the presence of ammonium salts, as in the case of the action of acids. Mg2+ ions also interfere with the detection of Na+ ions, since they form a crystalline precipitate with K, which can be mistaken for a crystalline Na+ precipitate.

Therefore, when detecting Na + ions using K, the following conditions should be met:

the test solution should not contain NH4+ and Mg2+ ions;

the solution should be neutral or slightly alkaline and fairly concentrated;

the reaction must be carried out in the cold.

2. Action of zinc-uranyl acetate Zn(UO2)3(CH3COO)8. Sodium ions with this reagent in neutral or acetic acid solutions form a pale yellow precipitate of sodium zinc uranyl acetate:

NaCl + Zn(UO2)3(CH3COO)8 + CH3COOH + 9H2O = NaZn(UO2)3(CH3COO)9 9H2Ov + HCl

Na+ +Zn2+ +3UO22+ +8CH3COO? +CH3COOH +9H2O =NaZn(UO2)3(CH3COO)9 9H2Ov+ H+

Under a microscope, NaZn(UO2)3(CH3COO)9 9H2O crystals look like regular octahedra or tetrahedra. The detection of Na+ ions in this case is not interfered with by K+ or NH4+ ions.

3. Flame color reaction (pharmacopoeial reaction). Sodium salts turn the burner flame yellow.

Reactions of ammonium ions NH4+

1. The action of alkali (pharmacopoeial reaction). Ammonium ions react with alkali solutions (KOH, NaOH). When heated, gaseous ammonia is released:

NH4+ + OH? = NH3^ + H2O

This reaction is specific and quite sensitive. Other cations do not interfere with the detection of ammonium ions.

Gaseous ammonia can be detected in several ways:

by smell;

on the blue of a red litmus paper moistened with distilled water;

corresponding chemical reactions, for example, the reaction between ammonia and mercury (I) nitrate proceeding according to this equation:

In this case, the disproportionation reaction of mercury (I) into mercury (II) and metallic mercury occurs. (The disproportionation reaction is the reaction of changing the oxidation state of the atoms of an element in a compound with the formation of two substances in which this element exhibits a higher and lower oxidation state compared to the initial oxidation state of the element in the original compound).

Filter paper moistened with a solution of mercury (I) nitrate turns black. The blackening of the filter paper is caused by the release of free metallic mercury.

2. Action of Nessler's reagent K2. Ammonium ions with Nessler's reagent (alkaline solution K2) form a red-brown amorphous precipitate of the amide complex of mercury (II), having the following formula:

This amide complex has the following name: diiododimercurammonium iodide.

NH4Cl + 2K2 + 2KOH = Iv + 5KI + KCl

NH4+ + 22? + 2OH? = IV + 5I?

The response is very sensitive. At low concentrations of ammonium ions, no precipitate is formed, and the solution turns yellow. In an acid solution, reagent K2 is destroyed with the formation of a red precipitate of HgI2. The reaction must be carried out in a neutral or alkaline medium. Reactions are hindered by cations forming colored precipitates of hydroxides

Cr(OH)3, Fe(OH)3, Ni(OH)2, etc..

3. The ratio of ammonium salts to heating. All ammonium salts decompose when heated. The process of decomposition of ammonium salts depends on the nature of the anion.

Ammonium salts that contain anions of volatile acids (HCl, HBr, HF, etc.) decompose into gaseous ammonia and volatile acid when heated, for example,

NH4Cl > NH3 + HCl

But when leaving the high temperature zone, the decomposition products recombine, forming an ammonium salt:

NH3 + HCl = NH4Cl.

If the composition of ammonium salts includes anions of non-volatile acids, then gaseous ammonia is released during calcination, and non-volatile acid remains:

(NH4)3PO4 = 3NH3^ + H3PO4

H3PO4 = H2O^ + HPO3

(NH4)3PO4 = 3NH3^ + H2O^ + HPO3

In cases where the salt anion has oxidizing properties, ammonia is oxidized to free nitrogen or to nitrogen oxides. For example:

(NH4)2Cr2O7 = N2 + 4H2O + Cr2O3

NH4NO3 = N2O + 2H2O

Examples of decomposition of some other ammonium salts:

NH4NO2 = N2 + 2H2O

3(NH4)2SO4 = N2 + 4NH3 + 6H2O + 3SO2

(NH4)2C2O4 = 2NH3 + H2O + CO + CO2

FROMthe systematic course of the analysis of a mixture of cations.PFirst Analytical Group

When analyzing cations of analytical group I, ammonium ions are first determined. To do this, an alkali solution is added to a small amount of the analyzed solution and heated. In the presence of ammonium ions, the smell of ammonia is felt. If ammonium ions are detected, then they must be removed from the solution, because they interfere with the determination of potassium and sodium ions. To open sodium ions, KOH or K2CO3 is added to a separate portion of the analyzed solution and boiled to remove ammonia. Then the solution is neutralized with acetic acid (CH3COOH), cooled and opened with Na+ by the action of a solution of K or Zn(UO2)3(CH3COO)8. To determine K+ ions, ammonia is removed from the solution by the action of NaOH or Na2CO3 while boiling the solution. Then the solution is neutralized with acetic acid and, after cooling, K+ is determined by the action of NaHC4H4O6 or Na3 solutions.

Practical recommendations for the analysis of a mixture of cations of the 1st analytical group

1. Determination of ammonium ions. To 2 - 3 drops of the solution to be determined, add 6 - 8 drops of NaOH solution and heat. Wet red litmus paper is brought to the opening of the test tube. If ammonium ions are detected, the ammonium ions must be removed before the determination of potassium or sodium ions (see the following paragraphs). If there are no ammonium ions, then points 2 and 5 do not need to be performed. Potassium ions are opened by performing step 3 or 4. Sodium ions are opened by performing step 6 or 7.

2. Preparation of a solution for the determination of potassium cations. To 5 drops of the test solution add 5 drops of Na2CO3 or NaOH solution. The test tube with the solution is heated until the ammonia is completely removed (odor disappears, wet red litmus paper should not turn blue). After removal of ammonium ions, a solution of acetic acid is added dropwise to the solution until it becomes acidic (the litmus paper should turn red) and cooled.

3. Determination of potassium cations by the action of NaHC4H4O6 solution. To 2 - 3 drops of a solution that does not contain NH4 + ions, add 3 - 4 drops of a NaHC4H4O6 solution, accelerating the precipitation by rubbing a glass rod against the walls of the test tube and cooling the solution.

4. Determination of potassium cations by the action of Na3 solution. 1 drop of a solution that does not contain NH4 + ions is applied to a glass slide, 1 drop of a Na3 solution is applied nearby. The drops are stirred with a glass rod.

5. Preparation of a solution for the determination of sodium cations. To 5 drops of the analyzed solution add 5 drops of K2CO3 or KOH solution. The tube is heated to remove the ammonia completely. After that, acetic acid is added until neutral.

6. Determination of sodium cations. To 3 - 4 drops of a solution that does not contain NH4 + ions, add 3 - 4 drops of K solution and rub the inner walls of the test tube with a glass rod.

7. Determination of sodium cations using a microcrystalline reaction. A drop of a solution that does not contain NH4+ ions is applied to a glass slide. Carefully evaporate it almost to dryness. A drop of Zn(UO2)3(CH3COO)8 solution is applied nearby and the drops are connected with each other with a glass rod. The formed crystals are examined under a microscope.

Table 4ToQualitative reactions of cations of the analytical group

		The reaction product and its properties
	(Pharm.) K(Sb(OH)6]	Nav; white; R. k. l.
	Zn(UO2)3(CH3COO)8 +	NaZn(UO2)3(CH3COO)9 9H2Ov; green-yellow;
	(Pharm.) Flame	yellow flame
	(Pharm.) NaHC4H4O6	KNS4H4O4v; white; R. k. sh.
	(Pharm.) Na3	K2nav; yellow; R. k. sh.,
	(Pharm.) Flame	purple flame color
	(Pharm.) NaOH heating.	NH3 > litmus paper turns blue 4NH3+2Hg2(NO3)2+ H2O >NO3v+ Hgv black NH3 + HCl >NH4Cl; White smoke
		v; brown

R. - soluble; to. - acids; sch. - alkalis, farm. - pharmacopoeial reaction.

Toations of the second analytical group.Ogeneral characteristic

The second analytical group of cations includes Pb2+, Ag+, Hg22+ cations. Cations of the second analytical group form insoluble halides (except silver fluoride) sulfates, sulfides, chromates, phosphates, arsenites, arsenates, hydroxides (oxides), carbonates. This is due to the high polarization ability of these cations.

The group reagent for the second analytical group is HCl solution. Under the action of HCl, chlorides of cations of only the second analytical group are precipitated. Cations of other analytical groups remain in solution.

The cations of the II analytical group are characterized by complex formation reactions, while the Hg22+ ions are characterized by oxidation-reduction reactions and disproportionation reactions. Therefore, the systematic course of the analysis of cations of the second analytical group is based on the reactions of precipitation, complexation, and redox. Most salts of cations of the II analytical group have no color. Colored salts are salts that contain colored anions, such as chromates.

Rreactions of cations of the second analytical group

1. The action of a solution of hydrochloric (hydrochloric) acid. Cations of the II analytical group form white precipitates with HCl.

Ag + + Cl? = AgClv PR = 1.78 10-10

Hg22+ +2Cl? \u003d Hg2Cl2v PR \u003d 1.3 10-18

Pb2+ + 2Cl? = PbCl2v PR = 1.6 10-5

Precipitates of chlorides dissolve in an excess of concentrated HCl to form complex ions

AgClv + 2HCl = H2

AgClv + 2Cl? = 2?

PbCl2v + 2HCl = H2

PbCl2v + 2Cl? = 2?

In this regard, a large excess of the group reagent is not allowed.

The most soluble of the chlorides of the II analytical group is lead chloride, which is noticeably soluble in hot water (at 1000C, 3.34 g of PbCl2 can be dissolved in 100 g of H2O). This is used to separate PbCl2 from other cations of this group.

Silver chloride is soluble in ammonia, unlike mercury (I) chloride:

AgClv + 2NH3 = Cl

AgClv + 2NH3 = + + Cl?

This reaction is used to separate AgCl from Hg2Cl2.

If the Hg2Cl2 precipitate is treated with an ammonia solution, it will turn black due to the formation of finely dispersed metallic mercury

Hg2Cl2v + 2NH3 = Clv + Hgv + NH4Cl.

Mercury amidochloride Cl, which is formed in this reaction, can be considered as ammonium chloride NH4Cl, in which two hydrogen atoms are replaced by one doubly charged mercury ion. This reaction is used to determine Hg22+ and separate from other cations during analysis.

2. The action of alkalis.

Lead cations with alkalis form a white precipitate Pb(OH)2.

Pb2+ + 2OH? = Pb(OH)2v

Lead hydroxide has amphoteric properties, therefore it dissolves both in nitric acid and in an excess of alkali:

Pb(OH)2v+ 2HNO3 = Pb(NO3)2+ 2H2O

Pb(OH)2v+ 2H+ = Pb2+ + 2H2O

Pb(OH)2v+ 2NaOH = Na2

Pb(OH)2v+ 2OH? = 2?

Silver cations with alkalis form a white precipitate of silver hydroxide AgOH, which rapidly decomposes to form silver oxide:

Ag+ +OH? = AgOHv

2AgOHv= Ag2Ov + H2O

Mercury (I) cations, when interacting with alkalis, form a black precipitate of mercury oxide (I):

Hg22+ + 2OH? = Hg2Ov + H2O

All oxides and hydroxides of cations of the second analytical group are soluble in nitric acid.

Ag2O +2HNO3 = 2AgNO3 + H2O

Hg2O+2HNO3 = Hg2(NO3)2 + H2O

Pb(OH)2 + 2HNO3 = Pb(NO3)2 + 2H2O

3. Action of potassium iodide solution.

Cations of the II analytical group form colored poorly soluble iodides:

Ag+ + I? = AgIv yellow

Pb2+ + 2I? = PbI2v golden yellow

Hg22+ + 2I? = Hg2I2v green.

Lead iodide is soluble in hot water acidified with acetic acid. Mercury (I) iodide Hg2I2 reacts with an excess of the reagent:

Hg2I2v+ 2I? = 2? + Hgv

4. Action of ammonia solution.

Silver cations form a white precipitate of silver hydroxide with an ammonia solution, which quickly turns brown, since the hydroxide turns into oxide. The precipitate is soluble in excess ammonia:

Ag+ + NH3 + H2O = AgOHv + NH4+

2AgOHv = Ag2Ov + H2O

Ag2Ov + 4NH3 + H2O = 2+ + 2OH?

In an acidic environment, the ammonia complex of silver is destroyed:

2H+ = Ag+ + 2NH4+

It is also destroyed by the action of iodide ions with the formation of a precipitate of silver iodide:

I? = AgIv+ 2NH3

Mercury (I) cations with ammonia solution form an ammonia complex of mercury (II) and metallic mercury. For example, with Hg2(NO3)2 the reaction proceeds in accordance with the equation

Lead cations form a white hydroxide with an ammonia solution, which does not dissolve in an excess of the reagent:

Pb2+ + 2NH3 + 2H2O = Pb(OH)2v+ 2NH4+

5. Action of chromates.

Cations of the II analytical group form colored precipitates under the action of K2CrO4 or Na2CrO4:

2Ag+ + CrO42? = Ag2CrO4v brick red;

Hg22+ + CrO42? = Hg2CrO4v red;

Рb2+ + CrO42? = PbCrO4 v yellow.

Silver chromate dissolves easily in ammonia solution:

Ag2CrO4v+ 4NH3 = 2+ + CrO42?.

The precipitate of lead chromate is soluble in potassium and sodium hydroxides:

PbCrO4v + 4OH? = 2? + CrO42?.

Precipitates of chromates are soluble in nitric acid:

2Ag2CrO4v+ 4HNO3 = 4AgNO3+ H2Cr2O7 + H2O

6. Action of carbonates.

Silver cations form a white precipitate with carbonate anions:

2Ag+ + CO32? = Ag2CO3v

Silver carbonate is soluble in nitric acid and ammonia solution:

Ag2CO3v+ 4NH3 = 2+ + CO32?

Ag2CO3v+ 2H+ = 2Ag+ + H2O + CO2^

Mercury (I) cations form a yellow precipitate with carbonate anions:

Hg22+ + CO32? = Hg2CO3v

Mercury carbonate (I) is unstable and decomposes:

Hg2CO3v = HgOv + Hgv + CO2^

Lead cations form a white precipitate of basic salt:

2Pb(NO3)2 + 3Na2CO3 + 2H2O = (PbOH)2CO3v + 2NaHCO3 + 4NaNO3

2Pb2+ + 3CO32? + 2H2O = (PbOH)2CO3v + 2HCO3?

The precipitate of the basic salt of lead is soluble in acids and alkalis:

(PbOH)2CO3 v+ 4H+ = 2Pb2+ + CO2 ^+ 3H2O

(PbOH)2CO3v+ 6OH? = 22? + CO32?

7. Action of sulfates.

Cations of the II analytical group form sparingly soluble white compounds:

2Ag+ + SO42? = Ag2SO4v

Hg22+ + SO42? = Hg2SO4v

Pb2+ + SO42? = PbSO4v

Lead sulfate is soluble in alkalis and 30% ammonium acetate solution:

PbSO4v + 4OH? = 2? +SO42?

PbSO4v + 2CH3COONH4 = Pb(CH3COO)2 + (NH4)2SO4.

This feature is used in the systematic course of the analysis of cations of I - VI analytical groups.

The effect of some reagents on cations of the II analytical group is presented in Table 5.

Table 5 Effect of some reagents on cations of the II analytical group



AgCl, white precipitate, soluble in NH3.	Hg2Cl2, white sediment, which, under the action of NH3, decomposes. on Hg and HgNH2Cl.	PbCl2, white precipitate, soluble in hot water.
Ag2S, black precipitate, dissolves in NH3.	HgS + Hg. Black precipitate, dissolves in aqua regia.	PbS, black precipitate, soluble in HNO3.
Ag2O, brown precipitate, soluble in NH3 or HNO3.	Hg2O, black precipitate, soluble in HNO3.	Pb(OH)2, white precipitate, soluble in HNO3.
AgI, yellow precipitate, insoluble in NH3.	Hg2I2, green precipitate, dissolves in excess reagent.	PbI2, golden yellow precipitate, dissolves in hot water, in excess of reagent and CH3COOH.
Ag2SO4, white precipitate, precipitates from concentrated solutions, dissolves in hot water.	Hg2SO4, white precipitate, dissolves in aqua regia.	PbSO4, white precipitate, soluble in alkalis and 30% ammonium acetate.

Thus, Ag+, Hg22+, Pb2+ cations belong to the second analytical group. When salts of cations of the II analytical group interact with HCl, white precipitates of AgCl, Hg2Cl2, PbCl2 are formed, which are sparingly soluble in water and acids. Precipitates of AgCl and Hg2Cl2 turn black due to decomposition and release of free metals (silver or mercury). AgCl dissolves in excess NH3 to form a colorless, water-soluble Cl complex. This complex compound decomposes under the action of nitric acid to form AgCl, which precipitates, and NH4NO3. This reaction is used to separate Ag+ from other group II cations. AgCl also visibly dissolves in an excess of chlorides to form complex compounds of type M

Hg2Cl2, when interacting with an ammonia solution, forms Cl and metallic mercury, as a result of which the precipitate turns black. The PbCl2 precipitate is slightly soluble in cold water and soluble in hot water. This property is used to separate Pb2+ from other group II cations.

FROMsystematic course of the analysis of cations of the 2nd analytical group

When analyzing cations of the II analytical group, mercury (I) is first opened by reaction with metallic copper. The group reagent (HCl solution) precipitates cations of the second analytical group in the form of chlorides. The Pb2+ ion is not completely precipitated. The precipitate of chlorides is treated with hot water and quickly filtered. Lead ions are discovered in the filtrate. If they are found, then the precipitate is washed several times with hot water until a negative reaction to Cl? (test with the addition of AgNO3). After separating PbCl2, the precipitate is treated with an ammonia solution. Silver chloride dissolves with the formation of silver ammonia Cl, and the precipitate of mercury chloride turns into a black mixture of NH2HgCl and Hg. Instantaneous blackening of the precipitate indicates the presence of Hg22+. Silver ions are opened in the filtrate: when nitric acid is added, the formation of a white precipitate indicates the presence of silver ions in the mixture: Cl + 2HNO3 = AgClv + 2NH4NO3 The precipitate dissolves in an ammonia solution.

To ations of the third analytical group. general characteristics

The 3rd analytical group of cations includes cations of alkaline earth metals: Ba2+, Sr2+, Ca2+, which belong to the main subgroup of the second group of D.I. Mendeleev. Most salts of these cations are poorly soluble in water: sulfates, carbonates, chromates, oxalates, phosphates. For cations of the III analytical group, oxidation-reduction reactions are not typical, since they have a constant degree of oxidation. The cations of this analytical group are colorless, most of their salts are colorless. Cations of the III analytical group form colored compounds only with colored anions, for example: the yellow color of BaCrO4 is due to the corresponding color of CrO42? ions.

The group reagent for cations of the III analytical group is a solution of sulfuric acid. To ensure complete precipitation of BaSO4, SrSO4 and CaSO4, ethyl alcohol is added to the solution. Cations of IV - VI analytical groups are not precipitated by sulfuric acid.

Rreactions of cations III of the analytical group

1. The action of a solution of sulfuric acid. Cations Ba2+, Sr2+, Ca2+ under the action of a solution of sulfuric acid form white precipitates of sulfates:

Ba2+ + SO42? = BaSO4v PR = 1.1 10-10

Sr2+ + SO42? = SrSO4v PR = 3.2 10-7

Ca2+ + SO42? = CaSO4v PR = 2.5 10-5

The solubility of strontium and calcium sulfates is quite high, therefore, to reduce their solubility under the action of a group reagent, ethyl alcohol is added to the solution. Sulfates do not dissolve in acids and alkalis. CaSO4 is soluble in concentrated solutions of (NH4)2SO4:

СaSO4 + (NH4)2SO4 = (NH4)2

CaSO4 + SO42? = 2?

This property is used to separate Ca2+ ions from Sr2+ when they are present simultaneously.

2. Action of gypsum water. Gypsum water (saturated CaSO4 solution) precipitates Ba2+ and Sr2+ ions in the form of sulfates:

BaCl2 + СaSO4 = BaSO4v + CaCl2

SrCl2 + СaSO4 = SrSO4v + CaCl2

The solubility product of BaSO4 is small, so the precipitate precipitates quickly. The precipitate of SrSO4 is formed slowly in the form of turbidity of the solution, since the solubility product of SrSO4 is greater than the solubility product of BaSO4, and, accordingly, the solubility of SrSO4 is greater.

3. Action of carbonates. Carbonate anions precipitate Ba2+, Sr2+, Ca2+ ions in the form of white crystalline precipitates:

Ba2+ + CO32? = BaCO3v PR = 4.0 10-10

Sr2+ + CO32? = SrCO3v PR = 1.1 10 -10

Ca2+ + CO32? = CaCO3v PR = 3.8 10-9

Precipitates are soluble in mineral acids (HCl, HNO3) and acetic acid, for example:

BaCO3 + 2H+ = Ba2+ + H2O + CO2^ BaCO3 + 2CH3COOH = Ba2+ + 2CH3COO?+ H2O + CO2^

4. Action of chromates. Chromate anions form yellow precipitates with Ba2+ and Sr2+ ions:

Ba2+ +CrO42? \u003d BaCrO4v PR \u003d 1.2 10-10

Sr2+ + CrO42? \u003d SrСrO4v PR \u003d 3.6 10-5

They are soluble in strong acids (HCl, HNO3)

2BaCrO4 + 2H+ = 2Ba2+ + Cr2O72? + H2O

Strontium chromate, unlike barium chromate, is soluble in acetic acid. This difference in the properties of chromates is used to detect and separate Ba2+ ions. In the presence of Ca2+, Sr2+, and Ba2+ ions in an acetic acid medium, only a BaCrO4 precipitate forms under the action of a K2CrO4 solution.

5. Action of oxalates. Oxalate ions (salts of oxalic acid H2C2O4) form white crystalline precipitates:

Ba2+ + C2O42? = BaC2O4v PR = 1.1 10-7

Sr2+ + C2O42? = SrC2O4v PR = 1.6 10-7

Ca2+ + C2O42? = CaC2O4v PR = 2.3 10-9

Precipitates are soluble in strong acids, but insoluble in dilute acetic acid:

BaC2O4 + 2H+ = Ba2+ + H2C2O4

This reaction can be used to open calcium ions. interfere with barium and strontium ions.

6. Flame color reaction. Barium salts color the colorless flame of a gas burner yellow-green; and salts of strontium and calcium - in red.

7. Microcrystalloscopic reaction for Ca2+. Calcium ions with a solution of sulfuric acid form characteristic crystals of gypsum CaSO4 2H2O. Under a microscope, they are easily distinguished from small crystals of BaSO4 and SrSO4. Such a study allows the discovery of calcium in the presence of strontium and barium.

8. Action of sodium rhodisonate. With cations of the III analytical group, sodium rhodizonate forms colored compounds under various conditions. This feature makes it possible to detect calcium, strontium and barium ions without their preliminary separation. With calcium ions in an alkaline environment (NaOH), sodium rhodizonate forms a precipitate of the basic calcium rhodizonate of violet color. The sensitivity of the reaction is 1 μg.

Rhodisonate sodium

With strontium ions, sodium rhodizonate forms a brown precipitate of strontium rhodizonate in a neutral medium:

The reaction is carried out by the drop method. A red-brown color is formed on filter paper during the interaction of solutions of strontium salts and sodium rhodisonate, which disappears when a drop of HCl is added (dissolution of the precipitate).

The presence of K2CrO4 (unlike Ba2+) does not interfere with the reaction with sodium rhodizonate. This property makes it possible to detect Sr2+ in the presence of Ba2+ (calcium cations give this reaction only in an alkaline medium). In the presence of salts of chromic acid, Ba2+ binds to a precipitate of BaCrO4, which does not react with sodium rhodizonate. The sensitivity of the reaction is 7 µg. Sodium rhodisonate forms a red precipitate of barium rhodisonate with barium salts. When a drop of a neutral solution of barium salt and sodium rhodisonate solution is applied to filter paper, a red-brown spot of barium rhodisonate precipitate appears.

When a drop of HCl is added, the spot turns red due to the transition of barium rhodizonate to barium hydrorhodizonate:

In the presence of K2CrO4, barium rhodizonate is not formed (Ba2+ binds to precipitate BaCrO4). The reaction is specific for Ba2+. The formation reaction of strontium rhodizonate, unlike Ba2+, takes place in the presence of potassium chromate. The reaction can be used to determine Ba2+ and Sr2+ in their total presence. A drop of a solution containing a mixture of Ba2+ and Sr2+ ions is placed on paper and a drop of sodium rhodisonate solution is added. The appearance of a red-brown color, which turns red upon addition of a drop of HC1, indicates the presence of Ba2+. If the color disappears when HC1 is added, then only Sr2+ ions are present in the solution. In the presence of Ba2+ ions, Sr2+ ions are determined as follows: a drop of potassium chromate solution, a drop of a solution of the analyzed mixture and a drop of sodium rhodisonate solution are applied to the paper. The appearance of a brown-red color of the spot indicates the presence of Sr2+, since BaCrO4 was formed with potassium chromate, which does not react with sodium rhodizonate. The sensitivity of the reaction is 0.25 μg. The effect of some reagents on cations of the third analytical group is given in Table. 6.

What does qualitative and quantitative analysis mean? Quantitative and qualitative analysis of the data obtained during the study. Qualitative analysis of inorganic substances

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