Analytical chemists expose the substances under study to other substances whose composition is known.

Substances that cause chemical transformations test substances with the formation of new compounds that differ characteristic properties are called chemical reagents. Currently, very much has been synthesized large number chemical reagents.

The meaning of chemical reagents. Chemical reagents are widely used for all types of chemical analysis. Chemical reagents in the hands of an analytical chemist are a means of research not only chemical composition, but also the structure of the analyzed compounds.

Classification of chemical reagents. Depending on the composition, reagents can be inorganic or organic.

According to the degree of purity, reagents are divided into “chemically pure”, “analytically pure” (analytical grade), “pure” and “technical”, used in laboratories and in production. For the vast majority of analyzes carried out in analytical laboratories, “analytically pure” reagents are quite suitable. The least amount of impurities is contained in brand reagents used for precise analytical work and special purposes.

There are no absolutely pure reagents, but the amount of impurities they contain can be so insignificant that it practically does not affect the analytical determinations. Working with highly contaminated reagents can lead to completely distorted analysis results.

Concepts, etc. are quite relative. In some cases (for certain purposes), “heavily contaminated reagents” turn out to completely satisfy the requirements. In other cases (for other purposes), even branded reagents turn out to be unsatisfactory. The point is that for analysis beyond pure substances used in nuclear and semiconductor technology, or when solving problems of creating microwave and light radiation generators, computers, etc., reagents beyond high purity, without which it is impossible to decide the most pressing problems modern science and new technology.

How high these requirements are can be judged by the fact that work with such ultrapure substances must be carried out in specialized premises, supplied with absolutely clean air, free from impurities of foreign substances, including microimpurities, with special laboratory equipment, special containers for storing reagents (ordinary glassware is unsuitable in this case), etc. The lack of proper “ultrapurity” in laboratory premises where work with ultrapure substances is carried out leads to contamination of both the analyzed objects themselves and those used for them highly pure reagents. As a result, the data of the analysis results, which have great importance for the production and use of highly pure materials.

Some reagents are known in analytical practice by the names of their authors. For example, L.A. Chugaev’s reagent for nickel ions is dimethyl glyoxime:

Nessler's reagent, which is an alkaline solution of potassium tetra-iodomercurate, etc.

Reagents used in analytical laboratories are divided into specific, selective, or selective, and group.

Specific reagents are designed to detect the desired ions in the presence of other ions. For example, it is a specific reagent for iron (III), with which it forms a blue precipitate of Prussian blue; is a specific reactant for iron (II), reacting with which it forms a blue precipitate of Turnboule blue; dimethylglyoxime (LA Chugaev's reagent) is the most specific reagent for nickel ions and forms a pink-red crystalline precipitate of nickel dimethylglyoximate in an ammonia medium.

Selective, or selective, reagents react with a limited number of individual ions, sometimes belonging to different groups. For example, -hydroxyquinoline

forms with different ions under certain conditions, poorly soluble compounds in which hydrogen hydroxyl group hydroxyquinoline is replaced by metal ions, such as or. From buffer acetic acid solutions, α-hydroxyquinoline quantitatively precipitates ions of copper, bismuth, cadmium, vanidium (V), aluminum, zinc and some others; from ammonia solutions it precipitates magnesium, beryllium, calcium, strontium, barium and tin ions.

Of particular importance in analytical practice are selective solvents, which are predominantly liquid organic compounds, dissolving (or extracting) one or more components from a complex mixture of substances.

Group reagents react with a whole group of ions.

Requirements for reagents. Value and practical significance The performance of analytical reagents is determined by a number of requirements placed on them. These requirements mainly include purity, sensitivity and specificity. When using contaminated reagents containing harmful impurities(or ions being detected), incorrect results are obtained. Therefore, the reagents must first of all be pure.

The maximum content of permissible impurities in reagents is regulated by the technical requirements given in GOST or TU (i.e., in state standards or technical specifications). However, it should be kept in mind that the reagents are of analytical grade. or . are not always required to carry out analytical reaction. In the initial reagent, the presence of only those impurities that complicate the analysis or distort its results is usually unacceptable. In all other cases, foreign impurities do not matter.

Lecture 3. Chemical reagents.

1. Chemical reagents: definition of the concept, classification according to various criteria.

2. Brands of chemical reagents: Kh., Ch.D.A., Kh.Ch.

3. Safety precautions when working with caustic, flammable, toxic substances.

4. Rules for storing reagents.

5. Various ways purification of chemical reagents: physical, chemical, using ion exchange resins.

6. Methods for purifying chemical reagents: recrystallization, distillation and distillation, sublimation; dehydration (absolutization) of alcohol, benzene, ether.

D.z. according to school Pustovalova pp. 101-109.

1. Chemical reagents: definition of the concept, classification according to various criteria.

Chemical reagents are substances that are used to carry out various syntheses, as well as for quantitative and qualitative analysis V laboratory conditions, in other words, help to qualitatively identify individual elements, their groups or whole molecules that are part of the substance under study. Often, chemical reagents involved in chemical reactions in the analysis and synthesis of various substances are called reagents.

Chemical reagents- substances used in laboratories for analysis, scientific research when studying methods of preparation, properties and transformations of various compounds. Typically, chemical reagents include both individual substances and some mixtures of substances (for example, petroleum ether). Chemical reagents are also called solutions complex composition special purpose(for example, Nessler's reagent - for the determination of ammonia).

Chemical reagents are divided into groups depending on their composition: inorganic reagents, organic reagents, reagents containing radioactive isotopes, etc. Among the chemical reagents according to their intended purpose, analytical reagents are distinguished, as well as indicators, chemical and organic solvents.

All chemicals are divided into groups:

• Self-igniting chemicals.
• Highly flammable liquid chemicals.
• Flammable solid chemicals.
• Flammable (oxidizing) chemicals.
• Substances that are physiologically active in relatively small doses.
• Other chemicals, low-hazard and practically safe.

1. Brands of chemical reagents: Kh., Ch.D.A., Kh.Ch.

Often the following are distinguished: degree of purity of chemical reagents: extra pure (marked “pure grade”), chemically pure (“reagent grade”), pure for analysis (“analytical grade”), pure (“pure grade”), purified (“analytical grade”) purified"), technical products packaged in small containers ("technical"). Many chemical reagents are specially produced for laboratory use, but purified chemicals are also used. chemical products, produced for industrial purposes. The purity of chemical reagents in Russia is regulated State standards(Guests technical specifications(THAT).

There is even such a widely used expression as Reagents grade (reactive purity). The expression “technical product” is used as a synonym for the definition of “unrefined”. But in most cases, this idea of ​​tech products is outdated.
According to the degree of purity, chemical reagents are divided into the following categories:

Technical products that are packaged in small containers (“technical”).
- purified (“purified”);
- clean (“h.”);

The qualification “pure” (pure grade) is assigned to chemical reagents containing basic. component not lower than 98.0%. For chemical reagents of the “pure for analysis” (analytical grade) qualification, the content of basic. component m.b. above or significantly below 98.0% depending on the application.
- pure for analysis (“analytical grade”), allowing you to successfully carry out most analytical definitions;
- chemically pure (“reagent grade”) and special purity products (ultra-high purification).

Chemicals of high purity are used for special purposes, for example, in optical glass melting or in fiber optics.
To distinguish subclasses of substances of special purity, labeling has been introduced. The container with the reagent of each subclass has a special color label:

There are other methods for classifying substances of special purity. Thus, the Research Institute of Chemical Reagents and Highly Pure Substances (IREA) proposed to characterize the purity of the drug by the total content a certain number microimpurities. For example, for especially pure SiO 2 ten impurities are standardized (Al, B, Fe, Ca, Mg, Na, P, Ti, Sn, Pb), and general content their number does not exceed 1·10 -5. For such a drug, the index “special grade 10-5” is established. To package high-purity drugs, it is necessary to completely abandon glassware, which is a source of contamination. Therefore, most often they use polyethylene cans; it is even better to use Teflon cans (fluoroplastic-4).

The value and practical significance of analytical chemical reagents are determined mainly by their sensitivity and selectivity. The sensitivity of chemical reagents is the smallest amount or lowest concentration of a substance (ion) that can be detected or quantified when a reagent is added. Specific chemical reagents, in turn, are those reagents that give characteristic reaction with an analyte or ion under known conditions, regardless of the presence of other ions.

For quality control drinking water and water supply sources use special sets of chemical reagents. The sets of chemical reagents include standard solutions of the ions being determined for calibrating measuring instruments and assessing the accuracy of measurements. The chemical reagents in the kits are packaged according to the principle of precise weighing (fixananals) and the preparation of working solutions is reduced to diluting the chemical reagents included in the kit with distilled water according to the instructions supplied with the kit.

1. Safety precautions - TB when working with caustic, flammable, toxic substances.

Many chemicals are dangerous not only to health, but also to human life. Their improper use can lead to irreversible consequences Therefore, it is extremely important to know and practice safety rules when working with chemicals.
Some drugs that are particularly sensitive to air, such as the metals rubidium and cesium, are stored in sealed glass ampoules that are filled with inert gas or hydrogen.
Any containers containing chemicals must have labels indicating the substances.

Vessels with chemical reagents should be grasped by the neck with one hand, holding the bottom by the bottom with the other hand.

Do not look into open heated containers from above to avoid injury if hot mass is released.
It is strictly forbidden to use any chemical containers for drinking - this can lead to severe poisoning.

Any experiments with substances that are hazardous to health, poisonous or have an unpleasant odor should certainly be carried out under traction.

Do not taste any chemicals under any circumstances. You should also not use your mouth to pipette caustic or poisonous liquids; you should use a bulb for this purpose.

Dilution of sulfuric acid should produced by adding acid to water and in no case vice versa. Heat-resistant glasses should be used as utensils because this process generates a significant amount of heat.
The aggressive chemicals HNO 3, H 2 SO 4 and HCl should be poured only when the draft is on in a special fume hood. Its doors should be closed if possible.

When working with strong acids, be sure to use safety glasses and, preferably, a long rubber apron.
It is strictly forbidden to heat combustible and flammable substances on a grid, on a bare fire, in open vessels or near an open flame, in particular benzene, ethyl alcohol, acetone, ethyl acetate, etc.
Volatile liquids of organic origin can easily ignite even in the absence of open fire, simply when it comes into contact with a hot surface. Flammable liquids should also not be poured into cans or trash cans - this can lead to a fire from an accidentally thrown match.

To drain waste liquids (aggressive, toxic and flammable), specially designed containers should be used.

1. Rules for storing reagents.

Handling many chemicals requires strict adherence to safety regulations. To ensure safety great value has proper placement, storage and use of chemicals.

Chemical reagents are placed according to certain schemes. Dry inorganic and organic chemicals are stored in separate cabinets. Acids are stored separately from other chemicals in the bottom of a fume hood. Poisonous, flammable and toxic substances are stored in a safe. Chemicals that spontaneously ignite on contact with water should be stored in a locked cabinet.

Each container containing a chemical reagent must have a label with full name And chemical formula drug, in addition, the bottle with flammable substances must indicate: “Flammable” on the label. Storage chemicals without labels is not allowed.

Reaction equations:

(CH 3 COO) 2 Ca → CaCO 3 + CH 3 COCH 3;

CH 3 SOSN 3 + ZS1 2 →CH 3 SOSS1 3 + ZNS1;

CH 3 COCC1 3 + Ca(OH) 2 → (CH 3 COO)2Ca + 2CHC1 3;

2СНС1 3 + О 2 → 2СОС1 2 + 2НС1;

COS1 2 + 2NN 3 → CO(NH 2) 2 + 2HC1;

I(1:2) K N E

N 2 + ZN 2 →2NH 3;

COC1 2 + HON → CO 2 + 2HC1;

H 2 + C1 2 → 2HC1.

Saturated hydrocarbon A can undergo substitution reactions and undergo cleavage at the carbon-carbon bond.

Let's consider the case when an excess of hydrocarbon A was taken. Then products B and D can be predominantly monosubstituted and hydrocarbon A contains two types of hydrogen atoms, B and D - the corresponding monosubstituted ones. But in this case they are isomers and, after reduction with zinc dust, should give equal amounts of products capable of reacting with an ammonia solution of silver oxide (Tollens reagent). Therefore, this option is not suitable.

Let us consider the case when two reactions occur simultaneously: splitting and substitution. The only two products that can be produced are ethane and cycloalkanes (most likely cyclopropane). If A is ethane, then the reaction occurs

C 2 H 6 → CH 3 X + C 2 H 5 X.

Since treatment with the Tollens reagent leads to the same precipitate, it should be assumed that CH 3 X and C 2 H 5 X entered into similar reactions, and the ratio of the masses of the precipitation is equal to the ratio of the molar masses of CH 3 X and C 2 H 5 X:

From here Mr(X)=46, i.e. X is NO 2. Consequently, the following reactions occurred:

N 2 O 4 ↔ 2NO 2 (formation of brown vapors);

C 2 H 6 + 2NO 2 → C 2 H 5 NO 2 + HNO 2;

C 2 H 6 + 2NO 2 → 2CH 3 NO 2;

RNO 2 + 2Zn + 8NH 4 C1 → RNHOH + 2Zn(NH 3) 4 C1 2 + 4HC1;

(R = CH 3, C 2 H 5)

2H 2 O + RNNНО + 2Аg(NН 3) 2 ОН → RNO + 2Аg↓ + 4NН 4 ОН.

Thus, for the case considered we have A - ethane, B - N 2 O 4, B - CH 3 NO 2, G - C 2 H 5 NO 2. With a different ratio of sediment masses, the option is suitable when A is cyclopropane, the reaction occurs:

(CH 2) 2 →C 3 H 5 X + CHN 2 CH 2 CH 2 X.

The ratio of precipitation masses is equal to the ratio of equivalents B and G:

From here Mr(X) = 30, liquid B - bromine. The following reactions took place:

(CH 2) 3 + Br 2 → C 3 H 5 Br + HBr;

(CH 2) 3 + Br 2 → B-CH 2 CH 2 CH 2 Br;

B-CH 2 CH 2 CH 2 Br + Zn → C 3 H 6 + ZnBr 2 (formation of cyclopropane);

Zn + 2Аg (NH 3) 2 ОН → Zn(NH 3) 4 (ОН) 2 + 2АgBr.

A - cyclopropane, B - bromine, C-1,3-dibromopropane, D - bromocyclopropane.

Assumptions that A is a cycloalkane with more than three carbon atoms in the ring do not lead to a reasonable answer.

6.2. Determination of one or more substances based on qualitative reactions

Solving qualitative problems of identifying substances found in bottles without labels involves carrying out a number of operations, the results of which can be used to determine which substance is in a particular bottle.

The first stage of the solution is a thought experiment, which is a plan of action and its expected results. To record a thought experiment, a special table-matrix is ​​used, in which the formulas of the substances being determined are indicated horizontally and vertically. At the intersection of the formulas of the interacting substances, the expected results of observations are recorded: - gas evolution, ↓ - precipitation, changes in color, odor or the absence of visible changes are indicated. If, according to the conditions of the problem, it is possible to use additional reagents, then it is better to write down the results of their use before compiling the table - the number of substances to be determined in the table can thus be reduced.

The solution to the problem will therefore consist of the following steps:

preliminary discussion of individual reactions and external characteristics substances;

recording formulas and expected results of pairwise reactions in a table;

conducting an experiment in accordance with the table (in the case of an experimental task);

analysis of reaction results and correlating them with specific substances;

formulation of the answer to the problem.

It must be emphasized that a thought experiment and reality do not always completely coincide, since real reactions are carried out at certain concentrations, temperatures, lighting (for example, at electric light AgC1 and AgBr are identical). A thought experiment often leaves out many small details. For example, Br 2 /aq is perfectly decolorized with solutions of Na 2 CO 3, Na 2 SiO 3, CH 3 COON; the formation of Ag 3 PO 4 precipitate does not occur in a strongly acidic environment, since the acid itself does not give this reaction; glycerol forms a complex with Cu(OH) 2, but does not form with (CuONH) 2 SO 4, if there is no excess alkali, etc. The real situation does not always agree with the theoretical prediction, and in this chapter the “ideal” matrix tables and “realities” will sometimes be different. And in order to understand what is really happening, look for every opportunity to work with your hands experimentally in a lesson or elective (remember the safety requirements).

Example 1. Numbered flasks contain solutions of the following substances: silver nitrate, hydrochloric acid, silver sulfate, lead nitrate, ammonia and sodium hydroxide. Without using other reagents, determine which bottle contains the solution of which substance.

Solution. To solve the problem, we will compose a matrix table in which we will enter in the appropriate squares below the diagonal that intersects it the observation data of the results of merging substances from one test tube with another.

Observation of the results of sequentially pouring the contents of some numbered test tubes into all others:

1+ 2- a white precipitate forms;

1+ 3 - no visible changes are observed;

1+ 4 - depending on the order in which the solutions are drained, a precipitate may form;

1+ 5 - a brown precipitate forms;

1+ 3 - a white precipitate forms;

2+4 - no visible changes are observed;

1+ 5 - no visible changes are observed;

3+4 - cloudiness is observed;

3+5 - a white precipitate forms;

4+5 - no visible changes are observed.

Let us further write down the equations of the ongoing reactions in cases where changes are observed in the reaction system (emission of gas, sediment, color change) and enter the formula of the observed substance and the corresponding square of the matrix table above the diagonal that intersects it:

1+2: AgNO 3 + HCl → AgCl↓ +HNO 3 ;

1+5: 2AgNO 3 + 2NaOH → Ag 2 O↓ + 2NaNO 3 +H 2 O;

(2AgOH → 2NaNO3 + H2O)

2+3: 2HCl + Pb(NO 2) 2 → PbCl 2 ↓+2HNO 3 ;

3+4: Pb(NO 3) 2 ↓ + 2NH 4 NO 3 →Pb(OH) 2 ↓+2NH 4 NO 3;

cloudiness

3+5: Pb(NO 3) 2 +2NaOH →Pb(OH) 2 ↓+2NaNO 3;

(when lead nitrate is added to excess alkali, the precipitate can immediately dissolve).

Thus, based on five experiments, we distinguish the substances in the numbered test tubes.

Example 2. Eight numbered test tubes (from 1 to 8) without inscriptions contain dry substances: silver nitrate (1), aluminum chloride (2), sodium sulfide (3), barium chloride (4), potassium nitrate (5), potassium phosphate (6 ), as well as solutions of sulfuric (7) and hydrochloric (8) acids. How, without any additional reagents other than water, can you distinguish between these substances?

Solution. First of all, let's dissolve the solids in water and mark the test tubes where they ended up. Let's create a matrix table (as in the previous example), in which we will enter observation data on the results of merging substances from one test tube with another below and above the diagonal that intersects it. On the right side of the table we will introduce an additional column “general result of observation”, which we will fill in after completing all experiments and summing up the results of observations horizontally from left to right (see, for example, p. 178).

1+2: 3AgNO 3 + AlCl 3 → 3AgCl↓+Al(NO 3) 3;

1+3: 2AgNO 3 + Na 2 S → Ag 2 S ↓ + 2NaNO 2 ;

1+4: 2AgNO 3 + BaCl 2 → AgCl↓+ Ba(NO 3) 2;

1+6: 3AgNO 3 + K 3 PO 4 → Ag 3 PO 4 ↓+ 3KNO 3 ;

1+7: 2AgNO 3 + H 2 SO 4 → Ag 2 SO 4 ↓+2HNO 3 ;

1+8: AgNO 3 +HCl → AgCl↓+ HNO 3 ;

2+3: 2AlCl 3 + 3Na 2 S + 6H 2 O → 2Al(OH) 3 + 3H 2 S + 6NaCl;

(Na 2 S + H 2 O) → NaOH + NaHS, hydrolysis);

2+6: AlCl 3 + K 3 PO 4 →AlPO 4 ↓+3KCl;

3+7: Na 2 S +H 2 SO 4 → Na 2 SO 4 +H 2 S;

3+8: Na 2 S +2HCl→ 2NaCl+H 2 S;

4+6: 3BaCl 2 + 2K 3 PO 4 → Ba 3 (PO 4) 2 + 6KCl;

4+7: BaCl 2 + H 2 SO 4 → BaSO 4 + 2KCl.

Visible changes do not occur only with potassium nitrate.

Based on the number of times a precipitate forms and gas is released, all reagents are uniquely identified. In addition, BaCl 2 and KzP0 4 are distinguished by the color of the precipitate with AgNO 3: AgCl is white, and Ag 3 P04 is yellow. In this problem, the solution may be simpler - any of the acid solutions allows you to immediately isolate sodium sulfide, which determines silver nitrate and aluminum chloride. Silver nitrate is determined among the remaining three solids barium chloride and potassium phosphate; barium chloride distinguishes between hydrochloric and sulfuric acids.

Example 3. Four unlabeled test tubes contain benzene, chlorhexane, hexane, and hexene. Using the minimum quantities and number of reagents, propose a method for determining each of the specified substances.

Solution. The substances being determined do not react with each other; there is no point in compiling a table of pairwise reactions.

There are several methods for determining these substances, one of them is given below.

Only hexene immediately discolors bromine water:

C 6 H 12 + Br 2 = C 6 H 12 Br 2.

Chlorhexane can be distinguished from hexane by passing their combustion products through a solution of silver nitrate (in the case of chlorhexane, a white precipitate of silver chloride precipitates, insoluble in nitric acid, unlike silver carbonate):

2C 6 H 14 + 19O 2 = 12CO 2 + 14H 2 O;

C 6 H 13 Cl + 9O 2 = 6CO 2 + 6H 2 O + HCl;

HC1 + AgNO 3 = AgCl↓ + HNO 3.

Benzene differs from hexane in freezing in ice water(C 6 H has 6 melting point = +5.5°C, and C 6 H has 14 melting point = - 95.3°C).

Tasks

Equal volumes are poured into two identical beakers: one of water, the other of a dilute solution of sulfuric acid. How can you distinguish between these liquids without having any chemical reagents at hand (you cannot taste the solutions)?

Four test tubes contain powders of copper(II) oxide, iron(III) oxide, silver, and iron. How to recognize these substances using only one chemical reagent? Recognition by appearance is excluded.

Four numbered test tubes contain dry copper(II) oxide, carbon black, sodium chloride, and barium chloride. How, using a minimum amount of reagents, can you determine which test tube contains which substance? Justify your answer and confirm it with the equations of the corresponding chemical reactions.

Six unlabeled test tubes contain anhydrous compounds: phosphorus (V) oxide, sodium chloride, copper sulfate, aluminum chloride, aluminum sulfide, ammonium chloride. How can you determine the contents of each test tube if all you have is a set of empty test tubes, water, and a burner? Propose an analysis plan.

Four unmarked test tubes contain aqueous solutions of sodium hydroxide, hydrochloric acid, potash and aluminum sulfate. Suggest a way to determine the contents of each test tube without using additional reagents.

The numbered test tubes contain solutions of sodium hydroxide, sulfuric acid, sodium sulfate and phenolphthalein. How to distinguish between these solutions without using additional reagents?

Unlabeled jars contain the following individual substances; iron, zinc powders, calcium carbonate, potassium carbonate, sodium sulfate, sodium chloride, sodium nitrate, as well as solutions of sodium hydroxide and barium hydroxide. There are no other chemical reagents at your disposal, including water. Make a plan to determine the contents of each jar.

Four numbered jars without labels contain solid phosphorus(V) oxide (1), calcium oxide (2), lead nitrate (3), calcium chloride (4). Determine which of the jars contains each of the indicated compounds, if it is known that substances (1) and (2) react violently with water, and substances (3) and (4) dissolve in water, and the resulting solutions (1) and ( 3) can react with all other solutions to form precipitation.

Five test tubes without labels contain solutions of hydroxide, sulfide, chloride, sodium iodide and ammonia. How to determine these substances using one additional reagent? Give equations for chemical reactions.

How to recognize solutions of sodium chloride, ammonium chloride, barium hydroxide, sodium hydroxide in containers without labels, using only these solutions?

Eight numbered test tubes contain aqueous solutions hydrochloric acid, sodium hydroxide, sodium sulfate, sodium carbonate, ammonium chloride, lead nitrate, barium chloride, silver nitrate. Using indicator paper and carrying out any reactions between solutions in test tubes, determine what substance is contained in each of them.

Two test tubes contain solutions of sodium hydroxide and aluminum sulfate. How to distinguish them, if possible, without the use of additional substances, having only one empty test tube or even without it?

Five numbered test tubes contain solutions of potassium permanganate, sodium sulfide, bromine water, toluene and benzene. How can you distinguish between them using only the named reagents? Use their characteristic features to detect each of the five substances (indicate them); give a plan for the analysis. Write diagrams of the necessary reactions.

Six unnamed bottles contain glycerin, an aqueous solution of glucose, butyraldehyde (butanal), 1-hexene, an aqueous solution of sodium acetate and 1,2-dichloroethane. With only anhydrous sodium hydroxide and copper sulfate as additional chemicals, determine what is in each bottle.

Solving qualitative problems of identifying substances found in bottles without labels involves carrying out a number of operations, the results of which can be used to determine which substance is in a particular bottle.

The first stage of the solution is a thought experiment, which is a plan of action and its expected results. To record a thought experiment, a special table-matrix is ​​used, in which the formulas of the substances being determined are indicated horizontally and vertically. In places where the formulas of interacting substances intersect, the expected results of observations are recorded: - gas evolution, - precipitation, changes in color, odor or the absence of visible changes are indicated. If, according to the conditions of the problem, it is possible to use additional reagents, then it is better to write down the results of their use before compiling the table - the number of substances to be determined in the table can thus be reduced.
The solution to the problem will therefore consist of the following steps:
- preliminary discussion of individual reactions and external characteristics of substances;
- recording formulas and expected results of pairwise reactions in a table,
- conducting an experiment in accordance with the table (in the case of an experimental task);
- analysis of reaction results and correlating them with specific substances;
- formulation of the answer to the problem.

It must be emphasized that a thought experiment and reality do not always completely coincide, since real reactions take place at certain concentrations, temperatures, and lighting (for example, under electric light, AgCl and AgBr are identical). A thought experiment often leaves out many small details. For example, Br 2 /aq is perfectly decolorized with solutions of Na 2 CO 3, Na 2 SiO 3, CH 3 COONa; the formation of Ag 3 PO 4 precipitate does not occur in a strongly acidic environment, since the acid itself does not give this reaction; glycerol forms a complex with Cu (OH) 2, but does not form with (CuOH) 2 SO 4, if there is no excess alkali, etc. Real situation does not always agree with the theoretical forecast, and in this chapter the tables-matrices of the “ideal” and “reality” will sometimes differ. And in order to understand what is really happening, look for every opportunity to work with your hands experimentally in a lesson or elective (remember the safety requirements).

Example 1. The numbered bottles contain solutions of the following substances: silver nitrate, hydrochloric acid, silver sulfate, lead nitrate, ammonia and sodium hydroxide. Without using other reagents, determine which bottle contains the solution of which substance.

Solution. To solve the problem, we will compose a matrix table in which we will enter in the appropriate squares below the diagonal that intersects it the observation data of the results of merging substances from one test tube with another.

Observation of the results of sequentially pouring the contents of some numbered test tubes into all others:

1 + 2 - a white precipitate forms; ;
1 + 3 - no visible changes are observed;

1 + 4 - depending on the order in which the solutions are drained, a precipitate may form;
1 + 5 - a brown precipitate forms;
2+3 - a white precipitate forms;
2+4 - no visible changes are observed;
2+5 - no visible changes are observed;
3+4 - cloudiness is observed;
3+5 - a white precipitate forms;
4+5 - no visible changes are observed.

Let us further write down the equations of the ongoing reactions in cases where changes are observed in the reaction system (emission of gas, sediment, color change) and enter the formula of the observed substance and the corresponding square of the matrix table above the diagonal that intersects it:

(when lead nitrate is added to excess alkali, the precipitate can immediately dissolve).
Thus, based on five experiments, we distinguish the substances in the numbered test tubes.

Example 2. Eight numbered test tubes (from 1 to 8) without inscriptions contain dry substances: silver nitrate (1), aluminum chloride (2), sodium sulfide (3), barium chloride (4), potassium nitrate (5), phosphate potassium (6), as well as solutions of sulfuric (7) and hydrochloric (8) acids. How, without any additional reagents other than water, can you distinguish between these substances?

Solution. First of all, let's dissolve the solids in water and mark the test tubes where they ended up. Let's create a matrix table (as in the previous example), in which we will enter data from observations of the results of merging substances from one test tube with another below and above the diagonal that intersects it. On the right side of the table we will introduce an additional column “general result of observation”, which we will fill in after completing all experiments and summing up the results of observations horizontally from left to right (see, for example, p. 178).

Visible changes do not occur only with potassium nitrate.

Based on the number of times a precipitate forms and gas is released, all reagents are uniquely identified. In addition, BaCl 2 and K 3 PO 4 are distinguished by the color of the precipitate with AgNO 3: AgCl is white, and Ag 3 PO 4 is yellow. In this problem, the solution may be simpler - any of the acid solutions allows you to immediately isolate sodium sulfide, which determines silver nitrate and aluminum chloride. Among the remaining three solids, barium chloride and potassium phosphate are determined by silver nitrate; hydrochloric and sulfuric acids are distinguished by barium chloride.

Example 3. Four unlabeled test tubes contain benzene, chlorhexane, hexane and hexene. Using minimum quantities and the number of reagents, suggest a method for determining each of these substances.

Solution. The substances being determined do not react with each other; there is no point in compiling a table of pairwise reactions.
There are several methods for determining these substances, one of them is given below.
Only hexene immediately discolors bromine water:

C 6 H 12 + Br 2 = C 6 H 12 Br 2.

Chlorhexane can be distinguished from hexane by passing their combustion products through a solution of silver nitrate (in the case of chlorhexane, a white precipitate of silver chloride precipitates, insoluble in nitric acid, as opposed to silver carbonate):

2C 6 H 14 + 19O 2 = 12CO 2 + 14H 2 O;
C 6 H 13 Cl + 9O 2 = 6 CO 2 + 6 H 2 O + HC1;
HCl + AgNO 3 = AgCl + HNO 3.

Benzene differs from hexane in freezing in ice water (C 6 H has 6 melting point = +5.5 ° C, and C 6 H has 14 melting point = -95.3 ° C).

1. Equal volumes are poured into two identical beakers: one of water, the other of a dilute solution of sulfuric acid. How can you distinguish between these liquids without having any chemical reagents at hand (you cannot taste the solutions)?

2. Four test tubes contain powders of copper(II) oxide, iron(III) oxide, silver, and iron. How to recognize these substances using only one chemical reagent? Recognition by appearance excluded.

3. Four numbered test tubes contain dry copper(II) oxide, carbon black, sodium chloride, and barium chloride. How, using a minimum amount of reagents, can you determine which test tube contains which substance? Justify your answer and confirm it with the equations of the corresponding chemical reactions.

4. Six unlabeled test tubes contain anhydrous compounds: phosphorus(V) oxide, sodium chloride, copper sulfate, aluminum chloride, aluminum sulfide, ammonium chloride. How can you determine the contents of each test tube if all you have is a set of empty test tubes, water, and a burner? Propose an analysis plan.

5 . Four unmarked test tubes contain aqueous solutions of sodium hydroxide, hydrochloric acid, potash and aluminum sulfate. Suggest a way to determine the contents of each test tube without using additional reagents.

6 . The numbered test tubes contain solutions of sodium hydroxide, sulfuric acid, sodium sulfate and phenolphthalein. How to distinguish between these solutions without using additional reagents?

7. Unlabeled jars contain the following individual substances: powders of iron, zinc, calcium carbonate, potassium carbonate, sodium sulfate, sodium chloride, sodium nitrate, as well as solutions of sodium hydroxide and barium hydroxide. There are no other chemical reagents at your disposal, including water. Make a plan to determine the contents of each jar.

8 . Four numbered jars without labels contain solid phosphorus (V) oxide (1), calcium oxide (2), lead nitrate (3), calcium chloride (4). Determine which jar contains each from of the indicated compounds, if it is known that substances (1) and (2) react violently with water, and substances (3) and (4) dissolve in water, and the resulting solutions (1) and (3) can react with all other solutions with formation of precipitation.

9 . Five test tubes without labels contain solutions of hydroxide, sulfide, chloride, sodium iodide and ammonia. How to determine these substances using one additional reagent? Give equations for chemical reactions.

10. How to recognize solutions of sodium chloride, ammonium chloride, barium hydroxide, sodium hydroxide contained in vessels without labels, using only these solutions?

11. . Eight numbered test tubes contain aqueous solutions of hydrochloric acid, sodium hydroxide, sodium sulfate, sodium carbonate, ammonium chloride, lead nitrate, barium chloride, and silver nitrate. Using indicator paper and carrying out any reactions between solutions in test tubes, determine what substance is contained in each of them.

12. Two test tubes contain solutions of sodium hydroxide and aluminum sulfate. How to distinguish them, if possible, without using additional substances, having only one empty test tube or even without it?

13. Five numbered test tubes contain solutions of potassium permanganate, sodium sulfide, bromine water, toluene and benzene. How can you distinguish between them using only the named reagents? Use them to detect each of the five substances characteristic features(specify them); give a plan for the analysis. Write diagrams of the necessary reactions.

14. Six unnamed bottles contain glycerin, an aqueous solution of glucose, butyraldehyde (butanal), 1-hexene, an aqueous solution of sodium acetate and 1,2-dichloroethane. With only anhydrous sodium hydroxide and copper sulfate as additional chemicals, determine what is in each bottle.

1. To determine water and sulfuric acid, you can use the difference in physical properties: boiling and freezing points, density, electrical conductivity, refractive index, etc. The strongest difference will be in electrical conductivity.

2. Add hydrochloric acid to the powders in test tubes. Silver will not react. When iron dissolves, gas will be released: Fe + 2HCl = FeCl 2 + H 2
Iron (III) oxide and copper (II) oxide dissolve without releasing gas, forming yellow-brown and blue-green solutions: Fe 2 O 3 + 6HCl = 2FeCl 3 + 3H 2 O; CuO + 2HCl = CuCl 2 + H 2 O.

3. CuO and C are black, NaCl and BaBr 2 are white. The only reagent may be, for example, diluted sulfuric acid H2SO4:

CuO + H 2 SO 4 = CuSO 4 + H 2 O (blue solution); BaCl 2 + H 2 SO 4 = BaSO 4 + 2HCl (white precipitate).
Dilute sulfuric acid does not react with soot and NaCl.

4 . Not large number We place each of the substances in water:

Two substances - sodium chloride and ammonium chloride - dissolve without reacting with water; they can be distinguished by heating the dry salts (ammonium chloride sublimes without residue): NH 4 Cl NH 3 + HCl; or by the color of the flame with solutions of these salts (sodium compounds color the flame yellow).

5. Let's make a table of pairwise interactions of the indicated reagents

3K 2 CO 3 + Al 2 (SO 4) 3 + 3H 2 O = 2 Al (OH) 3 + 3CO 2 + 3K 2 SO 4 ;

Based on the table presented, all substances can be determined by the number of precipitation and gas evolution.

6. All solutions are mixed in pairs. A pair of solutions that gives a raspberry color is NaOH and phenolphthalein. The raspberry solution is added to the two remaining test tubes. Where the color disappears is sulfuric acid, in the other is sodium sulfate. It remains to distinguish between NaOH and phenolphthalein (test tubes 1 and 2).
A. From test tube 1, add a drop of solution to a large amount of solution 2.
B. From test tube 2, a drop of solution is added to a large amount of solution 1. In both cases, the color is crimson.
Add 2 drops of sulfuric acid solution to solutions A and B. Where the color disappears, a drop of NaOH was contained. (If the color disappears in solution A, then NaOH - in test tube 1).

8 . React violently with water: P 2 O 5 and CaO with the formation of H 3 PO 4 and Ca(OH) 2, respectively:

P 2 O 5 + 3H 2 O = 2H 3 PO 4, CaO + H 2 O = Ca(OH) 2.
Substances (3) and (4) - Pb(NO 3) 2 and CaCl 2 - dissolve in water. Solutions can react with each other as follows:

Thus, solution 1 (H 3 PO 4) forms precipitates with all other solutions upon interaction. Solution 3 - Pb(NO 3) 2 also forms precipitates with all other solutions. Substances: I -P 2 O 5, II -CaO, III -Pb(NO 3) 2, IV-CaCl 2.
IN general case the precipitation of most precipitation will depend on the order in which the solutions are drained and the excess of one of them (in a large excess of H 3 PO 4, lead and calcium phosphates are soluble).

9. The problem has several solutions, two of which are given below.
A. Add a solution of copper sulfate to all test tubes:
2NaOH + CuSO 4 = Na 2 SO 4 + Cu(OH) 2 (blue precipitate);
Na 2 S + CuSO 4 = Na 2 SO 4 + CuS (black precipitate);
NaCl + CuSO 4 (no changes in a dilute solution);
4NaI+2CuSO 4 = 2Na 2 SO 4 + 2CuI+I 2 (brown precipitate);
4NH 3 + CuSO 4 = Cu(NH 3) 4 SO 4 (blue solution or blue precipitate, soluble in excess ammonia solution).

b. Add silver nitrate solution to all test tubes:
2NaOH + 2AgNO 3 = 2NaNO 3 + H 2 O + Ag 2 O (brown precipitate);
Na 2 S + 2AgNO 3 = 2NaNO 3 + Ag 2 S (black precipitate);
NaCl + AgNO 3 = NaN0 3 + AgCl (white precipitate);
NaI + AgNO 3 = NaNO 3 + AgI (yellow precipitate);
2NH 3 + 2AgNO 3 + H 2 O = 2NH 4 NO 3 + Ag 2 O (brown precipitate).
Ag 2 O dissolves in excess ammonia solution: Ag 2 0 + 4NH 3 + H 2 O = 2OH.

10 . To recognize these substances, all solutions should be reacted with each other:

NaOH and Ba(OH) 2 can be distinguished by their different flame colors (Na+ is yellow, and Ba 2+ is green).

11. Determine the acidity of solutions using indicator paper:
1) acidic environment -HCl, NH 4 C1, Pb(NO 3) 2;
2) neutral medium - Na 2 SO 4, BaCl 2, AgNO 3;
3) alkaline environment - Na 2 CO 3, NaOH. Let's make a table.

Work in chemical laboratory is inextricably linked with the use of various reagents, so each laboratory necessarily has a certain supply of them.

According to their purpose, reagents can be divided into two main groups: commonly used and special.

Commonly used reagents are available in any laboratory, and they include a relatively small group of chemicals: acids (hydrochloric, nitric and sulfuric), alkalis (ammonia solution, caustic soda and potassium), calcium and barium oxides, a number of salts, mainly inorganic, indicators ( phenolphthalein, methyl orange, etc.).

Special reagents are used only for certain jobs.

Based on their purity, reagents are divided into chemically pure (chemically pure), analytically pure (analytical grade), and pure (analytical grade).

In addition, there are reagents of the following standards: technical (technical), purified (pure), special purity (special purity), highest purification (high purity) and spectrally pure (sp. purity).

For reagents in each of these categories, a certain permissible content of impurities is established.

The most commonly used reagents, the consumption of which can be significant, especially in large enterprises, are purchased in large packages, in cans or bottles, sometimes containing several kilograms of the substance.

Rarely used and rare reagents usually have small packaging from 10 to 1 g and even smaller.

The most expensive and rare reagents are usually stored separately.

Those working in the laboratory must know the basic properties of the reagents they use, especially the degree of their toxicity and the ability to form explosive and flammable mixtures with other reagents.

In order to save reagents (especially the most valuable ones), solutions must be prepared in such quantities as are necessary for work. Preparing an excess solution is a waste of reagents. A solution left unused usually spoils; in addition, bottles containing unnecessary solutions clutter the laboratory .

When stored in jars, solid reagents can form dense lumps that are difficult to remove. Therefore, before taking a solid reagent from a jar, you need (with the cap closed) to shake the bike, hitting it, for example, with your palm on the side. If the caked reagent does not crumble, then, after opening the cork, loosen the top layer using a clean horn or porcelain spatula or glass rod. It is not recommended to use a metal spatula for this purpose.

Before taking the reagent from the jar, you need to inspect its neck and remove from it everything that could get into the poured substance and contaminate it (dust, paraffin, all kinds of putties, etc.). It is very convenient to take reagents from a jar using a porcelain spoon, a porcelain spatula, or pour them through a funnel for powders (Fig. 1). A funnel is inserted into the neck of the jar into which this or that substance is poured; The same funnel can be used when pouring very thick, viscous liquids.

Funnels for powders come in several sizes, with a wide part diameter from 50 to 200 mm and an end diameter from 20 to 38 mm and a height from 55 to 180 mm.

A reagent that spills onto the table (which inevitably becomes contaminated) cannot be poured back into the same jar where it is stored. Concern for maintaining the purity of reagents is the most important rule when working with them.

If there is very little reagent left in the jar, the remainder should be poured into smaller containers - this will free up space in the cabinet and reduce losses when taking the reagent.

It is necessary to ensure that all jars with reagents must have labels with the designation, what is in the jar, or inscriptions made with a wax pencil for glass. The place where the inscription will be should be slightly warmed with at least the palm of your hand. A wax pencil writes easier on a heated area and the inscription is more noticeable. If there is no label or inscription on the jar of reagent, such reagent cannot be used. IN such a case You need to establish exactly what is in the bank, as mistakes can lead to serious consequences.

Particular care must be taken when handling toxic substances(see Chapter 18 “Working with harmful and toxic substances”).

Before pouring the reagent into the jar, it must be thoroughly washed and dried, having first selected a stopper for it. You cannot pour the reagent into undried jars.

When weighing dry reagents, you should not pour them directly onto the scale pan, as this may damage the scales (see Chapter 5 “Balances and Weighing”).

When storing hygroscopic substances or those that can change upon contact with air, the jars must be sealed; for this purpose, their stoppers are filled with paraffin, Mendeleev's putty or sealing wax.

Special care is required when handling reagents stored in large glass containers, as these containers are very easy to break. If the glass stoppers “stick”, the bottle with the reagent is opened using one of the methods indicated in Chapter. 3 “Traffics and handling them.”

Rice. 1. Funnel

Figure 2. Liquid siphoning

Some reagents are sold and stored in sealed ampoules different sizes. Such an ampoule is opened as follows. At a distance of 1 cm from the end of the drawn part of the ampoule, very carefully make a scratch with a file or a special knife. It is useful to pre-moisten the incision site with water. When the incision is made, wipe the pulled end of the ampoule with clean cotton wool, holding the ampoule in your left hand so that its opened end is directed away from the worker and from neighbors, right hand break off the cut part with a quick jerk. If the drawn end has relatively thick walls, the scratch must be touched with the red-hot end of a drawn glass rod or with a red-hot iron wire.

When the ampoule contains liquid, you need to be especially careful when opening it; When breaking off the tip, the ampoule should not be turned over or tilted too much. If, after taking the reagent, part of it remains in the ampoule, the latter must be sealed again on a soldering torch.

It can be very difficult to pour a little liquid (0.5-1 cm3) from the ampoule. To achieve this, you can use a siphon made of a thinly drawn glass capillary with a diameter of about 0.2-0.25 mm. The tip of the ampoule is cut off and a capillary is immersed in it, as shown in Fig. 2. The liquid, rising through the capillary tube, flows out of it in drops.

The ampoules should be handled very carefully; They are best stored in cardboard boxes, wrapped in corrugated cardboard or lined with something soft.

Reagents that change under the influence of light are stored in yellow or dark bottles, sometimes inserted into a cardboard box.

Reagents that cannot be stored in glass containers are placed in containers made of materials that are resistant to the action of this reagent. For example, a solution of hydrofluoric acid is stored in vessels made of pure paraffin, ceresin, ebonite or polyethylene. However, paraffin and ceresin bottles have a number of disadvantages: they are not heat-resistant, have low mechanical strength, are fragile in the cold, are opaque, etc. Polyethylene bottles are mainly used for storing hydrofluoric acid.

Sometimes covered with paraffin inner surface glass bottles and flasks. Thus, perhydrol (30% hydrogen peroxide solution) and alkali solutions are best stored in such bottles.

Some reagents change or even decompose during prolonged storage, for example, aniline turns yellow during storage. Such reagents should be purified before use either by distillation or filtration through adsorbents ( activated carbon, silica gel, bleaching earths, etc.), or others. techniques, depending on the properties of the substance.

Some reagents have the ability to spontaneously ignite, these include white or yellow phosphorus, pyrophoric metals, organometallic compounds (for example, aluminum ethoxide), flammable reagents that require storage special conditions, include ethers (diethyl, amyl, etc.), alcohols (methyl, ethyl, butyl, etc.), hydrocarbons [(gasoline, gasoline, petroleum ether, kerosene, etc.), aromatic compounds (benzene, toluene, xylene), carbon disulfide, acetone, etc.

Do not store together reagents that can ignite or generate large amounts of heat upon interaction. For example, the metals sodium, potassium and lithium, as well as sodium peroxide and white phosphorus cannot be stored with flammable substances; metal sodium, potassium, lithium and calcium, as well as phosphorus - with elemental bromine and iodine.

Rice. 3. Riser for bottles, metal

Bertholet salt, potassium permanganate, sodium peroxide, hydrogen peroxide, concentrated perchloric acid and other oxidizing agents cannot be stored together with reducing agents - coal, sulfur, starch, phosphorus, etc.

Self-flammable and flammable substances should only be stored in appropriate containers.

It is absolutely unacceptable to mix and grind berthol's salt, potassium permanganate, sodium peroxide and other oxidizing agents with organic substances. Perchloric acid should be handled very carefully, as its vapors explode on contact with organic substances and easily oxidized compounds, for example, trivalent antimony salts, etc. Salts perchloric acid are also capable of exploding, sometimes even for no apparent reason. All these substances require special storage conditions. There should not be any in the laboratory large stock such substances.

Rice. 4. Device for tilting large bottles.

Rice. 5. Wooden riser for large bottles

Silver and copper acetylenides and azides are also explosive. heavy metals, salts of explosive acid, some nigro compounds, etc.

Stoppers from bottles containing different reagents should not be confused to avoid contamination of the latter.

When pouring liquids from large bottles, it is possible, especially if handled carelessly, that the liquid may spill and get on your clothes and hands. Therefore, in a laboratory or warehouse it is necessary to have special metal risers (Fig. 3), which make it possible to easily tilt the bottles. For tilting bottles with a capacity of 5-15 liters, the riser shown in Fig. 4.

A wooden device (Fig. 5) for bottles with a capacity of 20 liters or more can be made in any carpentry workshop. To transfer liquids, it is convenient to use nozzles (Fig. 6) on the neck of large bottles. Siphons are used for the same purpose.

When pouring liquids, be sure to use funnels.

The following must be remembered about reagents* and their handling:

1. Reagents must be protected from contamination.

2. Reagents should be used sparingly.

3. All bottles of reagents should always have labels indicating the name of the reagent and its degree of purity.

4. Reagents that change under the influence of light should be stored only in yellow or dark bottles.

5. Particular care should be taken when handling toxic, flammable or harmful substances, With concentrated acids and alkalis.

6. Work with flammable reagents away from fire and heating devices.