24.11.202227.05.2017

Chromium and its compounds are actively used in industrial production, in particular, in metallurgy, chemical and refractory industries.

Chromium Cr is a chemical element of Group VI of the Mendeleev periodic system, atomic number 24, atomic mass 51.996, atomic radius 0.0125, Cr2+ ion radii - 0.0084; Cr3+ - 0.0064; Cr4+ - 6.0056.

Chromium exhibits oxidation states +2, +3, +6, respectively, has valencies II, III, VI.

Chromium is a hard, ductile, rather heavy, malleable steel-gray metal.

Boils at 2469 0 C, melts at 1878 ± 22 0 C. It has all the characteristic properties of metals - it conducts heat well, almost does not resist electric current, and has a luster inherent in most metals. And at the same time, it is resistant to corrosion in air and in water.

Impurities of oxygen, nitrogen and carbon, even in the smallest quantities, dramatically change the physical properties of chromium, for example, making it very brittle. But, unfortunately, it is very difficult to obtain chromium without these impurities.

The structure of the crystal lattice is body-centered cubic. A feature of chromium is a sharp change in its physical properties at a temperature of about 37°C.

6. Types of chromium compounds.

Chromium oxide (II) CrO (basic) is a strong reducing agent, extremely unstable in the presence of moisture and oxygen. Has no practical value.

Chromium oxide (III) Cr2O3 (amphoteric) is stable in air and in solutions.

Cr2O3 + H2SO4 = Cr2(SO4)3 + H2O

Cr2O3 + 2NaOH = Na2CrO4 + H2O

It is formed by heating some chromium (VI) compounds, for example:

4CrO3 2Cr2O3 + 3O2

(NH4)2Cr2O7 Cr2O3 + N2 + 4H2O

4Cr + 3O2 2Cr2O3

Chromium(III) oxide is used to reduce low purity chromium metal with aluminum (aluminothermy) or silicon (silicothermy):

Cr2O3 +2Al = Al2O3 +2Cr

2Cr2O3 + 3Si = 3SiO3 + 4Cr

Chromium oxide (VI) CrO3 (acidic) - dark crimson needle-like crystals.

Obtained by the action of an excess of concentrated H2SO4 on a saturated aqueous solution of potassium bichromate:

K2Cr2O7 + 2H2SO4 = 2CrO3 + 2KHSO4 + H2O

Chromium oxide (VI) is a strong oxidizing agent, one of the most toxic chromium compounds.

When CrO3 is dissolved in water, chromic acid H2CrO4 is formed

CrO3 + H2O = H2CrO4

Acid chromium oxide, reacting with alkalis, forms yellow chromates CrO42

CrO3 + 2KOH = K2CrO4 + H2O

2. Hydroxides

Chromium (III) hydroxide has amphoteric properties, dissolving both in

acids (behaves like a base), and in alkalis (behaves like an acid):

2Cr(OH)3 + 3H2SO4 = Cr2(SO4)3 + 6H2O

Cr(OH)3 + KOH = K

When calcining chromium (III) hydroxide, chromium (III) oxide Cr2O3 is formed.

Insoluble in water.

2Cr(OH)3 = Cr2O3 + 3H2O

3. Acids

Chromium acids corresponding to its +6 oxidation state and differing in the ratio of the number of CrO3 and H2O molecules exist only in the form of solutions. When the acid oxide CrO3 is dissolved, monochromic acid (simply chromic) H2CrO4 is formed.

CrO3 + H2O = H2CrO4

Acidification of a solution or an increase in CrO3 in it leads to acids of the general formula nCrO3 H2O

at n=2, 3, 4, these are, respectively, di, tri, tetrachromic acids.

The strongest of them is dichromic, that is, H2Cr2O7. Chromic acids and their salts are strong oxidizers and poisonous.

There are two types of salts: chromites and chromates.

Chromites with the general formula RCrO2 are salts of chromic acid HCrO2.

Cr(OH)3 + NaOH = NaCrO2 + 2H2O

Chromites vary in color from dark brown to completely black and are usually found in solid masses. Chromite is softer than many other minerals, the melting point of chromite depends on its composition 1545-1730 0 C.

Chromite has a metallic luster and is almost insoluble in acids.

Chromates are salts of chromic acids.

Salts of monochromic acid H2CrO4 are called monochromates (chromates) R2CrO4, salts of dichromic acid H2Cr2O7 dichromates (bichromates) - R2Cr2O7. Monochromats are usually colored yellow. They are stable only in an alkaline environment, and upon acidification they turn into orange-red dichromates:

2Na2CrO4 + H2SO4 = Na2Cr2O7 + Na2SO4 + H2O

3.2.1; 3.3.1; 3.7.1; 3.8.1

3.2.1, 3.3.1; 3.4; 3.5

5. The limitation of the validity period was removed according to protocol N 3-93 of the Interstate Council for Standardization, Metrology and Certification (IUS 5-6-93)

6. REPUBLICATION (November 1998) with Amendments No. 1, 2, approved in March 1984, December 1988 (IUS 7-84, 3-89)

This standard applies to chromium (VI) oxide (chromic anhydride), which is a dark brown-red needle or prismatic crystals; soluble in water, hygroscopic.

Formula: CrO.

Molecular weight (according to international atomic masses 1971) - 99.99.

1. TECHNICAL REQUIREMENTS

1.1. Chromium oxide (VI) must be manufactured in accordance with the requirements of this standard according to the technological regulations approved in the prescribed manner.

(Changed edition, Rev. N 2).

1.2. According to chemical indicators, chromium oxide (VI) must comply with the standards specified in table.1.

Table 1


Name of indicator
	Clean for analysis (p.a.) OKP 26 1121 1062 08	Pure (h) OKP 26 1121 1061 09
1. Mass fraction of chromium oxide (VI) (СrО), %, not less than
2. Mass fraction of substances insoluble in water,%, no more
3. Mass fraction of nitrates (NO),%, no more		Not standardized
4. Mass fraction of sulfates (SO),%, no more
5. Mass fraction of chlorides (Сl), % , no more
6. Mass fraction of the sum of aluminum, barium, iron and calcium (Al + Ba + Fe + Ca),% , no more
7. Mass fraction of the sum of potassium and sodium (K ± Na),%, no more

2. ACCEPTANCE RULES

2.1. Acceptance rules - according to GOST 3885.

2.2. The determination of the mass fraction of nitrates and the amount of aluminum, barium, iron and calcium is carried out by the manufacturer in every 10th batch.

(Introduced additionally, Rev. N 2).

3. METHODS OF ANALYSIS

3.1a. General instructions for the analysis - according to GOST 27025.

When weighing, laboratory scales are used according to GOST 24104 * 2nd accuracy class with the largest weighing limit of 200 g and 3rd accuracy class with the largest weighing limit of 500 g or 1 kg or 4th accuracy class with the largest weighing limit of 200 g.
_______________
* Valid GOST 24104-2001. - Note "CODE".

It is allowed to use imported utensils according to the accuracy class and reagents in quality not lower than domestic ones.

3.1. Samples are taken according to GOST 3885.

The mass of the average sample must be at least 150 g.

3.2. Determination of the mass fraction of chromium oxide (VI)

3.1a-3.2. (Changed edition, Rev. N 2).

3.2.1. Reagents, solutions and glassware

Distilled water according to GOST 6709.

Potassium iodide according to GOST 4232, solution with a mass fraction of 30%, freshly prepared.

Hydrochloric acid according to GOST 3118.

Soluble starch according to GOST 10163, solution with a mass fraction of 0.5%.

GOST 27068, concentration solution (NaSO 5HO) = 0.1 mol / dm (0.1 N); prepared according to GOST 25794.2.

Burette with a capacity of 50 ml with a division value of 0.1 cm.

Flask Kn-1-500-29/32 THS according to GOST 25336.

Flask 2-500-2 according to GOST 1770.

Pipettes with a capacity of 2, 10 and 25 ml.

Stopwatch.

Cylinder 1(3)-100 according to GOST 1770.

(Changed edition, Rev. N 1,

3.2.2. Conducting an analysis

About 2.5000 g of the drug is placed in a volumetric flask, dissolved in a small amount of water, the volume of the solution is adjusted to the mark with water and mixed thoroughly.

25 ml of the resulting solution is transferred into a conical flask, 100 ml of water, 5 ml of hydrochloric acid, 10 ml of potassium iodide solution are added, mixed and left in the dark for 10 minutes. Then the stopper is washed off with water, 100 ml of water are added and the released iodine is titrated with a solution of 5-aqueous sodium sulphate, adding 1 ml of starch solution at the end of the titration, until a green color is obtained.

(Changed edition, Rev. N 2).

3.2.3. Results processing

The mass fraction of chromium oxide () as a percentage is calculated by the formula

where is the volume of a solution of 5-aqueous sodium sulphate of concentration exactly (NaSO 5HO) = 0.1 mol / dm (0.1 N) used for titration, cm;

Sample weight, g;

0.003333 - mass of chromium oxide (VI), corresponding to 1 cm3 of a solution of 5-aqueous sodium sulphate concentration exactly (NaSO 5HO) = 0.1 mol / dm (0.1 N), g.

At the same time, a control experiment is carried out with the same amounts of solutions of potassium iodide and hydrochloric acid and, if necessary, an appropriate correction is made to the result of the determination.

The result of the analysis is taken as the arithmetic mean of the results of two parallel determinations, the absolute discrepancy between which does not exceed the allowable discrepancy equal to 0.3%.

Permissible absolute total error of the analysis result is ±0.5% at a confidence level =0.95.

(Revised edition, from

m. N 1, 2).

3.3. Determination of the mass fraction of water-insoluble substances

3.3.1. Reagents and glassware

Distilled water according to GOST 6709.

Filtering crucible according to GOST 25336 type TF POR 10 or TF POR 16.

Glass V-1-250 THS according to GOST 25336.

Cylinder 1(3)-250 according to GOST 1770.

3.3.2. Conducting an analysis

30.00 g of the drug is placed in a glass and dissolved in 100 cm3 of water. The beaker is covered with a watch glass and incubated for 1 hour in a water bath. Then the solution is filtered through a filter crucible, previously dried to constant weight and weighed. The result of weighing the crucible in grams is recorded to the fourth decimal place. The residue on the filter is washed with 150 cm3 of hot water and dried in an oven at 105–110°C to constant weight.

The preparation is considered to comply with the requirements of this standard if the mass of the residue after drying does not exceed:

for the drug pure for analysis - 1 mg,

for the drug pure - 3 mg.

Permissible relative total error of the analysis result for the analytical preparation. ± 35%, for the preparation h. ± 20% with a confidence level = 0.95.

3.3.1, 3.3.2. (Changed edition, Rev. N 2).

3.4. Determination of the mass fraction of nitrates

The determination is carried out according to GOST 10671.2. At the same time, 1.50 g of the drug is placed in a flask Kn-2-100-34 (50) THS (GOST 25336), 100 cm3 of water are added, stirred until dissolved, 1.5 cm3 of concentrated sulfuric acid is added, carefully drop by drop with stirring 2 cm of ethyl alcohol rectified technical premium (GOST 18300) and heated in a boiling water bath for 15 minutes.

20 cm3 of water are added to the hot solution, and then, with stirring, about 14 cm3 of an ammonia solution with a mass fraction of 10% (GOST 3760) until the chromium is completely precipitated.

The contents of the flask are slowly heated to boiling and boiled for 10 minutes, to avoid ejection, pieces of unglazed porcelain and a glass rod are placed in the flask. Then the liquid is filtered through an ashless "blue tape" filter using a laboratory funnel with a diameter of 75 mm (GOST 25336) (the filter is pre-washed 4-5 times with hot water), the filtrate is collected in a 100 cm3 conical flask with a 60 cm mark. washed three times with hot water, collecting the washings in the same flask. The resulting solution is heated to a boil, boiled for 15 minutes, cooled, the volume of the solution is adjusted to the mark with water and stirred.

The solution is kept for the determination of chlorides according to clause 3.6.

5 cm3 of the resulting solution (corresponding to 0.125 g of the drug) are placed in a 50 cm3 conical flask, 5 cm3 of water are added, and then the determination is carried out by the method using indigo carmine.

The preparation is considered to comply with the requirements of this standard if the color of the analyzed solution observed after 5 minutes is not weaker than the color of the solution prepared at the same time and containing in the same volume:

for the drug pure for analysis 0.005 mg NO,

1 ml of sodium chloride solution, 1 ml of indigo carmine solution and 12 ml of concentrated sulfuric

acids.

3.5. Determination of the mass fraction of sulfates

The determination is carried out according to GOST 10671.5.

At the same time, 0.50 g of the drug is placed in a glass with a capacity of 50 cm3 and dissolved in 5 cm3 of water. The solution is transferred into a separating funnel with a capacity of 50 ml (GOST 25336), 5 ml of concentrated hydrochloric acid, 10 ml of tributyl phosphate are added and shaken.

After separation of the mixture, the aqueous layer is transferred to another identical separating funnel and, if necessary, the treatment of the aqueous layer with 5 ml of tributyl phosphate is repeated. The aqueous layer is separated into a separating funnel and washed with 5 ml of ether for anesthesia. After separation, the aqueous solution is transferred to an evaporating dish (GOST 9147), placed in an electric water bath, and the solution is evaporated to dryness.

The residue is dissolved in 10 cm3 of water, quantitatively transferred into a 50 cm3 conical flask (with a mark of 25 cm3), the volume of the solution is adjusted to the mark with water, mixed, and then the determination is carried out by the visual nephelometric method.

The drug is considered to comply with the requirements of this standard if the observed opalescence of the analyzed solution is not more intense than the opalescence of a solution prepared simultaneously with the analyzed one and containing in the same volume:

for the drug pure for analysis - 0.02 mg SO,

for pure preparation - 0.05 mg SO,

1 cm solution of hydrochloric acid with a mass fraction of 10%, 3 cm solution of starch and 3 cm solution of chloride

go barium.

3.6. Determination of the mass fraction of chlorides

The determination is carried out according to GOST 10671.7. In this case, 40 cm3 of the solution obtained according to clause 3.4. (corresponding to 1 g of the drug), placed in a conical flask with a capacity of 100 cm 3 and, if the solution is cloudy, add 0.15 cm optical density of solutions in cuvettes with a thickness of the light-absorbing layer of 100 mm) or by the visual nephelometric method.

The preparation is considered to comply with the requirements of this standard if the mass of chlorides does not exceed:

for the drug pure for analysis - 0.01 mg,

for the drug pure - 0.02 mg.

At the same time, under the same conditions, a control experiment is carried out to determine the mass fraction of chlorides in the amounts of alcohol and ammonia solution used for analysis, and if they are detected, the analysis results are corrected.

In case of disagreement in the assessment of the mass fraction of chlorides, the determination is carried out by the phototurbidimetric method.

3.4-3.6. (Changed edition, Rev. N 1, 2).

3.7. Determination of the mass fraction of aluminum, barium, iron and calcium

3.7.1. Equipment, reagents and solutions

ISP-30 spectrograph with a three-lens slit illumination system and a three-stage attenuator.

AC arc generator type DG-1 or DG-2.

Rectifier silicon type VAZ-275/100.

Microphotometer type MF-2 or MF-4.

Muffle furnace.

Stopwatch.

Spectroprojector type PS-18.

Organic glass mortars and agate.

Porcelain crucible according to GOST 9147.

Torsion scales VT-500 with a division value of 1 mg or others with a similar accuracy.

Coals graphitized for spectral analysis grade os.ch. 7-3 (carbon electrodes) with a diameter of 6 mm; the upper electrode is sharpened into a cone, the lower one has a cylindrical channel with a diameter of 3 mm and a depth of 4 mm.

Graphite powder, special purity grade, according to GOST 23463.

Spectral photographic plates of the SP-I type with a light sensitivity of 3-5 units. for aluminum, barium and calcium and spectral type SP-III, photosensitivity 5-10 units. for iron.

Ammonium dichromate according to GOST 3763.

Chromium (III) oxide obtained from chromium (VI) oxide according to this standard or ammonium dichromate, with a minimum content of detectable impurities, the determination of which is carried out by the method of additions under the conditions of this method; in the presence of impurities, they are taken into account when constructing a calibration curve.

Aluminum oxide for spectral analysis, chemically pure

Barium oxide grade os.h. 10-1.

Iron (III) oxide, special purity grades 2-4.

Calcium oxide, grade os.h. 6-2.

Ammonium chloride according to GOST 3773.

Distilled water according to GOST 6709.

Hydroquinone (paradioxybenzene) according to GOST 19627.

Potassium bromide according to GOST 4160.

Metol (4-methylaminophenol sulfate) according to GOST 25664.

Sodium sulfite 7-aqueous.

Sodium sulphate (sodium thiosulfate) 5-water according to GOST 27068.

Sodium carbonate according to GOST 83.

Sodium carbonate 10-water according to GOST 84.

Metol hydroquinone developer; prepare as follows: solution A-2 g of metol, 10 g of hydroquinone and 104 g of 7-aqueous sodium sulfite are dissolved in water, the volume of the solution is adjusted to 1 dm with water, stirred and, if the solution is cloudy, it is filtered; solution B-16 g of sodium carbonate (or 40 g of 10-aqueous sodium carbonate) and 2 g of potassium bromide are dissolved in water, the volume of the solution is adjusted to 1 dm with water, stirred and, if the solution is cloudy, it is filtered, then solutions A and B are mixed in equal volumes.

Fast fixer; prepared as follows: 500 g of 5-aqueous sodium sulphate and 100 g of ammonium chloride are dissolved in water, the volume of the solution is adjusted to 2 dm, stirred, and if the solution is cloudy, it is filtered.

Rectified technical ethyl alcohol in accordance with GOST 18300 of the highest grade.

(Changed edition, Rev. N 1, 2).

3.7.2. Preparation for analysis

3.7.2.1. Sample preparation

0.200 g of the drug is placed in a porcelain crucible, dried on an electric stove and calcined in a muffle furnace at 900 °C for 1 hour.

The resulting oxide of chromium (III) is ground in an agate mortar with powdered graphite in a ratio of 1:2.

3.7.2.2. Preparation of samples for building a calibration curve

Samples are prepared on the basis of chromium (III) oxide obtained from chromium (VI) oxide with a minimum content of detectable impurities. To obtain the base, a sample of chromium(VI) oxide is placed in a porcelain crucible, dried on an electric stove, and calcined in a muffle furnace at 900°C for 1 h (it is allowed to prepare samples based on chromium(III) oxide obtained from ammonium dichromate).

The head sample with a mass fraction of each impurity of 0.32% is prepared by grinding 0.0458 g of iron oxide (III), 0.0605 g of aluminum oxide, 0.0448 g of calcium oxide, 0.0357 g of barium oxide and 9.8132 g of chromium oxide (III) in a mortar made of organic glass or agate with 5 cm3 of ethyl alcohol for 1 hour, then dried under an infrared lamp or in an oven, and the mixture is triturated for 30 minutes.

By mixing the appropriate amounts of the head sample or the previous ones with the base, samples with a lower mass fraction of impurities indicated in Table 2 are obtained.

table 2


Sample number	Mass fraction of each impurity (Al, Ba, Fe, Ca) in samples in terms of metal, %

Each sample is mixed with powdered graphite in a ratio of 1:2.

3.7.2.1, 3.7.2.2. (Changed edition, Rev. N 2).

3.7.3. Conducting an analysis

The analysis is carried out in a DC arc under the conditions indicated below.


Current strength, A
Slot width, mm
Diaphragm height on the middle lens of the condenser system, mm
exposure, with

Before taking spectrograms, the electrodes are fired in a DC arc at a current strength of 10–12 A for 30 s.

After firing the electrodes, the analyzed sample or sample is introduced into the channel of the lower electrode (anode) to build a calibration graph. The sample weight is determined by the volume of the channel. The arc is ignited and the spectrogram is taken. The spectra of the analyzed sample and samples are taken on one photographic plate at least three times, each time placing a new pair of electrodes. The slot is opened before the arc is ignited.

The photographic plate with the taken spectra is developed, fixed, washed in running water and dried in air.

3.7.4. Results processing

Photometry of analytical spectral lines of the determined impurities and comparison lines is carried out using a logarithmic scale.


Analytical line impurities, nm		comparison line

	VA-233.527

		Cr-391.182 nm

For each analytical pair, the blackening difference () is calculated

where is the blackening of the impurity line;

- blackening of the comparison line or background.

Three values of the blackening difference determine the arithmetic mean value () for each element to be determined in the analyzed sample and the sample for constructing a calibration graph.

According to the values of the samples for constructing calibration graphs, a calibration graph is built for each element to be determined, plotting the concentration logarithms on the abscissa axis, and the arithmetic mean values of the blackening difference on the ordinate axis.

The mass fraction of each impurity is determined from the graph and the result is multiplied by 0.76.

The result of the analysis is taken as the arithmetic mean of the results of three parallel determinations, the relative discrepancy between the most different values of which does not exceed the allowable discrepancy of 50%.

Permissible relative total error of the analysis result is ±20% at a confidence level =0.95.

(Changed edition, Rev. N 2).

3.8. Determination of the mass fraction of the sum of sodium and potassium

3.8.1. Instruments, reagents, solutions and glassware

A flame photometer or a spectrophotometer based on the ISP-51 spectrograph with an FEP-1 attachment, with an appropriate photomultiplier, or a Saturn spectrophotometer. Other instruments providing similar sensitivity and accuracy may be used.

Propane-butane.

Compressed air for power supply of instrumentation.

Burner.

Spray.

Distilled water according to GOST 6709, secondarily distilled in a quartz distiller, or demineralized water.

Solutions containing Na and K; prepared according to GOST 4212, by appropriate dilution and mixing, a solution is obtained with a concentration of Na and K of 0.1 mg / cm - solution A.

Chromium (VI) oxide according to this standard, analytical grade, with the content of Na and K determined by the addition method (solution with a mass fraction of 10%) - solution B.

3.8.2. Preparation for analysis

3.8.2.1. Preparation of analyzed solutions

1.00 g of the drug is dissolved in water, quantitatively transferred into a volumetric flask, the volume of the solution is adjusted to the mark and mixed thoroughly.

3.8.2.2. Preparation of reference solutions

In six volumetric flasks, 10 cm 3 of solution B and the volumes of solution A indicated in Table 3 are introduced.

Table 3


Reference solution number	Volume of solution A, cm	Mass of each element (K, Na) added to 100 ml of reference solution, mg	Mass fraction of each impurity (K, Na) in terms of the preparation, %

The solutions are mixed, the volume of the solutions is brought to the mark and mixed again.

3.8.2.1, 3.8.2.2. (Changed edition, Rev. N 2).

3.8.3. Conducting an analysis

For analysis take at least two samples of the drug.

The radiation intensity of the resonance lines of sodium 589.0-589.6 nm and potassium 766.5 nm in the gas-air flame spectrum is compared when the analyzed solutions and reference solutions are introduced into it.

After preparing the device for analysis, photometry of the analyzed solutions and reference solutions is carried out in ascending order of the mass fraction of impurities. Then, photometry is carried out in reverse order, starting from the maximum content of impurities, and the arithmetic mean value of the readings for each solution is calculated, taking into account as a correction the reading obtained during photometry of the first reference solution. Spray water after each measurement.

3.8.4. Results processing

Based on the obtained data for reference solutions, a calibration graph is built, plotting the values of the radiation intensity on the ordinate axis, the mass fraction of sodium and potassium impurities in terms of the preparation on the abscissa axis.

The mass fraction of sodium and potassium is found according to the schedule.

The result of the analysis is taken as the arithmetic mean of the results of two parallel determinations, the relative discrepancy between which does not exceed the allowable discrepancy of 30%.

Permissible relative total error of the analysis result is ±15% at a confidence level =0.95.

(Changed edition, Rev. N 2).

4. PACKAGING, LABELING, TRANSPORT AND STORAGE

4.1. The drug is packaged and labeled in accordance with GOST 3885.

Type and type of container: 2-4, 2-5, 2-6, 11-6.

Packing group: V, VI, VII.

The product used as a technological raw material is packaged in liners made of a thin polymer film, inserted into metal drums of the BTPB-25, BTPB-50 type (GOST 5044) with a net weight of up to 70 kg.

The container is marked with a danger sign in accordance with GOST 19433 (class 5, subclass 5.1, classification code 5152).

(Changed edition, Rev. N 2).

4.2. The drug is transported by all means of transport in accordance with the rules for the carriage of goods in force on this type of transport.

4.3. The drug is stored in the manufacturer's packaging in covered warehouses.

5. MANUFACTURER WARRANTY

5.1. The manufacturer guarantees the compliance of chromium (VI) oxide with the requirements of this standard, subject to the conditions of transportation and storage.

5.2. Guaranteed shelf life - 3 years from the date of manufacture.

Sec. 5. (Changed edition, Rev. N 2).

6. SAFETY REQUIREMENTS

6.1. Chromium(VI) oxide is poisonous. The maximum permissible concentration in the air of the working area of industrial premises is 0.01 mg / m (1st hazard class). With an increase in concentration, it can cause acute and chronic poisoning with damage to vital organs and systems.

(Changed edition, Rev. N 2).

6.2. When working with the drug, it is necessary to use anti-dust respirators, rubber gloves and goggles, as well as observe the rules of personal hygiene; do not allow the drug to enter the body.

6.3. Maximum sealing of process equipment should be ensured.

6.4. The premises in which work with the drug is carried out should be equipped with general supply and exhaust ventilation, and places of the greatest dust - with shelters with local exhaust ventilation. The analysis of the drug should be carried out in a laboratory fume hood.

(Changed edition, Rev. N 2).

6.5. When analyzing the drug using combustible gases, fire safety rules should be observed.

The text of the document is verified by:
official publication
M.: IPK Standards Publishing House, 1999

Chromium is an element of a side subgroup of the 6th group of the 4th period of the periodic system of chemical elements of D. I. Mendeleev, with atomic number 24. It is designated by the symbol Cr (lat. Chromium). The simple substance chromium is a bluish-white hard metal.

Chemical properties of chromium

Under normal conditions, chromium reacts only with fluorine. At high temperatures (above 600°C) it interacts with oxygen, halogens, nitrogen, silicon, boron, sulfur, and phosphorus.

4Cr + 3O 2 – t° →2Cr 2 O 3

2Cr + 3Cl 2 – t° → 2CrCl 3

2Cr + N 2 – t° → 2CrN

2Cr + 3S – t° → Cr 2 S 3

In a hot state, it reacts with water vapor:

2Cr + 3H 2 O → Cr 2 O 3 + 3H 2

Chromium dissolves in dilute strong acids (HCl, H 2 SO 4)

In the absence of air, Cr 2+ salts are formed, and in air, Cr 3+ salts are formed.

Cr + 2HCl → CrCl 2 + H 2

2Cr + 6HCl + O 2 → 2CrCl 3 + 2H 2 O + H 2

The presence of a protective oxide film on the surface of the metal explains its passivity in relation to concentrated solutions of acids - oxidizing agents.

Chromium compounds

Chromium(II) oxide and chromium(II) hydroxide are basic.

Cr(OH) 2 + 2HCl → CrCl 2 + 2H 2 O

Chromium (II) compounds are strong reducing agents; pass into chromium (III) compounds under the action of atmospheric oxygen.

2CrCl 2 + 2HCl → 2CrCl 3 + H 2

4Cr(OH) 2 + O 2 + 2H 2 O → 4Cr(OH) 3

Chromium oxide (III) Cr 2 O 3 is a green, water-insoluble powder. It can be obtained by calcining chromium (III) hydroxide or potassium and ammonium dichromates:

2Cr(OH) 3 – t° → Cr 2 O 3 + 3H 2 O

4K 2 Cr 2 O 7 – t° → 2Cr 2 O 3 + 4K 2 CrO 4 + 3O 2

(NH 4) 2 Cr 2 O 7 - t ° → Cr 2 O 3 + N 2 + 4H 2 O (volcano reaction)

amphoteric oxide. When Cr 2 O 3 is fused with alkalis, soda and acid salts, chromium compounds are obtained with an oxidation state (+3):

Cr 2 O 3 + 2NaOH → 2NaCrO 2 + H 2 O

Cr 2 O 3 + Na 2 CO 3 → 2NaCrO 2 + CO 2

When fused with a mixture of alkali and an oxidizing agent, chromium compounds are obtained in the oxidation state (+6):

Cr 2 O 3 + 4KOH + KClO 3 → 2K 2 CrO 4 + KCl + 2H 2 O

Chromium (III) hydroxide C r (OH) 3 . amphoteric hydroxide. Grey-green, decomposes on heating, losing water and forming green metahydroxide CrO(OH). Does not dissolve in water. It precipitates from solution as a gray-blue and bluish-green hydrate. Reacts with acids and alkalis, does not interact with ammonia hydrate.

It has amphoteric properties - it dissolves in both acids and alkalis:

2Cr(OH) 3 + 3H 2 SO 4 → Cr 2 (SO 4) 3 + 6H 2 O Cr(OH) 3 + ZH + = Cr 3+ + 3H 2 O

Cr (OH) 3 + KOH → K, Cr (OH) 3 + ZON - (conc.) \u003d [Cr (OH) 6] 3-

Cr (OH) 3 + KOH → KCrO 2 + 2H 2 O Cr (OH) 3 + MON \u003d MCrO 2 (green) + 2H 2 O (300-400 ° C, M \u003d Li, Na)

Cr(OH) 3 →(120 o C – H 2 O) CrO(OH) →(430-1000 0 С –H 2 O) Cr2O3

2Cr(OH) 3 + 4NaOH (conc.) + ZN 2 O 2 (conc.) \u003d 2Na 2 CrO 4 + 8H 2 0

Receipt: precipitation with ammonia hydrate from a solution of chromium(III) salts:

Cr 3+ + 3(NH 3 H 2 O) = WITHr(OH) 3 ↓+ ЗНН 4+

Cr 2 (SO 4) 3 + 6NaOH → 2Cr(OH) 3 ↓+ 3Na 2 SO 4 (in excess of alkali - the precipitate dissolves)

Salts of chromium (III) have a purple or dark green color. By chemical properties, they resemble colorless aluminum salts.

Cr(III) compounds can exhibit both oxidizing and reducing properties:

Zn + 2Cr +3 Cl 3 → 2Cr +2 Cl 2 + ZnCl 2

2Cr +3 Cl 3 + 16NaOH + 3Br 2 → 6NaBr + 6NaCl + 8H 2 O + 2Na 2 Cr +6 O 4

Hexavalent chromium compounds

Chromium(VI) oxide CrO 3 - bright red crystals, soluble in water.

Prepared from potassium chromate (or dichromate) and H 2 SO 4 (conc.).

K 2 CrO 4 + H 2 SO 4 → CrO 3 + K 2 SO 4 + H 2 O

K 2 Cr 2 O 7 + H 2 SO 4 → 2CrO 3 + K 2 SO 4 + H 2 O

CrO 3 - acidic oxide, forms yellow chromates CrO 4 2- with alkalis:

CrO 3 + 2KOH → K 2 CrO 4 + H 2 O

In an acidic environment, chromates turn into orange dichromates Cr 2 O 7 2-:

2K 2 CrO 4 + H 2 SO 4 → K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O

In an alkaline environment, this reaction proceeds in the opposite direction:

K 2 Cr 2 O 7 + 2KOH → 2K 2 CrO 4 + H 2 O

Potassium dichromate is an oxidizing agent in an acidic environment:

K 2 Cr 2 O 7 + 4H 2 SO 4 + 3Na 2 SO 3 \u003d Cr 2 (SO 4) 3 + 3Na 2 SO 4 + K 2 SO 4 + 4H 2 O

K 2 Cr 2 O 7 + 4H 2 SO 4 + 3NaNO 2 = Cr 2 (SO 4) 3 + 3NaNO 3 + K 2 SO 4 + 4H 2 O

K 2 Cr 2 O 7 + 7H 2 SO 4 + 6KI = Cr 2 (SO 4) 3 + 3I 2 + 4K 2 SO 4 + 7H 2 O

K 2 Cr 2 O 7 + 7H 2 SO 4 + 6FeSO 4 = Cr 2 (SO 4) 3 + 3Fe 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O

Potassium chromate K 2 Cr About 4 . Oksosol. Yellow, non-hygroscopic. Melts without decomposition, thermally stable. Highly soluble in water yellow the color of the solution corresponds to the CrO 4 2- ion, slightly hydrolyzes the anion. In an acidic environment, it passes into K 2 Cr 2 O 7. Oxidizing agent (weaker than K 2 Cr 2 O 7). Enters into ion exchange reactions.

Qualitative reaction on the ion CrO 4 2- - precipitation of a yellow precipitate of barium chromate, decomposing in a strongly acidic environment. It is used as a mordant for dyeing fabrics, a leather tanning agent, a selective oxidizing agent, and a reagent in analytical chemistry.

Equations of the most important reactions:

2K 2 CrO 4 + H 2 SO 4 (30%) = K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O

2K 2 CrO 4 (t) + 16HCl (conc., horizon) \u003d 2CrCl 3 + 3Cl 2 + 8H 2 O + 4KCl

2K 2 CrO 4 +2H 2 O+3H 2 S=2Cr(OH) 3 ↓+3S↓+4KOH

2K 2 CrO 4 +8H 2 O+3K 2 S=2K[Сr(OH) 6]+3S↓+4KOH

2K 2 CrO 4 + 2AgNO 3 \u003d KNO 3 + Ag 2 CrO 4 (red) ↓

Qualitative response:

K 2 CrO 4 + BaCl 2 \u003d 2KSl + BaCrO 4 ↓

2ВаСrO 4 (t) + 2НCl (razb.) = ВаСr 2 O 7(p) + ВаС1 2 + Н 2 O

Receipt: sintering of chromite with potash in air:

4(Cr 2 Fe ‖‖)O 4 + 8K 2 CO 3 + 7O 2 = 8K 2 CrO 4 + 2Fe 2 O 3 + 8СO 2 (1000 °С)

Potassium dichromate K 2 Cr 2 O 7 . Oksosol. technical name chrompeak. Orange-red, non-hygroscopic. Melts without decomposition, decomposes on further heating. Highly soluble in water orange the color of the solution corresponds to the ion Cr 2 O 7 2-). In an alkaline medium, it forms K 2 CrO 4 . A typical oxidizing agent in solution and when fused. Enters into ion exchange reactions.

Qualitative reactions- blue coloring of an ether solution in the presence of H 2 O 2, blue coloring of an aqueous solution under the action of atomic hydrogen.

It is used as a leather tanning agent, a mordant for dyeing fabrics, a component of pyrotechnic compositions, a reagent in analytical chemistry, a metal corrosion inhibitor, mixed with H 2 SO 4 (conc.) - for washing chemical dishes.

Equations of the most important reactions:

4K 2 Cr 2 O 7 \u003d 4K 2 CrO 4 + 2Cr 2 O 3 + 3O 2 (500-600 o C)

K 2 Cr 2 O 7 (t) + 14HCl (conc) \u003d 2CrCl 3 + 3Cl 2 + 7H 2 O + 2KCl (boiling)

K 2 Cr 2 O 7 (t) + 2H 2 SO 4 (96%) ⇌2KHSO 4 + 2CrO 3 + H 2 O (“chromium mixture”)

K 2 Cr 2 O 7 +KOH (conc) \u003d H 2 O + 2K 2 CrO 4

Cr 2 O 7 2- + 14H + + 6I - \u003d 2Cr 3+ + 3I 2 ↓ + 7H 2 O

Cr 2 O 7 2- + 2H + + 3SO 2 (g) \u003d 2Cr 3+ + 3SO 4 2- + H 2 O

Cr 2 O 7 2- + H 2 O + 3H 2 S (g) \u003d 3S ↓ + 2OH - + 2Cr 2 (OH) 3 ↓

Cr 2 O 7 2- (conc) + 2Ag + (razb.) \u003d Ag 2 Cr 2 O 7 (so red) ↓

Cr 2 O 7 2- (razb.) + H 2 O + Pb 2+ \u003d 2H + + 2PbCrO 4 (red) ↓

K 2 Cr 2 O 7 (t) + 6HCl + 8H 0 (Zn) \u003d 2CrCl 2 (syn) + 7H 2 O + 2KCl

Receipt: treatment of K 2 CrO 4 with sulfuric acid:

2K 2 CrO 4 + H 2 SO 4 (30%) = K 2Cr 2 O 7 + K 2 SO 4 + H 2 O

The discovery of chromium belongs to the period of rapid development of chemical-analytical studies of salts and minerals. In Russia, chemists took a special interest in the analysis of minerals found in Siberia and almost unknown in Western Europe. One of these minerals was the Siberian red lead ore (crocoite), described by Lomonosov. The mineral was investigated, but nothing but oxides of lead, iron and aluminum was found in it. However, in 1797, Vauquelin, by boiling a finely ground sample of the mineral with potash and precipitating lead carbonate, obtained an orange-red colored solution. From this solution, he crystallized a ruby-red salt, from which an oxide and a free metal, different from all known metals, were isolated. Vauquelin called him Chromium ( Chrome ) from the Greek word- coloring, color; True, here it was not the property of the metal that was meant, but its brightly colored salts.

Finding in nature.

The most important chromium ore of practical importance is chromite, the approximate composition of which corresponds to the formula FeCrO 4.

It is found in Asia Minor, in the Urals, in North America, in southern Africa. The above-mentioned mineral crocoite - PbCrO 4 - is also of technical importance. Chromium oxide (3) and some of its other compounds are also found in nature. In the earth's crust, the chromium content in terms of metal is 0.03%. Chromium is found on the Sun, stars, meteorites.

Physical properties.

Chromium is a white, hard and brittle metal, exceptionally chemically resistant to acids and alkalis. It oxidizes in air and has a thin transparent oxide film on the surface. Chromium has a density of 7.1 g / cm 3, its melting point is +1875 0 C.

Receipt.

With strong heating of chromium iron ore with coal, chromium and iron are reduced:

FeO * Cr 2 O 3 + 4C = 2Cr + Fe + 4CO

As a result of this reaction, an alloy of chromium with iron is formed, which is characterized by high strength. To obtain pure chromium, it is reduced from chromium(3) oxide with aluminum:

Cr 2 O 3 + 2Al \u003d Al 2 O 3 + 2Cr

Two oxides are usually used in this process - Cr 2 O 3 and CrO 3

Chemical properties.

Thanks to a thin protective oxide film covering the surface of chromium, it is highly resistant to aggressive acids and alkalis. Chromium does not react with concentrated nitric and sulfuric acids, as well as with phosphoric acid. Chromium interacts with alkalis at t = 600-700 o C. However, chromium interacts with dilute sulfuric and hydrochloric acids, displacing hydrogen:

2Cr + 3H 2 SO 4 \u003d Cr 2 (SO 4) 3 + 3H 2
2Cr + 6HCl = 2CrCl 3 + 3H 2

At high temperatures, chromium burns in oxygen to form oxide(III).

Hot chromium reacts with water vapor:

2Cr + 3H 2 O \u003d Cr 2 O 3 + 3H 2

Chromium also reacts with halogens at high temperatures, halogens with hydrogens, sulfur, nitrogen, phosphorus, coal, silicon, boron, for example:

Cr + 2HF = CrF 2 + H 2
2Cr + N2 = 2CrN
2Cr + 3S = Cr2S3
Cr + Si = CrSi

The above physical and chemical properties of chromium have found their application in various fields of science and technology. For example, chromium and its alloys are used to obtain high-strength, corrosion-resistant coatings in mechanical engineering. Alloys in the form of ferrochrome are used as metal cutting tools. Chrome-plated alloys have found application in medical technology, in the manufacture of chemical process equipment.

The position of chromium in the periodic table of chemical elements:

Chromium heads the side subgroup of group VI of the periodic system of elements. Its electronic formula is as follows:

24 Cr IS 2 2S 2 2P 6 3S 2 3P 6 3d 5 4S 1

In filling the orbitals with electrons at the chromium atom, the regularity is violated, according to which the 4S orbital should have been filled first to the state 4S 2 . However, due to the fact that the 3d orbital occupies a more favorable energy position in the chromium atom, it is filled up to the value 4d 5 . Such a phenomenon is observed in the atoms of some other elements of the secondary subgroups. Chromium can exhibit oxidation states from +1 to +6. The most stable are chromium compounds with oxidation states +2, +3, +6.

Divalent chromium compounds.

Chromium oxide (II) CrO - pyrophoric black powder (pyrophoric - the ability to ignite in air in a finely divided state). CrO dissolves in dilute hydrochloric acid:

CrO + 2HCl = CrCl 2 + H 2 O

In air, when heated above 100 0 C, CrO turns into Cr 2 O 3.

Divalent chromium salts are formed by dissolving chromium metal in acids. These reactions take place in an atmosphere of an inactive gas (for example, H 2), because in the presence of air, Cr(II) is easily oxidized to Cr(III).

Chromium hydroxide is obtained in the form of a yellow precipitate by the action of an alkali solution on chromium (II) chloride:

CrCl 2 + 2NaOH = Cr(OH) 2 + 2NaCl

Cr(OH) 2 has basic properties, is a reducing agent. The hydrated Cr2+ ion is colored pale blue. An aqueous solution of CrCl 2 has a blue color. In air in aqueous solutions, Cr(II) compounds transform into Cr(III) compounds. This is especially pronounced for Cr(II) hydroxide:

4Cr(OH) 2 + 2H 2 O + O 2 = 4Cr(OH) 3

Trivalent chromium compounds.

Chromium oxide (III) Cr 2 O 3 is a refractory green powder. It is close to corundum in hardness. In the laboratory, it can be obtained by heating ammonium dichromate:

(NH 4) 2 Cr 2 O 7 \u003d Cr 2 O 3 + N 2 + 4H 2

Cr 2 O 3 - amphoteric oxide, when fused with alkalis, forms chromites: Cr 2 O 3 + 2NaOH \u003d 2NaCrO 2 + H 2 O

Chromium hydroxide is also an amphoteric compound:

Cr(OH) 3 + HCl = CrCl 3 + 3H 2 O
Cr(OH) 3 + NaOH = NaCrO 2 + 2H 2 O

Anhydrous CrCl 3 has the appearance of dark purple leaves, is completely insoluble in cold water, and dissolves very slowly when boiled. Anhydrous chromium sulfate (III) Cr 2 (SO 4) 3 pink, also poorly soluble in water. In the presence of reducing agents, it forms purple chromium sulfate Cr 2 (SO 4) 3 *18H 2 O. Green chromium sulfate hydrates are also known, containing a smaller amount of water. Chrome alum KCr(SO 4) 2 *12H 2 O crystallizes from solutions containing violet chromium sulfate and potassium sulfate. A solution of chromic alum turns green when heated due to the formation of sulfates.

Reactions with chromium and its compounds

Almost all chromium compounds and their solutions are intensely colored. Having a colorless solution or a white precipitate, we can conclude with a high degree of probability that chromium is absent.

We strongly heat in the flame of a burner on a porcelain cup such an amount of potassium dichromate that will fit on the tip of a knife. Salt will not release water of crystallization, but will melt at a temperature of about 400 0 C with the formation of a dark liquid. Let's heat it for a few more minutes on a strong flame. After cooling, a green precipitate forms on the shard. Part of it is soluble in water (it turns yellow), and the other part is left on the shard. The salt decomposed when heated, resulting in the formation of soluble yellow potassium chromate K 2 CrO 4 and green Cr 2 O 3 .
Dissolve 3g of powdered potassium dichromate in 50ml of water. To one part add some potassium carbonate. It will dissolve with the release of CO 2 , and the color of the solution will become light yellow. Chromate is formed from potassium dichromate. If we now add a 50% solution of sulfuric acid in portions, then the red-yellow color of the bichromate will appear again.
Pour into a test tube 5 ml. potassium dichromate solution, boil with 3 ml of concentrated hydrochloric acid under draft. Yellow-green poisonous gaseous chlorine is released from the solution, because chromate will oxidize HCl to Cl 2 and H 2 O. The chromate itself will turn into green trivalent chromium chloride. It can be isolated by evaporating the solution, and then, fusing with soda and nitrate, converted to chromate.
When a solution of lead nitrate is added, yellow lead chromate precipitates; when interacting with a solution of silver nitrate, a red-brown precipitate of silver chromate is formed.
Add hydrogen peroxide to a solution of potassium bichromate and acidify the solution with sulfuric acid. The solution acquires a deep blue color due to the formation of chromium peroxide. Peroxide, when shaken with some ether, will turn into an organic solvent and turn it blue. This reaction is specific for chromium and is very sensitive. It can be used to detect chromium in metals and alloys. First of all, it is necessary to dissolve the metal. With prolonged boiling with 30% sulfuric acid (hydrochloric acid can also be added), chromium and many steels partially dissolve. The resulting solution contains chromium (III) sulfate. To be able to conduct a detection reaction, we first neutralize it with caustic soda. Gray-green chromium (III) hydroxide precipitates, which dissolves in excess NaOH and forms green sodium chromite. Filter the solution and add 30% hydrogen peroxide. When heated, the solution will turn yellow, as chromite is oxidized to chromate. Acidification will result in a blue color of the solution. The colored compound can be extracted by shaking with ether.

Analytical reactions for chromium ions.

To 3-4 drops of a solution of chromium chloride CrCl 3 add a 2M solution of NaOH until the initial precipitate dissolves. Note the color of the sodium chromite formed. Heat the resulting solution in a water bath. What is happening?
To 2-3 drops of CrCl 3 solution add an equal volume of 8M NaOH solution and 3-4 drops of 3% H 2 O 2 solution. Heat the reaction mixture in a water bath. What is happening? What precipitate is formed if the resulting colored solution is neutralized, CH 3 COOH is added to it, and then Pb (NO 3) 2 ?
Pour 4-5 drops of solutions of chromium sulfate Cr 2 (SO 4) 3, IMH 2 SO 4 and KMnO 4 into a test tube. Heat the reaction site for several minutes on a water bath. Note the change in color of the solution. What caused it?
To 3-4 drops of K 2 Cr 2 O 7 solution acidified with nitric acid, add 2-3 drops of H 2 O 2 solution and mix. The blue color of the solution that appears is due to the appearance of perchromic acid H 2 CrO 6:

Cr 2 O 7 2- + 4H 2 O 2 + 2H + = 2H 2 CrO 6 + 3H 2 O

Pay attention to the rapid decomposition of H 2 CrO 6:

2H 2 CrO 6 + 8H+ = 2Cr 3+ + 3O 2 + 6H 2 O
blue color green color

Perchromic acid is much more stable in organic solvents.

To 3-4 drops of K 2 Cr 2 O 7 solution acidified with nitric acid, add 5 drops of isoamyl alcohol, 2-3 drops of H 2 O 2 solution and shake the reaction mixture. The layer of organic solvent that floats to the top is colored bright blue. The color fades very slowly. Compare the stability of H 2 CrO 6 in organic and aqueous phases.
When CrO 4 2- and Ba 2+ ions interact, a yellow precipitate of barium chromate BaCrO 4 precipitates.
Silver nitrate forms brick red precipitate of silver chromate with CrO 4 2 ions.
Take three test tubes. Place 5-6 drops of K 2 Cr 2 O 7 solution in one of them, the same volume of K 2 CrO 4 solution in the second, and three drops of both solutions in the third. Then add three drops of potassium iodide solution to each tube. Explain the result. Acidify the solution in the second tube. What is happening? Why?

Entertaining experiments with chromium compounds

A mixture of CuSO 4 and K 2 Cr 2 O 7 turns green when alkali is added, and turns yellow in the presence of acid. By heating 2 mg of glycerol with a small amount of (NH 4) 2 Cr 2 O 7 and then adding alcohol, a bright green solution is obtained after filtration, which turns yellow when acid is added, and turns green in a neutral or alkaline medium.
Place in the center of the can with thermite "ruby mixture" - thoroughly ground and placed in aluminum foil Al 2 O 3 (4.75 g) with the addition of Cr 2 O 3 (0.25 g). So that the jar does not cool down longer, it is necessary to bury it under the upper edge in the sand, and after the thermite is ignited and the reaction begins, cover it with an iron sheet and cover it with sand. Bank to dig out in a day. The result is a red-ruby powder.
10 g of potassium bichromate is triturated with 5 g of sodium or potassium nitrate and 10 g of sugar. The mixture is moistened and mixed with collodion. If the powder is pressed in a glass tube, and then the stick is pushed out and set on fire from the end, then a “snake” will begin to crawl out, first black, and after cooling - green. A stick with a diameter of 4 mm burns at a speed of about 2 mm per second and lengthens 10 times.
If you mix solutions of copper sulfate and potassium dichromate and add a little ammonia solution, then an amorphous brown precipitate of the composition 4СuCrO 4 * 3NH 3 * 5H 2 O will fall out, which dissolves in hydrochloric acid to form a yellow solution, and in excess of ammonia a green solution is obtained. If further alcohol is added to this solution, a green precipitate will form, which, after filtration, becomes blue, and after drying, blue-violet with red sparkles, clearly visible in strong light.
The chromium oxide left after the “volcano” or “pharaoh snake” experiments can be regenerated. To do this, it is necessary to fuse 8 g of Cr 2 O 3 and 2 g of Na 2 CO 3 and 2.5 g of KNO 3 and treat the cooled alloy with boiling water. Soluble chromate is obtained, which can also be converted into other Cr(II) and Cr(VI) compounds, including the original ammonium dichromate.

Examples of redox transitions involving chromium and its compounds

1. Cr 2 O 7 2- -- Cr 2 O 3 -- CrO 2 - -- CrO 4 2- -- Cr 2 O 7 2-

a) (NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 + 4H 2 O b) Cr 2 O 3 + 2NaOH \u003d 2NaCrO 2 + H 2 O
c) 2NaCrO 2 + 3Br 2 + 8NaOH = 6NaBr + 2Na 2 CrO 4 + 4H 2 O
d) 2Na 2 CrO 4 + 2HCl = Na 2 Cr 2 O 7 + 2NaCl + H 2 O

2. Cr(OH) 2 -- Cr(OH) 3 -- CrCl 3 -- Cr 2 O 7 2- -- CrO 4 2-

a) 2Cr(OH) 2 + 1/2O 2 + H 2 O = 2Cr(OH) 3
b) Cr(OH) 3 + 3HCl = CrCl 3 + 3H 2 O
c) 2CrCl 3 + 2KMnO 4 + 3H 2 O = K 2 Cr 2 O 7 + 2Mn(OH) 2 + 6HCl
d) K 2 Cr 2 O 7 + 2KOH = 2K 2 CrO 4 + H 2 O

3. CrO - Cr (OH) 2 - Cr (OH) 3 - Cr (NO 3) 3 - Cr 2 O 3 - CrO - 2
Cr2+

a) CrO + 2HCl = CrCl 2 + H 2 O
b) CrO + H 2 O \u003d Cr (OH) 2
c) Cr(OH) 2 + 1/2O 2 + H 2 O = 2Cr(OH) 3
d) Cr(OH) 3 + 3HNO 3 = Cr(NO 3) 3 + 3H 2 O
e) 4Cr (NO 3) 3 \u003d 2Cr 2 O 3 + 12NO 2 + O 2
f) Cr 2 O 3 + 2 NaOH = 2NaCrO 2 + H 2 O

Chrome element as an artist

Chemists quite often turned to the problem of creating artificial pigments for painting. In the 18th-19th centuries, the technology for obtaining many pictorial materials was developed. Louis Nicolas Vauquelin in 1797, who discovered the previously unknown element chromium in Siberian red ore, prepared a new, remarkably stable paint - chrome green. Its chromophore is aqueous chromium (III) oxide. Under the name "emerald green" it began to be produced in 1837. Later, L. Vauquelen proposed several new paints: barite, zinc and chrome yellow. Over time, they were replaced by more persistent yellow, orange pigments based on cadmium.

Chrome green is the most durable and lightfast paint that is not affected by atmospheric gases. Rubbed in oil, chrome green has great hiding power and is capable of drying quickly, therefore, since the 19th century. it is widely used in painting. It is of great importance in porcelain painting. The fact is that porcelain products can be decorated with both underglaze and overglaze painting. In the first case, paints are applied to the surface of only a slightly fired product, which is then covered with a layer of glaze. This is followed by the main, high-temperature firing: for sintering the porcelain mass and melting the glaze, the products are heated to 1350 - 1450 0 C. Very few paints can withstand such a high temperature without chemical changes, and in the old days there were only two of them - cobalt and chromium. Black oxide of cobalt, applied to the surface of a porcelain item, fuses with the glaze during firing, chemically interacting with it. As a result, bright blue cobalt silicates are formed. This cobalt blue chinaware is well known to everyone. Chromium oxide (III) does not interact chemically with the components of the glaze and simply lies between the porcelain shards and the transparent glaze with a "deaf" layer.

In addition to chrome green, artists use paints derived from Volkonskoite. This mineral from the group of montmorillonites (a clay mineral of the subclass of complex silicates Na (Mo, Al), Si 4 O 10 (OH) 2) was discovered in 1830 by the Russian mineralogist Kemmerer and named after M.N. Volkonskaya, the daughter of the hero of the Battle of Borodino, General N N. Raevsky, wife of the Decembrist S. G. Volkonsky Volkonskoite is a clay containing up to 24% chromium oxide, as well as oxides of aluminum and iron (III). determines its diverse coloration - from the color of a darkened winter fir to the bright green color of a marsh frog.

Pablo Picasso turned to the geologists of our country with a request to study the reserves of Volkonskoite, which gives the paint a uniquely fresh tone. At present, a method has been developed for obtaining artificial wolkonskoite. It is interesting to note that, according to modern research, Russian icon painters used paints from this material as early as the Middle Ages, long before its “official” discovery. Guinier's green (created in 1837), whose chromoform is a hydrate of chromium oxide Cr 2 O 3 * (2-3) H 2 O, where part of the water is chemically bound and part adsorbed, was also popular with artists. This pigment gives the paint an emerald hue.

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"National Research Tomsk Polytechnic University"

Institute of Natural Resources Geoecology and Geochemistry

Chromium

By discipline:

Chemistry

Completed:

student of group 2G41 Tkacheva Anastasia Vladimirovna 10/29/2014

Checked:

teacher Stas Nikolay Fedorovich

Position in the periodic system

Chromium- an element of a side subgroup of the 6th group of the 4th period of the periodic system of chemical elements of D. I. Mendeleev with atomic number 24. It is indicated by the symbol Cr(lat. Chromium). simple substance chromium- hard bluish-white metal. Chromium is sometimes referred to as a ferrous metal.

The structure of the atom

17 Cl) 2) 8) 7 - diagram of the structure of the atom

1s2s2p3s3p - electronic formula

The atom is located in period III, and has three energy levels

The atom is located in VII in the group, in the main subgroup - at the external energy level of 7 electrons

Element properties

Physical properties

Chromium is a white shiny metal with a cubic body-centered lattice, a = 0.28845 nm, characterized by hardness and brittleness, with a density of 7.2 g / cm 3, one of the hardest pure metals (second only to beryllium, tungsten and uranium), with a melting point of 1903 degrees. And with a boiling point of about 2570 degrees. C. In air, the surface of chromium is covered with an oxide film, which protects it from further oxidation. The addition of carbon to chromium further increases its hardness.

Chemical properties

Chromium under normal conditions is an inert metal, when heated it becomes quite active.

Interaction with non-metals

When heated above 600°C, chromium burns in oxygen:

4Cr + 3O 2 \u003d 2Cr 2 O 3.

It reacts with fluorine at 350°C, with chlorine at 300°C, with bromine at a red heat temperature, forming chromium (III) halides:

2Cr + 3Cl 2 = 2CrCl 3 .

It reacts with nitrogen at temperatures above 1000°C to form nitrides:

2Cr + N 2 = 2CrN

or 4Cr + N 2 = 2Cr 2 N.

2Cr + 3S = Cr 2 S 3 .

Reacts with boron, carbon and silicon to form borides, carbides and silicides:

Cr + 2B = CrB 2 (the formation of Cr 2 B, CrB, Cr 3 B 4, CrB 4 is possible),

2Cr + 3C \u003d Cr 2 C 3 (the formation of Cr 23 C 6, Cr 7 B 3 is possible),

Cr + 2Si = CrSi 2 (possible formation of Cr 3 Si, Cr 5 Si 3, CrSi).

It does not interact directly with hydrogen.

Interaction with water

In a finely ground hot state, chromium reacts with water, forming chromium (III) oxide and hydrogen:

2Cr + 3H 2 O \u003d Cr 2 O 3 + 3H 2

Interaction with acids

In the electrochemical series of voltages of metals, chromium is before hydrogen, it displaces hydrogen from solutions of non-oxidizing acids:

Cr + 2HCl \u003d CrCl 2 + H 2;

Cr + H 2 SO 4 \u003d CrSO 4 + H 2.

In the presence of atmospheric oxygen, chromium (III) salts are formed:

4Cr + 12HCl + 3O 2 = 4CrCl 3 + 6H 2 O.

Concentrated nitric and sulfuric acids passivate chromium. Chromium can dissolve in them only with strong heating, chromium (III) salts and acid reduction products are formed:

2Cr + 6H 2 SO 4 = Cr 2 (SO 4) 3 + 3SO 2 + 6H 2 O;

Cr + 6HNO 3 \u003d Cr (NO 3) 3 + 3NO 2 + 3H 2 O.

Interaction with alkaline reagents

In aqueous solutions of alkalis, chromium does not dissolve; it slowly reacts with alkali melts to form chromites and release hydrogen:

2Cr + 6KOH \u003d 2KCrO 2 + 2K 2 O + 3H 2.

Reacts with alkaline melts of oxidizing agents, such as potassium chlorate, while chromium passes into potassium chromate:

Cr + KClO 3 + 2KOH = K 2 CrO 4 + KCl + H 2 O.

Recovery of metals from oxides and salts

Chromium is an active metal, capable of displacing metals from solutions of their salts: 2Cr + 3CuCl 2 = 2CrCl 3 + 3Cu.

Properties of a simple substance

Stable in air due to passivation. For the same reason, it does not react with sulfuric and nitric acids. At 2000 °C, it burns out with the formation of green chromium (III) oxide Cr 2 O 3, which has amphoteric properties.

Synthesized compounds of chromium with boron (borides Cr 2 B, CrB, Cr 3 B 4, CrB 2, CrB 4 and Cr 5 B 3), with carbon (carbides Cr 23 C 6, Cr 7 C 3 and Cr 3 C 2), with silicon (silicides Cr 3 Si, Cr 5 Si 3 and CrSi) and nitrogen (nitrides CrN and Cr 2 N).

Cr(+2) compounds

The oxidation state +2 corresponds to the basic oxide CrO (black). Cr 2+ salts (blue solutions) are obtained by reducing Cr 3+ salts or dichromates with zinc in an acidic environment (“hydrogen at the time of isolation”):

All these Cr 2+ salts are strong reducing agents, to the extent that they displace hydrogen from water upon standing. Oxygen in the air, especially in an acidic environment, oxidizes Cr 2+, as a result of which the blue solution quickly turns green.

Brown or yellow Cr(OH) 2 hydroxide precipitates when alkalis are added to solutions of chromium(II) salts.

Chromium dihalides CrF 2 , CrCl 2 , CrBr 2 and CrI 2 were synthesized

Cr(+3) compounds

The +3 oxidation state corresponds to the amphoteric oxide Cr 2 O 3 and the hydroxide Cr (OH) 3 (both green). This is the most stable oxidation state of chromium. Chromium compounds in this oxidation state have a color from dirty purple (ion 3+) to green (anions are present in the coordination sphere).

Cr 3+ is prone to the formation of double sulfates of the form M I Cr (SO 4) 2 12H 2 O (alum)

Chromium (III) hydroxide is obtained by acting with ammonia on solutions of chromium (III) salts:

Cr+3NH+3H2O→Cr(OH)↓+3NH

Alkali solutions can be used, but in their excess a soluble hydroxo complex is formed:

Cr+3OH→Cr(OH)↓

Cr(OH)+3OH→

By fusing Cr 2 O 3 with alkalis, chromites are obtained:

Cr2O3+2NaOH→2NaCrO2+H2O

Uncalcined chromium (III) oxide dissolves in alkaline solutions and in acids:

Cr2O3+6HCl→2CrCl3+3H2O

When chromium(III) compounds are oxidized in an alkaline medium, chromium(VI) compounds are formed:

2Na+3HO→2NaCrO+2NaOH+8HO

The same thing happens when chromium (III) oxide is fused with alkali and oxidizing agents, or with alkali in air (the melt becomes yellow in this case):

2Cr2O3+8NaOH+3O2→4Na2CrO4+4H2O

Chromium compounds (+4)[

With careful decomposition of chromium oxide (VI) CrO 3 under hydrothermal conditions, chromium oxide (IV) CrO 2 is obtained, which is ferromagnetic and has metallic conductivity.

Among chromium tetrahalides, CrF 4 is stable, chromium tetrachloride CrCl 4 exists only in vapor.

Chromium compounds (+6)

The +6 oxidation state corresponds to acidic chromium oxide (VI) CrO 3 and a number of acids between which there is an equilibrium. The simplest of them are chromic H 2 CrO 4 and two-chrome H 2 Cr 2 O 7. They form two series of salts: yellow chromates and orange dichromates, respectively.

Chromium oxide (VI) CrO 3 is formed by the interaction of concentrated sulfuric acid with solutions of dichromates. A typical acid oxide, when interacting with water, it forms strong unstable chromic acids: chromic H 2 CrO 4, dichromic H 2 Cr 2 O 7 and other isopoly acids with the general formula H 2 Cr n O 3n+1. An increase in the degree of polymerization occurs with a decrease in pH, that is, an increase in acidity:

2CrO+2H→Cr2O+H2O

But if an alkali solution is added to an orange solution of K 2 Cr 2 O 7, how does the color turn yellow again, since chromate K 2 CrO 4 is formed again:

Cr2O+2OH→2CrO+HO

It does not reach a high degree of polymerization, as occurs in tungsten and molybdenum, since polychromic acid decomposes into chromium (VI) oxide and water:

H2CrnO3n+1→H2O+nCrO3

The solubility of chromates roughly corresponds to the solubility of sulfates. In particular, yellow barium chromate BaCrO 4 precipitates when barium salts are added to both chromate and dichromate solutions:

Ba+CrO→BaCrO↓

2Ba+CrO+H2O→2BaCrO↓+2H

The formation of a blood-red, poorly soluble silver chromate is used to detect silver in alloys using assay acid.

Chromium pentafluoride CrF 5 and unstable chromium hexafluoride CrF 6 are known. Volatile chromium oxyhalides CrO 2 F 2 and CrO 2 Cl 2 (chromyl chloride) have also been obtained.

Chromium(VI) compounds are strong oxidizing agents, for example:

K2Cr2O7+14HCl→2CrCl3+2KCl+3Cl2+7H2O

The addition of hydrogen peroxide, sulfuric acid, and an organic solvent (ether) to dichromates leads to the formation of blue chromium peroxide CrO 5 L (L is a solvent molecule), which is extracted into the organic layer; this reaction is used as an analytical one.