Quantitative analysis. Chemical methods of analysis

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Introduction

1.2 Basic techniques and methods for analyzing an unknown sample

Conclusion

List of information sources used

Introduction

Analytical chemistry is of great practical importance in the life of modern society, since it creates the means for chemical analysis and ensures its implementation.

Chemical analysis is an important means of production control and product quality assessment in a number of industrial sectors, such as ferrous and non-ferrous metallurgy, mechanical engineering, production of pure and ultrapure materials for the radio-electronic industry, mining, chemical oil refining, petrochemical, pharmaceutical and food industries, geological service, etc. Without chemical analysis, it is impossible to solve the problems of environmental protection, the functioning of the agro-industrial complex, medical diagnostics, and the development of biotechnology.

The scientific basis of chemical analysis is analytical chemistry, which develops the theoretical foundations of methods of analysis or borrows them from related fields of chemical and physical science and adapts them to their goals. Analytical chemistry determines the limits of applicability of methods, evaluates their metrological characteristics, and develops methods for analyzing various objects. So, analytical chemistry is a field of scientific knowledge, a section of chemical science, and an analytical service is a system for meeting the needs of society in chemical analysis.

The purpose of the course work in the discipline "Analytical chemistry and physical and chemical methods of analysis" is to master the basic principles of qualitative and quantitative analysis.

The goal is achieved by solving a specific task for the analysis of an unknown substance, carrying out a calculation using a titrimetric method of analysis and the construction of the corresponding titration curve.

1. Qualitative analysis of an unknown substance

1.1 Theoretical information on qualitative analysis

Qualitative analysis is a section of analytical chemistry devoted to establishing the qualitative composition of substances, that is, the detection of elements and the ions they form, which are part of both simple and complex substances. This is done using chemical reactions characteristic of a given cation or anion, which make it possible to detect them both in individual substances and in mixtures.

Chemical reactions suitable for qualitative analysis must be accompanied by a noticeable external effect. It can be: evolution of gas, change in the color of the solution, precipitation, dissolution of the precipitate, formation of crystals of a characteristic shape.

In the first four cases, the course of the reaction is observed visually, the crystals are examined under a microscope.

To obtain correct results, reactions are required that are not interfered with by other ions present. This requires specific (interacting only with the ion being determined) or at least selective (selective) reagents.

Unfortunately, selective, especially specific reagents are very is small, therefore, when analyzing a complex mixture, one has to resort to masking interfering ions, converting them into a reaction-inert form, or, more often, to separating a mixture of cations or anions into constituent parts, called analytical groups. Do this is with the help of special (group) reagents, which, reacting with a number of ions under the same conditions, form compounds with similar properties - sparingly soluble precipitates or stable soluble complexes. This allows you to divide a complex mixture into simpler components. Qualitative analysis consists of the following steps:

preliminary observations;

preliminary tests;

Action of acids on a dry sample;

Transfer of the analyzed sample into solution ;

Systematic (or fractional) qualitative analysis of cations and

When carrying out analytical reactions, it is necessary to adhere to certain conditions. These include the concentration of reactants, the reaction of the medium, temperature.

1.2 Basic techniques and methods for the analysis of an unknown sample. Preparation of a substance for analysis

When starting to study the chemical composition of a given substance, it is first necessary to carefully consider it, determining its appearance, color, smell, degree of grinding (powder, coarse-grained or fine-grained mixture, solid mass, etc.), the presence of crystalline or amorphous phases and prepare accordingly for analysis, and only after that proceed to the establishment of its chemical composition.

The preparation of the test substance for analysis is a very important part of the entire study.

By the color of the analyzed sample, one can make assumptions about the presence or absence of certain cations in it. If, for example, the analyzed object is a colorless transparent or white mass, then this indicates the absence of significant amounts of colored cations in it - chromium (III) Cr 3+ (blue-violet color), manganese (II) Mn 2+ (light pink), iron (III) Fe 3+ (yellow-brown), cobalt (II) Co 2+ (pink), nickel (II) Ni 2+ (green), copper (II) Cu 2+ (blue). If the sample is colored, then it can be assumed that it contains one or more of the above cations. For a complete analysis of the test substance, it is necessary to take a small amount of it, measured in milligrams. Qualitative analysis is performed in two stages. First, preliminary tests are carried out, a, then they proceed to a systematic analysis of cations and anions.

Preliminary tests

Preliminary tests make it possible to establish the presence of some elements, the detection of which is difficult during the systematic course of the analysis.

Flame coloring

For the flame color test, a wire 60 mm long and 2-3 mm in diameter is taken. One of its horses is bent into a loop, the other end is soldered into a glass rod, which serves as a handle. The wire must be well cleaned by repeated annealing in the hottest non-luminous flame of the burner. The wire is immersed in hydrochloric acid and calcined in a burner flame, then cooled to room temperature. Several crystals of the analyte are placed on the wire prepared in this way and brought into the flame of the burner. Various ions color the flame in the following colors:

Carmine red………………………Sr 2+ ,Li 2+

Brick red……………………….Ca 2+

Yellow…………………………………….Na +

Yellow-green……………………………Ba 2+

Blue-green…………………………......Those

Light blue……………………………As,Sb,Pb 2+

Bright blue………………………………Cu 2+ ,Se

Violet ……………………………….K +, Rb + or Cs +

Wetting the wire with hydrochloric acid is carried out in order to obtain volatile chlorides of the cations present in the sample (if it contains a non-volatile or hardly volatile component) in a flame.

By the nature of the products of thermolysis (calcination) of a sample of a solid analyte, one can sometimes judge the presence of some cations and anions in the analyte.

To carry out this test, a small portion of the analyzed substance is placed on the bottom of a refractory test tube (~7 cm long) and the sample is heated, fixing the test tube in a horizontal position, in the flame of a gas burner. During thermal decomposition of the sample, gaseous thermolysis products are released, some of which condense at the cold end of the tube.

Based on the color of the sublime, some preliminary conclusions can be drawn:

Sublimation color Possible products of thermolysis

White …………………………………… Ammonium salts, Hg 2 Cl 2, HgCl 2,

Yellow…………………………………...HgI 2 , As 2 S 3 , S

Mirror metal ……………. Arsenic or mercury (plaque)

During thermal decomposition, along with sublimation, the release of vapors and gases can occur. The appearance of water droplets on the walls of the cold part of the test tube (tube) indicates that either the test sample contains water of crystallization, or water is formed during sample thermolysis (hydroxides, acidic and basic salts, organic compounds decompose with the release of water).

The evolution of violet vapors of iodine and their condensation in the form of dark crystals indicates the possibility of the presence of iodide ions or other iodine-containing anions :

In addition to violet vapors of iodine, brown vapors of bromine can be released (possibly present bromide ions and other bromine-containing anions), yellow-brown vapors of nitrogen oxides (possibly the presence of nitrates and nitrites), as well as gaseous CO (possibly the presence of oxalates), CO 2 (possibly the presence of carbonates, oxalates), C1 2 (possibly the presence of chloride ions and other chloride-containing anions), SO 2 (the presence of sulfites, thio-sulfates is possible), SO 3 (the presence of sulfates is possible), NH 3 (the presence of ammonium salts is possible), O 2 (the presence of peroxides, nitrates, chromates is possible, dichromates, etc.).

Action diluted hydrochloric acid (~1 mol/l)

Dilute sulfuric acid displaces weak acids from their salts - carbonates, sulfites, thiosulfates, sulfides, cyanides, nitrites, acetates. The released weak acids, unstable in an acidic environment, either volatilize or decompose with the formation of gaseous products.

In the presence of carbonates in the analyzed sample, gaseous carbon dioxide CO 2 is released (colorless and odorless). In the presence of sulfites and thiosulfates, sulfur dioxide SO 2 is released with the smell of burning sulfur; in the presence of sulfides - hydrogen sulfide H 2 S with the smell of rotten eggs; in the presence of cyanides - a pair of hydrocyanic acid HCN with the smell of bitter almonds; in the presence of nitrites - brown vapors of nitrogen dioxide NO 2, in the presence of acetates - vapors of acetic acid CH 3 COOH with the smell of vinegar.

The test is carried out as follows: a small amount of the analyte is taken into a test tube and dilute sulfuric acid is added dropwise to it. Emission of gases indicates the presence in the analyzed sample of the above anions of weak, unstable acids in an acidic environment.

Concentrated sulfuric acid, when interacting with the analyte, can release gaseous reaction products also from fluorides, chlorides, bromides, iodides, thiocyanates, oxalates, nitrates .

In the presence of fluorides in the analyzed substance, hydrogen fluoride vapors HF are released; in the presence of chlorides -- vapors of HC1 and gaseous chlorine C1 2 ; in the presence of bromides -- pairs of HBr and yellow gaseous bromine Br 2 ; in the presence of iodides -- violet vapors of iodine J 2 ; in the presence of thiocyanates -- gaseous sulfur dioxide SO 2 ; in the presence of oxalates - colorless gaseous oxide CO and carbon dioxide CO 2 carbon.

The test is carried out as follows. To a small mass of solid analyte (0.010 g) in a test tube, slowly, carefully, dropwise add concentrated sulfuric acid. If gas evolution is observed, then this indicates the presence of the above anions in the analyzed sample .

To carry out this test, a mixture of dilute H 2 SO 4 with KJ is taken, several crystals of the test substance, previously crushed to a powder state, or 3-4 drops of a solution of the analyte (if the substance is soluble) are added. In the presence of oxidizing agents, free iodine is released, which is detected by the brown color of the solution or with the help of starch. This reaction is given by NO 2 -, NO 3 -, MnO 4 -, CrO 4 2-, Fe 3+, Cu 2+ ions.

To detect reducing agents, a mixture of dilute solutions of KMnO 4 + is taken H 2 SO 4. The discoloration of this solution is caused by SO 3 2-, S 2-, S 2 O 3 2-, J -, NO 2 -, Cl -, Fe 2+, Cr 3+ -ions:

Dissolution in water

A small amount of the analyte is introduced into a test tube, a few milliliters of distilled water are added, and the mixture is stirred for some time. If the substance is completely dissolved in water, then most of the substance selected for analysis is dissolved in the minimum possible volume of distilled water, and the resulting solution is analyzed further. A small portion of the original solid test sample is retained for retesting or verification tests, if necessary.

Cation analysis

Analytical group - a group of cations, which with any one reagent (under certain conditions) can give similar analytical reactions. Division of cations into analytical groups based on their relationship to various anions. Two classifications are accepted: sulfide and acid-base.

According to the acid-base classification, cations are divided into six analytical groups (Table 1)

Table 1 - Separation of cations into groups according to acid-base classification

	Group	Received connections	group characteristic
K + , Na + , NH 4 +			Chlorides, sulfates and hydroxides are soluble in water
		Precipitate AgCl, PbCl 2	Chlorides are insoluble in water
		Precipitate BaSO 4 , CaSO 4	Sulfates are insoluble (or poorly soluble) in water and acids
Zn 2+, Al 3+, Cr 3+,	Excess 4n KOH or NaOH	Solution ZnO 2 2-, AlO 2 -, CrO 2 -,	Hydroxides are soluble in excess of caustic alkali
Mg 2+ , Mn 2+ , Fe 2+ , Fe 3+	Excess 25% NH 3	Precipitate Mg (OH) 2, Mn (OH) 2, Fe (OH) 2, Fe (OH) 3	Hydroxides are insoluble in excess caustic alkali.
Ni 2+ , Co 2+ , Cu 2+	Excess 25% NH 3	Ni(NH 3) 4 2+, Co(NH 3) 4 2+, Cu(NH 3) 6 2+	Hydroxides are soluble in excess ammonia

Anion Analysis The classification of anions is based on the difference in the solubility of barium and silver salts. According to the most common classification, anions are divided into three analytical groups, as shown in Table 2.

Table 2 - Classification of anions

Usually, the object is first examined for cations. From individual samples of the solution, using group reagents, it is determined which cations of which analytical groups are present in the solution, and then anions are already determined in it.

1.3 Progress in determining the composition of an unknown sample

A substance was issued for analysis, which is a mixture of two salts (test tube No. 13). According to the condition, only the following ions can be included in the composition of salts:

1. K +, Na +, NH 4 +

4. Zn 2+, Al 3+, Cr 3+

5.Mg 2+ ,Fe 2+ ,Fe 3+

6. Cu 2+ ,Co 2+ ,Ni 2+

1. SO 4 2-, SO 3 2-, CO 3 2-, PO 4 2-

3. NO 3 -, NO 2 -, CH 3 COO -

The analysis of the substance is carried out in accordance with the scheme described in paragraph 1.2.

Preliminary tests

The given substance is a fine-grained mixture of colorless crystals and grains. By the color of the substance, it can be assumed that there are no Fe 3+, Cr 2+, Cu 2+, Co 2+, Ni 2+ cations in it.

Flame coloring

Nichrome wire soaked in dilute hydrochloric acid is calcined in a burner flame, then cooled to room temperature. We place several crystals of the analyte on the wire prepared in this way. The burner flame turns pale blue, which indicates the possible presence of the Pb 2+ cation in the analyzed substance and the absence of K + , Ba 2+ , Ca 2+ , Cu 2+ cations

Thermal decomposition product test

A small portion of the analyzed substance is placed on the bottom of a refractory test tube and heated in a burner flame. We observe the release of yellow vapors, on the basis of this we can make an assumption about the possible presence of nitrates in the analyzed sample. Equations (1,2) for the formation of these substances are given below:

Decomposition of nitrates:

a) from alkaline earth to copper (inclusive)

Me(NO 3) 2 > 2MeO + + 2NO2 + O2 (1)

b) nitrates of silver, mercury, etc.

2MeNO 3 >2Me + 2NO 2 + O 2 (2)

The absence of dark plaque on the walls of the cold part of the tube also indicates the absence of iodides in the presence of oxidizing agents.

Conclusion: the analyzed substance may contain nitrates and lack iodine-containing ions.

The action of dilute sulfuric acid

We add a few drops of dilute H 2 SO 4 to a small amount of the given substance and heat it in a burner flame. A gas is released with a characteristic smell of vinegar.

The chemistry of the process is given below (equation (3)):

CH 3 COO - + H + > CH 3 COOH ^ (3)

Therefore, the anion CH 3 COO - may be present in the analyte.

The action of concentrated sulfuric acid

Slowly add concentrated sulfuric acid to a small mass of the analyzed sample. Colorless vapors are released with a characteristic odor of acetic acid, which once again confirms the presence in the analyzed sample of the anion CH 3 COO -

Emissions of vapors with a characteristic smell of chlorine and violet vapors of iodine in accordance with equations (4-6):

Cl - + H + > HCl ^ (4)

2Cl - + SO 4 2- + 2H + > Cl 2 ^ + SO 3 2- + H 2 O (5)

2J - + H 2 SO 4 > J 2 + SO 3 2- + H 2 O (6)

we do not observe, therefore, in the analyzed substance, there may be no anions Cl - ,I - .

Test for the presence of oxidizing agents

We take a mixture of H 2 SO 4 with KI , add a few crystals of the analyte. The release of free iodine, which causes the solution to turn brown in accordance with equations (7-9), does not occur, on the basis of which it can be assumed that NO 2 -, Fe 3+, Cu 2+ anions are absent in this substance

Process chemistry:

2J - + 2NO 2 - + 4H + > J 2 + 2NO + 2H 2 O (7)

2J - + 2Fe 3+ > J 2 + 2Fe 2+ (8)

4J - + 2Cu 2+ > J 2 + 2CuJv (9)

Test for the presence of reducing agents

Add a mixture of dilute solutions of KMnO 4 +H 2 SO 4 to a small portion of the analyte. Discoloration of the solution in accordance with the following equations (10-14) is not observed, which indicates the possible absence in the analyzed sample

NO 2 - , SO 3 2- , J - , Cl - , Fe 2+

2J - + 2NO 2 - + 4H + > J 2 + 2NO + 2H 2 O (10)

5SO 3 2- + 2MnO 4 - + 6H + > 5SO 4 2- + 2Mn 2+ + 3H 2 O (11)

16H + + 10J - + 2MnO 4 - > 5J 2 + 2Mn 2+ + 8H 2 O (12)

16H + + 10Cl - + 2MnO 4 - > 5Cl 2 + 2Mn 2+ + 8H 2 O (13)

5Fe 2+ + MnO 4 - + 8H + > 5Fe 3+ + Mn 2+ + 4H 2 O (14)

Dissolution in water

The analyte is completely soluble in water. Based on this, we can make an assumption about the simultaneous presence of Ag, Pb 2+, CH 3 COO -, NO 3 ions in the solution - (because only with these anions, the lead cation discovered in preliminary tests completely dissolves in water).

Sample for the presence of NH 4

We add a few drops of caustic soda to the analyzed mixture and heat it in the flame of a gas burner, the smell of ammonia is not felt, therefore the NH 4 + anion is absent.

Test for Fe 2+

We add a few drops of HCl solution to the test tube with the analyzed substance and a solution of red blood salt K 3, blue coloring of the solution in accordance with the equation (15) below is not observed, therefore, the Fe 2+ cation is absent.

3- + Fe 2+ >Fe 3 2 (15)

Test for Fe 3+

Add a few drops of water and a few drops of a concentrated solution of ammonium thiocyanate to a test tube with a solution of the analyte. Blood-red coloration in accordance with equation (16) is not observed, therefore, the Fe 3+ cation is absent.

Fe 3+ +3CNS - >Fe(CNS) 3 (16)

Conclusion: based on the results of preliminary tests, we can make an assumption about the presence of the following ions in the analyzed mixture: Pb 2+ ,CH 3 COO - ,NO 3 -

Systematic analysis

Cation test

Test for cations of the second analytical group

We add a few drops of hydrochloric acid HCl to the analyzed sample, we observe precipitation in accordance with equations (17,18), which confirms the possible presence of Pb 2+, Ag + cations in this substance

Process chemistry:

Pb 2+ +2HCl>PbCl 2 v (17)

Ag + +HCl>AgClv (18)

Check the formed precipitate for dissolution in hot water. Add some hot water to the resulting sediment. The precipitate dissolves, therefore, the Ag 2+ cation is absent.

In order to accurately verify the presence of the Pb 2+ cation in the analyzed sample, we will carry out the following experiment. Add the same amount of KI to a few drops of the analyte solution. A yellow precipitate is formed (Equation (19)).

Pb 2+ +2KI>PbI 2 v +2K + (19)

We add a few drops of water and a 2M solution of CH 3 COOH to the test tube, heat it, and the precipitate dissolves. Immerse the test tube in cold water. Brilliant golden crystals fall out in accordance with equation (20).

PbI 2 v + CH 3 COOH> I + HI. (twenty)

Thus, the presence of a lead cation in the analyzed substance was proved, which is consistent with preliminary tests (flame color test).

Since the lead cation interferes with the opening of the cations of the third and first analytical groups, it must be separated. To do this, add a few drops of 10 N HCl to the solution of the analyte, mix with a glass rod and filter. Wash the precipitate with acidified water 2N. hydrochloric acid solution (to reduce the solubility of lead chloride). Filtrate No. 1 may contain the following cations Ca 2+, Ba 2+, K +, Na +, as well as a small amount of the already discovered Pb 2+ cation. Then add a few drops of ammonium sulfate solution (NH 4) 2 SO 4 to the filtrate, heat in a boiling water bath for several minutes, let it stand for a while, and filter again. Filtrate No. 2 possibly contains K +, Na +, Ca 2+ cations. The precipitate containing Pb 2+ and possibly containing Ba 2+, Ca 2+ cations is treated with a hot 30% solution of CH 3 COONH 4 until PbSO 4 is completely removed, filtered , the precipitate is washed with distilled water and transferred to a porcelain cup, add a few milliliters of potassium carbonate solution K 2 CO 3, boil for several minutes, heating on an asbestos grid in the flame of a gas burner. After cooling, add a few milliliters of water to a porcelain cup, mix, let it stand and drain the transparent layer of liquid. Then add potassium carbonate K 2 CO 3 again, heat again for a few minutes, and filter. The precipitate is washed with warm water until the anions SO 4 2- are completely removed. The precipitate is dissolved in a test tube in a small portion of acetic acid and washed with a small amount of distilled water. Next, we will analyze for the presence of the Ba 2+ cation, for this we add a few drops of a solution of potassium chromate K 2 CrO 4 to the resulting solution. No precipitate is formed, therefore, the Ba 2+ cation is absent. We check the resulting solution for the presence of the Ca 2+ cation, add sodium carbonate, mix with a glass rod, we do not observe the formation of a precipitate, therefore, the Ca 2+ cation is absent. Let's check the filtrate No. 2 for the presence of the K + cation; for this, add a solution of Na 3 and a little acetic acid to the filtrate, a yellow precipitate of the cobalt complex salt is not formed, therefore, the K + cation is absent. Let's check the filtrate No. 2 for the presence of the Na + cation, add a few drops of KH 2 SbO 4 solution, no white crystalline precipitate is formed, therefore the Na + cation is absent. To open the cations of the fourth, fifth and sixth analytical groups, we add sodium hydroxide to the filtrate left after the separation of lead, no precipitation is observed, therefore, there are no cations in the analyzed mixture: Cu 2+, Zn 2+, Al 3+,

Mg 2+ ,Cr 3+ ,Ni 2+ ,Co 2+

Anion test

The presence of the Pb 2+ cation excludes the presence of anions of the first and second analytical groups in the analyzed substance; otherwise, precipitation would be observed when dissolved in water.

Despite the fact that in the preliminary tests we did not assume the presence of the NO 2 - anion, we will check the analyzed mixture for the presence of this anion. Let us add a few drops of the Griess-Ilosvay solution to the solution of the analyzed mixture, we do not observe red coloring of the solution, therefore, the NO 2 anion is indeed absent in this mixture.

Qualitative reactions to anions of the third analytical group

Let us confirm the presence of NO 3- anion in the analyzed substance. Let's carry out the following reaction: add 2-3 drops of dephenylamine and 5 drops of concentrated sulfuric acid to a few drops of a solution of an unknown substance. A dark blue color of the formed diphenylbenzidine is observed (equation (21)):

2(C 6 H 5) 2 NHC 6 H 5 -N -C 6 H 4 -C 6 H 4 -NH-C 6 H 5 C 6 H 5 -N \u003d C 6 H 4 \u003d C 6 H 4 \u003d N- C 6 H 5 (21)

According to the condition of the task in the issued mixtures can be present two anion. According to the results of preliminary tests the presence of anions NO 2 - , SO 4 2- , CO 3 2- , SO 3 2- , PO 4 3- , Cl - , I - - expelled Consequently, anion is present in the analyzed mixture CH 3 COO - , the presence of which confirms the release of vinegar vapor under the action of dilute sulfuric acid (preliminary tests equation (3) ).

Based on the above experiments, it can be concluded that the analyzed mixture contains the Pb 2+ cation and CH 3 COO - ,NO 3 - anions.

After analyzing the experimental data and preliminary observations, we conclude that this mixture consists of two salts Pb(NO 3) 2 and (CH 3 COO) 2 Pb.

Let us analyze the physical properties of these compounds.

Lead(II) acetate Рb(ОСОСН 3) 2 - colorless crystals; m.p. 280 °С; -- 960.90 kJ/mol; during melting, it partially evaporates, at higher temperatures it decomposes to Pb, CO 2 , H 2 O and acetone. Solubility in water (g per 100 g): 29.3 (10 ° C), 55.2 (25 ° C) and 221.0 (50 ° C);

Lead nitrate Pb(NO 3) 2 , colorless crystals. When heated above 200 ° C, it begins to decompose without melting with the release of NO 2 and O 2 and the sequential formation of oxonitrates Pb (NO 3) 2 2PbO, Pb (NO 3) 2, 5PbO and PbO oxide at 500-550 ° C. Solubility in water (g per 100 g): 45.5 (10°C), 58.5 (25°C), 91.6 (60°C) and 116.4 (80°C).

Indeed, the given substance, presumably consisting of salts of Pb(NO 3) 2 and (CH 3 COO) 2 Pb, is a mixture of colorless crystals, which is consistent with the above reference data. The flame of the burner (during the test for staining the tribe) is painted in a pale blue color, which indicates the presence of lead in the issued sample. When ignited, the analyte decomposes with the release of yellow fumes corresponding to equation (22), this confirms the presence of lead nitrate in this mixture.

Pb(NO 3) 2 > 2PbO + 2NO2 + O2 (22)

Under the action of dilute sulfuric acid on a dry sample, the release of vapors with a characteristic smell of vinegar was observed, therefore, lead acetate is present in this mixture. Thus, comparing the reference data, the results of preliminary observations and experimental data, we conclude that the earlier assumption about the composition of the mixture is confirmed.

unknown sample sulfuric acid reaction

2. Calculation of the theoretical titration curve

2.1 Theoretical foundations of titrimetric analysis

Titrimetric analysis is based on measuring the amount (volume or mass) of a titrant solution (reagent of precisely known concentration) spent on the reaction with the component being determined. The reagent solution is added until its amount is equivalent to the amount of the analyte. The reagent solution used in titrimetric analysis is called titrated or standard. the concentration of solutions in the titrimetric analysis is expressed as the number of gram equivalents per liter of solution.

Titrimetric methods are divided into two large groups. The first group includes methods based on ionic reactions: neutralization, precipitation, and complexation. The second group includes redox methods based on redox reactions, which are associated with the transfer of electrons from one particle to another. The applied reactions must satisfy a number of requirements. The reaction must proceed quantitatively according to a certain equation without side reactions. The reaction must proceed at a sufficient rate, so it is necessary to create optimal conditions that ensure the rapid course of the reaction. Establishing the equivalence point should be done fairly reliably.

Methods of neutralization. These include definitions based on the interaction of acids and alkalis. Neutralization methods are usually divided into acidimetry (determination of bases), alkalimetry (determination of acids) and halometry (determination of salts).

Precipitation methods are divided into argentometry, which makes it possible to determine by titration with a solution of silver nitrate, chlorides, iodides, cyanides, thiocyanates; on mercurometry based on titration with a solution of ferrous mercury nitrate.

Methods of complex formation are based on the use of reactions in which complex compounds are formed. They are subdivided into mercurymetry, based on titration with a solution of mercury(II) nitrate, resulting in the formation of slightly dissociated mercury(II) chloride, complexometry, based on the use of organic reagents-complesons; fluorometry based on the use of NaF.

Redox methods are based on the use of various oxidizing and reducing agents for titration.

Permanganatometry. The method was proposed in 1846. F. Margeritt for titration of solutions of iron (II) salts.

Bromatometry is a method based on oxidation with a solution of KBrO 3 in an acidic environment. Cerimetry. 1861. L. Lange proposed a solution of Ce (SO 4) 2 as an oxidizing agent. Cerium sulfate is used for titration of many reducing agents in strongly acidic solutions of iron (II) salts, arsenic, oxalic acids, etc.

Titanometry. Titanium(III) salts are used as vigorous reducing agents in the determination of mainly organic substances.

Nitritometry is based on titration with a standard solution of sodium nitrite. Nitritometry is most often used to determine organic substances by the reaction of diazotization or nitrosation.

Ascorbinometry is based on the use of ascorbic acid as a reducing agent. It is used for direct titration of various oxidizing agents.

2.2 Complexometric titration

Complexonometry (chelatometry), a titrimetric method of analysis based on the formation of strong intercomplex compounds (chelates) between metal cations and complexones. most commonly used are iminodiacetic, nitrilotriacetic (complexon I) and ethylenediaminetetraacetic (complexon II) acids, the disodium salt of the latter (complexon III, EDTA), as well as 1,2-diaminocyclohexanetetraacetic acid (complexon IV). The widespread use of complexones II and III is due to the fact that their reactions with metal cations proceed completely and in accordance with stoichiometry, their solutions are stable during storage; these reagents are available and high purity preparations can be obtained. The end point of the titration is set visually by changing the color of complexometric indicators (metal indicators), as well as potentiometrically, photometrically, amperometrically, or by other methods.

Complexones are non-selective reagents. The selectivity of complexones is increased by various methods: by lowering the pH of the medium, by separating (precipitating, extracting) the ion being determined, by masking, by changing the degree of oxidation of the cation, etc. .

Practical use

The high stability of coordination compounds of metals with Y 4- opens up the fundamental possibility of titrimetric determination of a large group of cations. Various methods of complexometric titration can be as follows: direct, reverse, displacement, etc.

In direct titration, a standard complexone solution is added in small portions to the solution of the ion under study. The pH value during titration must be greater than 7. But this can cause precipitation of metal hydroxides. Ammonia buffer (for nickel, copper, zinc, and cadmium) is used as a warning, and tartrates or citrates (for manganese and lead) are also added. Since the concentration of the ion being determined sharply decreases at the equivalence point, this point must be fixed by the change in color of the indicator that forms an intracomplex compound with the metal cation. The indicator reacts to a change in the concentration of the metal cation pMe in the same way as a pH indicator reacts to a change in pH. Thus, the ions Ca, Sr, Ba, Cu, Mg, Mn, Zn, etc. are determined. Prior to the complexometric method, there were no sufficiently reliable methods for analyzing compounds containing these metals.

Back titration is used when the pH required for complex formation causes precipitation of the metal to be determined, and also when there is no reliable indicator for the metal ion. Titrated EDTA solution is added in a slight excess to the analyzed salt solution. Set by introducing a buffer solution, the desired pH. The excess EDTA is titrated with a solution of magnesium chloride or zinc chloride. The equivalence point is fixed by changing the color of the indicator. Back titrations are also used. When the metal ion interacts with EDTA or a metal indicator in a slow manner, such as in the case of a nickel ion. This method is used when direct titration is not possible due to the formation of sparingly soluble precipitates of metal cations with anions present in the solution, for example, PbSO 4 , CaC 2 O 4 2H 2 O. The precipitates must dissolve during the titration.

Titration by displacement of one cation by another is used in the case when it is not possible to select the appropriate indicator for the ion being determined, or when the metal cation at a given pH cannot be transferred from the precipitate to the solution. In this case, a compound with a complexone can be obtained by an exchange reaction during titration of a salt of the metal being determined with a solution of a compound of some other metal with EDTA. For example, titrate with a solution of magnesium or zinc complexonate. To apply this method, it is necessary that the resulting compound of the metal to be determined with the complexone be stronger than the magnesium or zinc complexonate. At present, complexometric techniques have been developed for the analysis of very many objects.

The determination of water hardness was the first practical application of EDTA in analytical chemistry.

Water hardness is characterized by the molar concentration of calcium and magnesium equivalents.

Complexometric titration is also used for the analysis of various alloys, the determination of sulfates, phosphates and other anions, and for the analysis of organic compounds.

Physico-chemical methods for establishing the equivalence point in complexometry

Various physicochemical methods are commonly used to establish optimal titration conditions.

In addition, using physicochemical methods, it is possible to determine elements for which color indicators have not yet been found.

Potentiometric titration with a complexone is performed using ion-selective electrodes or using inert electrodes made of noble metals that respond to changes in the redox potential of the system.

Using a bimetallic pair of platinum-tungsten electrodes, lead, copper, zinc, nickel, cadmium and other elements can be titrimetrically determined.

The amperometric titration of EDTA is widely used for the determination of nickel, zinc, cadmium, and lead.

Conductometric, photometric, thermometric and other types of complexone titration with physicochemical indication of the equivalence point are used.

2.3 Calculation of the titration curve by complexometry

Evaluate the possibility of titrimetric determination and plot a titration curve for the following data 0.05M ZnCl 2 0.025M Na 2 H 2 Y, pH 9, ammonia concentration 0.1 mol/l.

We write the equation of the titrimetric reaction:

Zn 2+ + H 2 Y 2->ZnY 2- +2H +

The calculation of the titration curve is reduced to the calculation of the exponential concentration of Zn 2+ depending on the volume of the titrant. The stability of ZnY 2- depends on the acidity of the medium (the higher the acidity, the lower the stability), therefore, to bind hydrogen ions, the quantitative determination of ZnCl 2 is carried out in an ammonium buffer medium.

Calculate the volume of the titrant according to the law of equivalents:

The presence of the H ion + in an environment where Trilon B is present, leads to the following competing reactions:

Y4- +H + HY 3- , = K 4 ;

HY 3- +H + H 2 Y 2- , = K 3 ;

H 2 Y 2- +H + H 3 Y - , = K 2 ;

H 3 Y - +H + H 4 Y , = K 1 ;

where K 1, K 2, K 3, K 4 - stepwise dissociation constants H 4 Y (K 1 \u003d 1.0. 10 -2, K 2 \u003d 2.1. 10 -3, K 3 \u003d 6.9. 10 -7, K 4 \u003d 5.5. 10 -11).

Let us calculate the conditional stability constant, which expresses the strength of zinc complexes with Trilon B:

Calculate the coefficients of competing reactions:

Zn 2+ is also involved in competing reactions for the formation of complex compounds with ammonia NH 3 in accordance with the following reaction equations:

Zn 2 + +NH 3 Zn (NH 3) 2+,

Zn 2 + +2NH 3 Zn(NH 3) 2 2+,

Zn 2+ +3NH 3 Zn(NH 3) 3 2+,

Zn 2+ +4NH 3 Zn(NH 3) 4 2+,

According to a literary source

Substituting expressions (4) and (5) into the stability constant equation (3), we obtain:

1) before the start of titration, in the absence of competing reactions involving zinc, the concentration of Zn 2+ ions is equal to the concentration of salt ZnCl 2

ZnCl 2 >Zn 2+ +2Cl -

C=0.05 mol/l

2) up to the equivalence point, the value of pZn is determined by the concentration of the untitrated zinc ion, equation (a), so the dissociation of the complexonate formed according to equation (b) with an excess of zinc ions can be neglected.

a) Zn 2 + + H 2 Y 2-> ZnY 2- + 2H +

b) ZnY 2- -Zn 2 + + Y 4-.

Let's calculate for points

3) At the equivalence point, the calculation of the concentration of Zn 2 + ions is carried out taking into account the reaction equation for the dissociation of the complex:

ZnY 2- -Zn 2+ +Y 4-

This equilibrium is quantitatively described by a constant:

1,8 10 -5

4) after the equivalence point, the concentration of the metal complexonate remains constant

The concentration of ligand ions is determined by the excess of added titrant:

For the found values and the values of pZn 2+ and pY 4- are calculated and a titration curve is plotted in the coordinates pZn 2+ - V of the titrant. The titration curve is analyzed, the titration jump is calculated, and the indicator is selected.

Table 3 presents the calculation data for changes in the concentration of ions of the analyte and titrant depending on the volume of the added titrant (provided that the volume of the solution does not change during the titration).

Table 3 - Change in pZn during titration with Trilon B.

Let's analyze the resulting curve. As can be seen, in the region of the equivalence point, there is a sharp change in the concentration of zinc ions, which can be noted using the corresponding indicator. The titration jump is pZn 2+ =6.5-3.6=2.9, that is, the value is sufficient to fix the equivalence point. Based on this, it can be concluded that the complexometric determination of zinc is possible in the range of given concentrations.

Indicators in complexometry are metal indicators that form intensely colored compounds with metal ions, the stability constants of which, however, are lower than the constants of colorless complexes of Trilon B with metal ions.

The selection of the indicator is carried out in accordance with the titration conditions described in the Lurie reference book. Comparing the titration conditions presented in the problem with the data from the handbook, we come to the conclusion that, in this case, the indicator is a 0.1% aqueous solution of acidic chromium blue K, which provides a color transition from pink to gray-blue.

2.4 Determination of the anionic composition of wastewater

In the overwhelming majority of cases, the salt composition of natural waters is determined by Ca 2+, Mg 2+, Na +, K + cations and HCO 3 - , Cl - , SO 4 2- anions. These ions are called the main water ions or macrocomponents; they determine the chemical type of water. The remaining ions are present in much smaller amounts and are called microcomponents; they do not determine the chemical type of water.

According to the predominant anion, waters are divided into three classes: bicarbonate, sulfate and chloride. The waters of each class are divided, in turn, according to the predominant cation into three groups: calcium, magnesium and sodium.

Dissolved gases are also present in natural waters. Basically, these are gases that diffuse into the water from the air atmosphere, such as oxygen, carbon dioxide, nitrogen. But at the same time, in underground waters or waters of non-centralized sources of water supply, in mineral and thermal waters, hydrogen sulfide, radioactive gas radon, as well as inert and other gases may be present.

There are several methods for determining the anionic composition of water.

Method of complexometric titration

The determination of many anions is based on the precipitation of their sparingly soluble compounds with a titrated solution of some cation, the excess of which is then titrated with EDTA. Sulfate by this method is precipitated in the form of BaSO 4 with barium chloride and subsequent complexometric titration of excess Ba 2+ ions by a special method. The phosphate is precipitated as MgNH 4 PO 4 and the amount of magnesium remaining in the solution is determined complexometrically.

Chromatography

Ion chromatography is a method for the qualitative and quantitative determination of ions in solutions. It allows you to determine inorganic and organic anions, alkali and alkaline earth metal cations, transition metal cations, amines and other organic compounds in ionic form. Throughout the world, ion chromatography is used more than other methods, providing the detection of many components in any water. Ion chromatographs are used for analysis. The main element of any chromatograph is a separating analytical column. The analysis of inorganic anions such as fluoride, chloride, nitrite, nitrate, sulfate and phosphate by ion chromatography has been the most common method in the world for many years. In addition to ionochromatographic columns, high-performance columns have been developed and successfully used to determine the main inorganic anions; along with standard anions, they also detect oxyanions such as oxychalides: chlorite, chlorate, bromate, etc.

Argentometry.

Argentometry (from lat. argentum - silver and Greek metreo - measure), titrimetric method for determining anions (Hal -, CN -, PO 4 3-, CrO 4 2-, etc.), forming sparingly soluble compounds or stable complexes with Ag ions + The test solution is titrated with a standard solution of AgNO3 or the excess of the latter introduced into the analyzed solution is titrated with a standard solution of NaCl (the so-called back titration).

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Almost all methods are based on the relationship between the composition of a substance and its properties. To detect a component or its amount, measure analytical signal.

Analytical signal is the average of the measurements of the physical quantity at the final stage of the analysis. The analytical signal is functionally related to the content of the determined component. This may be the current strength, EMF of the system, optical density, radiation intensity, etc.

If it is necessary to detect any component, the appearance of an analytical signal is usually recorded - the appearance of a precipitate, a color, a line in the spectrum, etc. The appearance of an analytical signal must be reliably recorded. At a certain amount of the component, the magnitude of the analytical signal is measured: the mass of the deposit, the current strength, the intensity of the lines of the spectrum, etc. Then the content of the component is calculated using the functional dependence analytical signal - content: y=f(c), which is established by calculation or experience and can be presented in the form of a formula, table or graph.

In analytical chemistry, there are chemical, physical and physico-chemical methods of analysis.

In chemical methods of analysis, the element or ion being determined is converted into a compound that has one or another characteristic property, on the basis of which it can be established that this particular compound was formed.

Chemical Methods analysis have a specific scope. Also, the speed of performing analyzes using chemical methods does not always satisfy the needs of production, where it is very important to get analyzes in a timely manner, while it is still possible to regulate the technological process. Therefore, along with chemical methods, physical and physico-chemical methods of analysis are becoming more widespread.

Physical Methods analyzes are based on the measurement of some

a system parameter that is a function of composition, such as emission absorption spectra, electrical or thermal conductivity, potential of an electrode immersed in a solution, permittivity, refractive index, nuclear magnetic resonance, etc.

Physical analysis methods make it possible to solve problems that cannot be resolved by chemical analysis methods.

For the analysis of substances, physicochemical methods of analysis are widely used, based on chemical reactions, the course of which is accompanied by a change in the physical properties of the analyzed system, for example, its color, color intensity, transparency, thermal and electrical conductivity, etc.

Physical and chemical methods of analysis are characterized by high sensitivity and rapid execution, make it possible to automate chemical-analytical determinations and are indispensable in the analysis of small amounts of substances.

It should be noted that it is not always possible to draw a strict boundary between physical and physicochemical methods of analysis. Sometimes they are combined under the general name "instrumental" methods, because. to perform certain measurements, instruments are required that allow one to measure with great accuracy the values of certain parameters that characterize certain properties of a substance.

INTRODUCTION

Subject and tasks of modern analytical chemistry. The value of analytical chemistry in the development of various fields of natural science. The concept of differentiation and integration of natural sciences. Chemistry and geology. The laws of chemistry and their significance for the earth sciences. The role of analytical chemistry in solving the problems of geology, geochemistry, space research: determining the material composition of the Earth, the earth's crust, studying the geological processes of the external dynamics and geological activity of natural waters, etc.
Modern methods for studying the composition of substances. Qualitative and quantitative analysis of inorganic and organic substances. Chemical, physico-chemical and physical methods of analysis. Characteristics of methods and examples of their application in geology (geological research). The choice of the method for determining an element in an object, depending on its composition and the task of analysis.

I. THEORETICAL FOUNDATIONS OF ANALYTICAL CHEMISTRY

Chemical equilibria in a homogeneous system
The main types of homogeneous equilibria used in analytical chemistry: acid-base, redox, complexation equilibrium.
Mass action law. The equilibrium constant of a reversible chemical reaction. The concept of ideal and real systems. Causes of deviation from ideality. Activity, activity coefficient, its connection with ionic strength. Ionic state of elements. The concentration is general and equilibrium. α-factor (mole fraction). Thermodynamic constants, real, conditional, their connection.
Acid-base balance. Modern concepts of acids and bases. Protolithic Bronsted-Lowry theory. Acid-base pairs, acidity and basicity constants, their relationship. Processes of ionization and dissociation.
Solvent types, autoprotolysis reaction. Ionic product of the solvent. Leveling and differentiating effects of solvents.
Calculation of pH in solutions of acids, bases and ampholytes. Buffer solutions and their properties.
Equilibrium of complexation. Classification of complex compounds. Chelates, intercomplex compounds. changes in the potential of the redox system. Quantitative characteristics of the stability of complex compounds - general and stepwise stability constants. Types of complex compounds used in analytical chemistry and their characteristics. The use of complexation to detect, separate, mask and unmask ions, dissolve precipitates,
Theoretical foundations of the interaction of organic reagents with inorganic ions. Functional-analytical groups, chromophore groups. Cycloformation rule L.A. Chugaev. The main factors affecting the stability of chelates are the nature of the metal ion, the basicity and denticity of the ligand, the spatial factor, etc.
The main directions of the use of organic reagents in chemical analysis (detection, determination and masking of ions). The most common organic reagents are dimethylglyoxime, 8-hydroxyquinoline, etc.
Complexons. General properties of complexones and complexonates. Main directions of use of disodium salt of ethylenediaminetetraacetic acid (EDTA) for detection, masking and quantification of ions.
Redox balance. Reversible and irreversible redox systems. Equilibrium electrode potential. Nernst equation. Standard potential redox system. The concept of the real (formal) potential of the system. Factors affecting the value of the formal potential. Direction of oxidation - reduction reactions. Equilibrium constants of redox reactions. Connection of the equilibrium constant with standard potentials.
The rate of redox reactions. Catalytic, induced reactions in redox processes. The main oxidizing and reducing agents used in the analysis.
Redox reactions in the processes of external dynamics during the formation of sedimentary and metamorphic rocks.

Equilibrium in a heterogeneous system

Equilibrium in the system solid phase - solution. Precipitation - dissolution reactions in analytical chemistry. Thermodynamic equilibrium constant of the reaction of precipitation - dissolution (thermodynamic product of solubility). Influence of conditions on the equilibrium state of the reaction of precipitation - dissolution (real and conditional solubility products). Use of the solubility product rule in analytical chemistry.
Conditions for the formation and dissolution of precipitates. Crystalline and amorphous sediments. Dependence of the sediment structure on the nature and conditions of sedimentation. Colloidal state as an intermediate stage of sediment formation. Precipitation cleanliness. Coprecipitation. The use of this phenomenon for the concentration of microimpurities. Law V.G. Khlopin. The phenomenon of isomorphism in silicates and other minerals.
Calculation of solubility under various conditions (influence of pH, complexation, oxidation-reduction reactions, ionic strength of the solution and temperature). The influence of the ion of the same name. salt effect.
Equilibrium between two liquid phases. Extraction and its use in analytical chemistry. Distribution law. Distribution coefficient. Equilibrium constants in the liquid-liquid system (extraction constant). The use of extraction in the practice of chemical analysis.

Sample preparation for analysis and analysis.

Preliminary macro - and microscopic studies. Sampling for analysis of homogeneous and heterogeneous substances, average sample.
The choice of scheme and method of analysis depending on the composition of the analyte. Decomposition of the analyzed sample. Methods for transferring hardly soluble objects into solution: dissolution in acids and alkalis, fusion with acidic and alkaline fluxes. Analysis of various objects: minerals, ores, rocks, natural and waste water, air.

Metrological bases of analytical chemistry.

Characteristics of analytical methods. Determination of concentration by the calibration curve method and the method of additives. Limit of detection, lower and upper limits of determined concentrations, sensitivity coefficient, selectivity, time required for analysis (quickness).
Classification of errors. Systematic and random errors. Correctness and reproducibility. Statistical processing of measurement results. The law of normal distribution of random variables. Mean, variance, standard deviation. Validation of correctness. Comparison of variances and means of two methods of analysis. Ways to improve the reproducibility and correctness of the analysis.

II. ANALYSIS METHODS

Detection methods

Tasks and choice of detection method. Chemical, physico-chemical and physical detection methods. Qualitative analysis. Characteristics of analytical reactions. Selective and specific reagents. Methods for lowering the detection limit and increasing selectivity: the use of complexation, co-precipitation, extraction, flotation, etc. Application of precipitate formation reactions, colored compounds, gas evolution. Microcrystalloscopic, drop, luminescent, spectral analysis; powder trituration analysis. Use of organic reagents.
Analytical classification of ions. Acid-base and hydrogen sulfide analysis schemes. Systematic and fractional course of analysis. Express qualitative analysis in the field.

Separation and concentration methods.

Basic methods of separation and concentration.
Separation of elements using precipitation reactions. Application of organic and inorganic reagents for precipitation. Group reagents and conditions for their use. Characterization of poorly soluble compounds most commonly used in analysis: carbonates, chromates, phosphates, oxalates, hydroxides, sulfides. Conditions for the formation and dissolution of metal sulfides. The role of the processes of sedimentation and dissolution of sediments in the study of the laws of migration (concentration and dispersion) of elements in nature.
Chromatographicanalysis. Basic principles of the method. Classification of chromatography methods according to the state of aggregation of the phases, according to the mechanisms of separation and the technique of performing the experiment. Methods for obtaining chromatograms.
The most important theoretical provisions. Theory of theoretical plates and kinetic theory. Basic equations of chromatography.
Ion exchange and ion exchange chromatography. Gas chromatography. Liquid chromatography, partition chromatography on paper. Using paper chromatography to separate and detect cations.
Extraction. The concepts of extractant, diluent, extract, re-extraction. Extraction conditions, quantitative characteristics of extraction. extraction rate. Classification of extraction systems according to the type of extractable compounds. extraction methods. Separation of elements by extraction method. Increasing the selectivity of separation by selecting organic solvents, pH, masking. Devices for carrying out extraction.

Chemical methods of quantitative analysis

Gravimetric methods of analysis

The essence of gravimetric analysis. Direct and indirect methods of analysis. The most important inorganic and organic precipitants. Precipitated and gravimetric forms. Requirements for the precipitated and gravimetric forms. Precipitation, filtration and washing of precipitates.
Examples of gravimetric determinations (determination of crystallization and hygroscopic water, carbon dioxide, sulfur, iron, aluminum, barium, calcium, magnesium, phosphorus).
Analysis of carbonate rock: determination of the amount of sesquioxides, determination of calcium oxide and magnesium oxide.

Titrimetric methods of analysis.

Basic provisions and methods of titrimetric analysis.
Requirements for reactions in titrimetric analysis. Measuring utensils. Methods for expressing the concentrations of solutions. Expression of equivalent masses in various methods of titrimetric analysis. Titer. Titration. Equivalence point and end point of the titration. Chemical and physico-chemical methods for detecting the end point of titration.
Primary and secondary standard solutions. Primary standards and requirements for them. Fixanals. Separate weighing method and pipetting method for determining the concentration of working solutions. Calculation of analysis results.
Acid-base titration. The essence of the method of acid-base titration. working solutions. Primary standard solutions of acids and bases.
Calculation of pH at various points in the titration. Titration curves for strong and weak acids and bases.
Indicators in the method of acid-base titration. Theory of indicators. Interval of color transition of the indicator. Titration index. Select an indicator to establish the end point of the titration. Titration errors.
Practical application of the acid-base titration method. Determination of removable and permanent water hardness. Analysis of a mixture of carbonate and alkali, carbonate and bicarbonate. Determination of ammonium salts.
Change in redox potential during titration. Titration curve. Factors affecting the titration jump. Methods for detecting the end point of the titration. Redox indicators.
Basic redox methods of titrimetric analysis: iodometry, permanganatometry, dichromatometry. Iodometric determination of copper (II). Permanganometric determination of iron, water oxidizability. Dichromatometric determination of iron.
Complexation reactions used in titrimetry and requirements for them. Complexometry. Titration curve. Factors affecting the magnitude of the titration jump. Indicators in complexometry. Complexometric determination of water hardness, calcium, magnesium.
Metrological characteristics of chemical methods of quantitative analysis.

Physical - chemical and physical methods of analysis

Basic principles of physico-chemical and physical methods of analysis. Their brief description and meaning. Classification of methods of analysis. Electrochemical and spectroscopic methods. Their role for geochemical research. Analysis without destroying the analyte.
The choice of the method of analysis depending on the task at hand in the analysis of rocks, ores and minerals. Analysis of rocks for impurities (mass spectrometry, isotope analysis, X-ray methods of analysis). Processing and presentation of analysis results.

Electrochemical methods of analysis.

General characteristics of electrochemical methods of analysis. Their classification. Potential measurement. Electrochemical cell. Reversible and irreversible electrochemical reactions. Sensitivity and selectivity of electrochemical methods of analysis.
Potentiometry. Direct potentiometry. Classification and characteristics of electrodes. Indicator electrodes and reference electrodes. Ionometry: basic concepts and principles of the method. Classification of ion-selective electrodes. Selectivity factor. Potentiometric determination of medium acidity (pH), nitrate fluorides and some other ions (sodium, potassium) using ion-selective electrodes. Determination of concentration by the electrode calibration method and the addition method.
Potentiometric titration . Requirements for chemical and electrochemical (indicator) reactions. The use of reactions of various types: acid-base, precipitation, complex formation and oxidation - recovery. Indicator electrodes and reference electrodes. Schematic diagram of a potentiometer, pH meters. Examples of practical application (determination of a mixture of acids, cobalt, etc.).
Voltammetry. Polarographic method of analysis. polarographic cell. Indicator electrode and reference electrodes. Indicator electrodes in voltammetry. Obtaining and characteristics of the polarogram. Ilkovich equation. Polarographic wave equation. half wave potential. Qualitative and quantitative polarographic analysis. Possibilities, advantages and disadvantages of polarographic analysis. Modern varieties of polarography. Examples of the practical application of voltammetry to determine the main components and impurities in minerals, ores, natural waters, and environmental objects.
Amperometric titration. The essence of the method. indicator electrodes. Selection of the potential of the indicator electrode. Type of titration curves. Examples of practical use.
Coulometry. Theoretical basis. Methods for determining the amount of electricity in potentiostatic and galvanostatic coulometry. Direct coulometry and coulometric titration. Determination of the end point of the titration. Electrochemical generation of titrants. Practical application of the method, its advantages and disadvantages. Determination of small amounts of acid, alkali, determination of oxidizing agents, etc.
Metrological characteristics of electrochemical methods of analysis.

Spectroscopic methods of analysis.

Obtaining chemical-analytical information in the interaction of electromagnetic radiation with matter. Classification of spectroscopic methods of analysis according to the types of spectra and methods of their excitation.
Atomic emission spectroscopy. Emission spectra. Arc and spark discharge as sources of excitation. Plasmatron, inductively coupled plasma. Factors affecting the intensity of spectral lines. Practice of emission spectroscopy. Sample preparation and its introduction into the discharge. Qualitative and quantitative analysis. Chemical - spectral methods of analysis.
Emission photometry of a flame. Flame as a source of excitation. Processes that take place in a flame. Chemical reactions in flames. Factors affecting the degree of atomization. Dependence of the radiation intensity on the concentration of elements in the solution.
Examples of practical application of emission methods of analysis. Determination of alkali and alkaline earth elements. Determination of traces of metals in rocks, ores, minerals, water. Application of atomic emission methods in the study of environmental objects.
Atomic absorption spectroscopy. Fundamentals of the method. The law of absorption of electromagnetic radiation. Methods for obtaining an absorbing layer of atoms (flame and electrothermal atomization). Radiation sources, their characteristics (hollow cathode lamp, laser). The principle of atomic absorption measurements. Possibilities, advantages and disadvantages of the method. Examples of practical application of the atomic absorption method in geology.
Molecular absorption spectroscopy (spectrophotometry). Theoretical foundations of spectrophotometric analysis. Basic laws of light absorption. The Bouguer-Lambert-Beer law. Values characterizing light absorption: optical density and transmission. Molar absorption coefficient. The concept of true and apparent (average) molar absorption coefficient. Causes of deviation from the laws of absorption. Methods for determining concentrations by photometric method: calibration graph method, additive method, differential photometry method.
Choice of optimal conditions for carrying out the photometric reaction. Stages of photometric analysis. Photometric determination of some elements (iron, titanium, nickel, phosphorus, silicon, etc.).
Luminescence. The main characteristics of the method. Various types of luminescence and their classification. Basic regularities of molecular luminescence. Stokes-Lommel law. Rule of mirror symmetry of absorption and luminescence spectra. Examples of practical application (determination of rare earth elements, uranium, aluminum, etc.).
Metrological characteristics of spectroscopic methods of analysis.

III. WORKSHOPS
Methods for detecting and separating elements.

Study of characteristic reactions of some cations and anions. Separation and detection of cations using precipitation methods - dissolution, extraction and chromatography on paper. Detection of the main components and impurities in minerals, rocks, ores (test).

Methods for the quantitative determination of elements.
Chemical methods of analysis

Gravimetric methods of analysis. Determination of barium and sulfate ion in the sample (test).
Variants of work: Determination of calcium. Definition of iron. Definition of aluminum. Determination of sesquioxides in carbonate rock. Determination of water of crystallization in minerals.
Titrimetricanalysis methods. Acid-base titration. Preparation of a secondary standard solution of alkali and a primary standard solution of oxalic acid. Standardization of alkali solution.
Determination of the concentration of hydrochloric acid. (test). Statistical processing of measurement results. Variants of work: Determination of ammonium salts.
complexometric titration. Complexometric determination of calcium and magnesium in carbonate rock (test).
Variants of work: Complexometric determination of the total hardness of water.
Redox titration. Iodometric determination of copper (II) (test). Dichromatometric determination of iron (test).

Physico-chemical methods of analysis.

Potentiometric determination of cobalt (test). Variants of work: potentiometric titration of phosphoric acid.
Determination of fluoride ion (or individual ions: nitrates, sodium, potassium) in natural waters using an ion-selective electrode (test).
Removal and interpretation of the voltammetric spectrum (copper, cadmium, lead, nickel, zinc) (test).
Quantitative voltammetric analysis. Determination of the concentration of substances by the method of calibration curve or the method of additives (test).
Coulometric titration of thiosulfate ion (or hydrochloric acid) (test).
Amperometric titration of zinc. (optional work).
Photometric determination of an element (iron, nickel, manganese, titanium, silicon or phosphorus) (test).
Determination of high concentrations of elements (manganese, nickel, copper, etc.) by differential spectrophotometric method (test).
Luminescent determination of zirconium or organic dyes (test).
Atomic absorption determination of copper (zinc, manganese, iron)
Atomic emission (flame) determination of sodium and potassium.
Gas chromatographic determination of a mixture of alcohols (hydrocarbons).

Fundamentals of analytical chemistry (under the editorship of Yu.A. Zolotov). In 2 books. General issues. Separation methods. Methods of chemical analysis. M.: Higher school. 2004. 361, 503 pp. Series "Classical university textbook".
Fundamentals of analytical chemistry. Practical guide. Textbook for universities. Ed. Yu.A. Zolotova. M.: Higher school. 2001. 463 p.
Fundamentals of analytical chemistry. Tasks and questions. Textbook for universities. Ed. Yu.A. Zolotova. M.: Higher school. 2004. 412 p.
E.N. Dorohova, G.V. Prokhorov. Analytical chemistry. Physico-chemical methods of analysis. Moscow: Higher school, 1991.

additional literature

D. Skoog, D. West. Fundamentals of Analytical Chemistry: in 2 hours. M.: 1979
V.P.Vasiliev. Analytical chemistry. Parts 1-2 M.: Higher school, 1989.

The program is drawn up
Assoc. Viter I.P.
Editor
prof. Shekhovtsova T.N.

Subject of Analytical Chemistry

There are various definitions of the concept of "analytical chemistry", for example:

Analytical chemistry - it is the science of the principles, methods and means of determining the chemical composition and structure of substances.

Analytical chemistry - is a scientific discipline that develops and applies methods, instruments and general approaches to obtain information about the composition and nature of matter in space and time(definition adopted by the Federation of European Chemical Societies in 1993).

The task of analytical chemistry is the creation and improvement of its methods, the determination of the limits of their applicability, the assessment of metrological and other characteristics, the development of methods for analyzing specific objects.

A system that provides a specific analysis of certain objects using the methods recommended by analytical chemistry is called analytical service.

The main task of the pharmaceutical analytical service is to control the quality of medicines produced by the chemical-pharmaceutical industry and prepared in pharmacies. Such control is carried out in analytical laboratories of chemical and pharmaceutical plants, control and analytical laboratories and in pharmacies.

Principle, method and methodology of analysis

Analysis- a set of actions, the purpose of which is to obtain information about the chemical composition of the object.

Principle of analysis - a phenomenon that is used to obtain analytical information.

Analysis method - a summary of the principles underlying the analysis of the substance (without specifying the component and object being determined).

Analysis Method - a detailed description of performing an analysis of a given object using the selected method, which provides specified characteristics of correctness and reproducibility.

Several different analysis methods may have the same principle. Many different analysis methods can be based on the same analysis method.

The analysis methodology may include the following steps:

A particular analysis technique does not have to include all of the above steps. The set of operations performed depends on the complexity of the composition of the analyzed sample, the concentration of the analyte, the goals of the analysis, the permissible error of the analysis result, and on which analysis method is supposed to be used.

Types of analysis

Depending on the purpose, there are:

Depending on which components should be detected or determined, the analysis can be:

· isotopic(individual isotopes);

· elemental(elemental composition of the compound);

· structural-group /functional/(functional groups);

· molecular(individual chemical compounds characterized by a certain molecular weight);

· phase(individual phases in an inhomogeneous object).

Depending on the mass or volume of the analyzed sample, there are:

· macroanalysis(> 0.1 g / 10 - 10 3 ml);

· semi-microanalysis(0.01 - 0.1 g / 10 -1 - 10 ml),

· microanalysis (< 0,01 г / 10 -2 – 1 мл);

· submicroanalysis(10 -4 – 10 -3 g /< 10 -2 мл);

· ultramicroanalysis (< 10 -4 г / < 10 -3 мл).

Methods of analytical chemistry

Depending on the nature of the property being measured (the nature of the process underlying the method) or the method of recording the analytical signal, the determination methods are:

Physical methods of analysis, in turn, are:

· spectroscopic(based on the interaction of matter with electromagnetic radiation);

· electrometric (electrochemical)(based on the use of processes occurring in an electrochemical cell);

· thermometric(based on the thermal effect on the substance);

· radiometric(based on nuclear reaction).

Physical and physico-chemical methods of analysis are often combined under the general name " instrumental methods of analysis».

CHAPTER 2

2.1. Analytical reactions

Chemical methods for detecting substances are based on analytical reactions.

Analyticalcall chemical reactions, the result of which carries certain analytical information, for example, reactions accompanied by precipitation, gas evolution, the appearance of an odor, a change in color, the formation of characteristic crystals.

The most important characteristics of analytical reactions are selectivity and detection limit. Depending on the selectivity(the number of substances that enter into a given reaction or interact with a given reagent) analytical reactions and the reagents that cause them are:

Limit of detection(m min , P or C min , P) - smallest mass or concentration of a substance, which with a given confidence probability P can be distinguished from the signal of the control experiment(See Chapter 10 for more details).

2.2. Systematic and fractional analysis

Detection of elements in the joint presence can be carried out by fractional and systematic methods of analysis.

Systematic called a method of qualitative analysis based on the separation of a mixture of ions using group reagents into groups and subgroups and the subsequent detection of ions within these subgroups using selective reactions.

The name of systematic methods is determined by the group reagents used. Known systematic methods of analysis:

· hydrogen sulfide,

· acid-base,

· ammonium phosphate.

Each systematic method of analysis has its own group analytical classification. The disadvantage of all systematic methods of analysis is the need for a large number of operations, duration, bulkiness, significant losses of detectable ions, etc.

Fractionalcalled a qualitative analysis method that involves the detection of each ion in the presence of others using specific reactions or carrying out reactions under conditions that exclude the influence of other ions.

Usually, the detection of ions by the fractional method is carried out according to the following scheme - first, the influence of interfering ions is eliminated, then the desired ion is detected using a selective reaction.

The elimination of the interfering effect of ions can be carried out in two ways.

For example

· complex formation

· pH change

· redox reactions

· precipitation

· extraction

2.3. General characteristics, classification and methods for detecting cations

According to acid-base classification cations, depending on their relationship to solutions of HCl, H 2 SO 4 , NaOH (or KOH) and NH 3, are divided into 6 groups. Each of the groups, with the exception of the first, has its own group reagent.

First analytical group of cations

The first analytical group of cations includes cations K + , Na + , NH 4 + , Li + . They do not have a group reagent. Ions NH 4 + and K + form sparingly soluble hexanitrocobaltates, perchlorates, chloroplatinates, as well as sparingly soluble compounds with some large organic anions, for example, dipicrylamine, tetraphenylborate, hydrotartrate. Aqueous solutions of salts of group I cations, with the exception of salts formed by colored anions, are colorless.

Hydrated ions K + , Na + , Li + are very weak acids, acidic properties are more pronounced in NH 4 + (рК a = 9.24). Not prone to complex formation reactions. Ions K + , Na + , Li + do not participate in redox reactions, since they have a constant and stable oxidation state, NH 4 + ions have reducing properties.

The detection of cations of the I analytical group is carried out according to the following scheme

The detection of K + , Na + , Li + interfere with the cations of p- and d-elements, which are removed by precipitating them (NH 4) 2 CO 3 . The detection of K + is interfered with by NH 4 +, which is removed by calcining the dry residue or binding with formaldehyde:

4 NH 4 + + 6CHOH + 4OH - ® (CH 2) 6 N 4 + 10H 2 O

Similar information.

Quantitative analysis is expressed by a sequence of experimental methods that determine the content (concentrations) of individual components and impurities in a sample of the material under study. Its task is to determine the quantitative ratio of chemical compounds, ions, elements that make up samples of the substances under study.

Tasks

Qualitative and quantitative analysis are branches of analytical chemistry. In particular, the latter solves various issues of modern science and production. This technique determines the optimal conditions for conducting chemical-technological processes, controls the quality of raw materials, the degree of purity of finished products, including drugs, establishes the content of components in mixtures, the relationship between the properties of substances.

Classification

Methods of quantitative analysis are divided into:

physical;
chemical (classic);
physical and chemical.

chemical method

It is based on the use of various types of reactions that quantitatively occur in solutions, gases, bodies, etc. Quantitative chemical analysis is divided into:

Gravimetric (weight). It consists in the exact (strict) determination of the mass of the analyzed component in the test substance.
Titrimetric (volumetric). The quantitative composition of the test sample is determined by strict measurements of the volume of a reagent of known concentration (titrant), which interacts in equivalent quantities with the substance to be determined.
Gas analysis. It is based on the measurement of the volume of gas that is formed or absorbed as a result of a chemical reaction.

Chemical quantitative analysis of substances is considered classical. It is the most developed method of analysis and continues to evolve. It is accurate, easy to perform, does not require special equipment. But its use is sometimes associated with some difficulties in the study of complex mixtures and a relatively small feature of sensitivity.

physical method

This is a quantitative analysis based on the measurement of the values of the physical parameters of the investigated substances or solutions, which are a function of their quantitative composition. Subdivided into:

Refractometry (measurement of refractive index values).
Polarimetry (measurement of optical rotation values).
Fluorimetry (determination of fluorescence intensity) and others

Physical methods are characterized by rapidity, low limit of determination, objectivity of results, and the possibility of automating the process. But they are not always specific, since the physical quantity is affected not only by the concentration of the test substance, but also by the presence of other substances and impurities. Their application often requires the use of sophisticated equipment.

Physical and chemical methods

The tasks of quantitative analysis are the measurement of the values of the physical parameters of the system under study, which appear or change as a result of chemical reactions. These methods are characterized by a low detection limit and speed of execution, require the use of certain instruments.

gravimetric method

It is the oldest and most developed quantitative analysis technology. In fact, analytical chemistry began with gravimetry. A set of actions allows you to accurately measure the mass of the determined component, separated from other components of the system under test in a constant form of a chemical element.

Gravimetry is a pharmacopoeial method, which is characterized by high accuracy and reproducibility of results, ease of execution, but laborious. Includes tricks:

deposition;
distillation;
discharge;
electrogravimetry;
thermogravimetric methods.

Deposition method

Quantitative precipitation analysis is based on the chemical reaction of the analyte with a precipitant to form a poorly soluble compound, which is separated, then washed and calcined (dried). At the finish, the selected component is weighed.

For example, in the gravimetric determination of Ba 2+ ions in salt solutions, sulfuric acid is used as a precipitant. The reaction produces a white crystalline precipitate of BaSO 4 (precipitated form). After roasting this sediment, the so-called gravimetric form is formed, which completely coincides with the precipitated form.

When determining Ca 2+ ions, oxalic acid can be used as a precipitant. After analytical treatment of the precipitate, the precipitated form (CaC 2 O 4) is converted into the gravimetric form (CaO). Thus, the precipitated form can either coincide with or differ from the gravimetric form in terms of the chemical formula.

Scales

Analytical chemistry requires highly accurate measurements. In the gravimetric method of analysis, very accurate scales are used as the main instrument.

Weighing at the required accuracy of ± 0.01 g is carried out on a pharmacy (manual) or technochemical scales.
Weighing at the required accuracy of ±0.0001 g is carried out on an analytical balance.
With an accuracy of ± 0.00001 g - on microteres.

Weighing technique

Carrying out a quantitative analysis, the determination of the mass of a substance on technochemical or technical scales is carried out as follows: the object under study is placed on the left pan of the balance, and the balancing weights on the right. The weighing process is completed when the balance pointer is in the middle position.

In the process of weighing on a pharmacy scale, the central ring is held with the left hand, with the elbow resting on the laboratory table. The damping of the arm during weighing can be accelerated by lightly touching the bottom of the weighing pan to the surface of the table.

Analytical balances are mounted in separate allotted laboratory rooms (weight rooms) on special monolithic shelves-stands. To prevent the influence of air fluctuations, dust and moisture, the scales are protected by special glass cases. When working with an analytical balance, the following requirements and rules should be observed:

before each weighing, check the condition of the balance and set the zero point;
weighed substances are placed in a container (bottle, watch glass, crucible, test tube);
the temperature of the substances to be weighed is brought to the temperature of the balance in the weighing room for 20 minutes;
The balance must not be loaded beyond the specified limit loads.

Stages of gravimetry according to the precipitation method

Gravimetric qualitative and quantitative analysis includes the following steps:

calculation of weighed masses of the analyzed sample and the volume of the precipitant;
weighing and dissolving the sample;
deposition (obtaining a precipitated form of the component to be determined);
removing precipitation from the mother liquor;
sediment washing;
drying or calcining the precipitate to constant weight;
weighing gravimetric form;
calculation of analysis results.

The choice of precipitator

When choosing a precipitant - the basis of quantitative analysis - take into account the possible content of the analyzed component in the sample. To increase the completeness of sediment removal, a moderate excess of the precipitant is used. The precipitant used must have:

specificity, selectivity relative to the ion being determined;
volatility, easily removed by drying or calcining the gravimetric form.

Among the inorganic precipitants, the most common solutions are: HCL; H 2 SO 4 ; H3PO4; NaOH; AgNO 3 ; BaCL 2 and others. Among organic precipitants, preference is given to solutions of diacetyldioxime, 8-hydroxyquinoline, oxalic acid, and others that form intra-complex stable compounds with metal ions, which have the following advantages:

Complex compounds with metals, as a rule, have a slight solubility in water, ensuring complete precipitation of metal ions.
The adsorption capacity of intra-complex precipitates (molecular crystal lattice) is lower than the adsorption capacity of inorganic precipitates with an ionic structure, which makes it possible to obtain a pure precipitate.
Possibility of selective or specific precipitation of metal ions in the presence of other cations.
Due to the relatively large molecular weight of gravimetric forms, the relative error of determination is reduced (as opposed to the use of inorganic precipitants with a small molar mass).

Deposition process

This is the most important step in the characterization of quantitative analysis. When obtaining a precipitated form, it is necessary to minimize costs due to the solubility of the precipitate in the mother liquor, to reduce the processes of adsorption, occlusion, co-precipitation. It is required to obtain sufficiently large sediment particles that do not pass through the filtration pores.

Requirements for the precipitated form:

The component that is determined must quantitatively precipitate and correspond to the value of Ks≥10 -8 .
The sediment should not contain foreign impurities and be stable relative to the external environment.
The precipitated form should be converted as completely as possible to the gravimetric form upon drying or calcination of the test substance.
The aggregate state of the precipitate must correspond to the conditions of its filtration and washing.
Preference is given to a crystalline precipitate containing large particles, having a lower absorption capacity. They are easier to filter without clogging the filter pores.

Obtaining a crystalline precipitate

Conditions for obtaining an optimal crystalline precipitate:

Precipitation is carried out in a dilute solution of the test substance with a diluted solution of the precipitant.
Add the precipitant solution slowly, drop by drop, with gentle stirring.
Precipitation is carried out in a hot solution of the test substance with a hot solvent.
Sometimes precipitation is carried out in the presence of compounds (for example, a small amount of acid), which slightly increase the solubility of the precipitate, but do not form soluble complex compounds with it.
The precipitate is left in the initial solution for some time, during which the “precipitation of the precipitate” occurs.
In cases where the precipitated form is formed as an amorphous precipitate, attempts are made to make it thicker to facilitate filtration.

Obtaining an amorphous precipitate

Conditions for obtaining an optimal amorphous precipitate:

A hot concentrated solution of the precipitant is added to the hot concentrated solution of the test substance, which promotes coagulation of the particles. The sediment gets thicker.
Add precipitant quickly.
If necessary, a coagulant - electrolyte is introduced into the test solution.

Filtration

Quantitative analysis methods include such an important step as filtration. Filtration and washing of precipitates is carried out using either glass filters or paper filters that do not contain ash. Paper filters vary in density and pore size. Dense filters are marked with blue tape, less dense - with black and red. The diameter of ash-free paper filters is 6-11 cm. Before filtration, the clear solution above the precipitate is drained.

Electrogravimetry

Quantitative analysis can be carried out by electrogravimetry. The test drug is removed (most often from solutions) during electrolysis on one of the electrodes. After the reaction is complete, the electrode is washed, dried and weighed. By increasing the mass of the electrode, the mass of the substance formed on the electrode is determined. This is how an alloy of gold and copper is analyzed. After separating gold in solution, copper ions accumulated on the electrode are determined.

Thermogravimetric method

It is carried out by measuring the mass of a substance during its continuous heating in a certain temperature range. Changes are recorded by a special device - a derivatograph. It is equipped with continuous weighing thermometers, an electric furnace for heating the test sample, a thermocouple for measuring temperatures, a standard and a continuous recorder. The change in the mass of the sample is automatically recorded in the form of a thermogravigram (derivatogram) - a curve of mass change built in the coordinates:

time (or temperature);
mass loss.

Conclusion

Quantitative results must be accurate, correct and reproducible. For this purpose, appropriate analytical reactions or physical properties of the substance are used, all analytical operations are correctly performed and reliable methods of measuring the results of the analysis are used. During the performance of any quantitative determination, an assessment of the reliability of the results must be carried out.

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Tasks

Classification

chemical method

physical method

Physical and chemical methods

gravimetric method

Deposition method

Scales

Weighing technique

Stages of gravimetry according to the precipitation method

The choice of precipitator

Deposition process

Obtaining a crystalline precipitate

Obtaining an amorphous precipitate

Filtration

Electrogravimetry

Thermogravimetric method

Conclusion

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