Chromium(lat. Cromium), Cr, a chemical element of Group VI of the Mendeleev periodic system, atomic number 24, atomic mass 51.996; steel-blue metal.

Natural stable isotopes: 50 Cr (4.31%), 52 Cr (87.76%), 53 Cr (9.55%) and 54 Cr (2.38%). Of the artificial radioactive isotopes, the most important is 51 Cr (half-life T ½ = 27.8 days), which is used as an isotope tracer.

History reference. Chromium was discovered in 1797 by LN Vauquelin in the mineral crocoite - natural lead chromate РbCrО 4 . Chrome got its name from the Greek word chroma - color, paint (because of the variety of colors of its compounds). Independently of Vauquelin, chromium was discovered in crocoite in 1798 by the German scientist M. G. Klaproth.

Distribution of Chromium in nature. The average content of Chromium in the earth's crust (clarke) is 8.3·10 -3%. This element is probably more characteristic of the Earth's mantle, since the ultramafic rocks, which are believed to be closest in composition to the Earth's mantle, are enriched in Chromium (2·10 -4%). Chromium forms massive and disseminated ores in ultramafic rocks; the formation of the largest deposits of Chromium is associated with them. In basic rocks, the content of Chromium reaches only 2 10 -2%, in acidic rocks - 2.5 10 -3%, in sedimentary rocks (sandstones) - 3.5 10 -3%, shale - 9 10 -3 %. Chromium is a comparatively weak water migrant; Chromium content in sea water is 0.00005 mg/l.

In general, Chromium is a metal of the deep zones of the Earth; stony meteorites (analogues of the mantle) are also enriched in Chromium (2.7·10 -1%). Over 20 chromium minerals are known. Only chrome spinels (up to 54% Cr) are of industrial importance; in addition, chromium is contained in a number of other minerals that often accompany chromium ores, but are of no practical value in themselves (uvarovite, volkonskoite, kemerite, fuchsite).

Physical properties of Chromium. Chromium is a hard, heavy, refractory metal. Pure Chrome is plastic. Crystallizes in a body-centered lattice, a = 2.885Å (20 °C); at 1830°C, transformation into a modification with a face-centered lattice is possible, a = 3.69Å.

Atomic radius 1.27 Å; ionic radii Cr 2+ 0.83Å, Cr 3+ 0.64Å, Cr 6+ 0.52 Å. Density 7.19 g/cm 3 ; t pl 1890 °C; t kip 2480 °C. Specific heat capacity 0.461 kJ/(kg K) (25°C); thermal coefficient of linear expansion 8.24 10 -6 (at 20 °C); thermal conductivity coefficient 67 W/(m K) (20 °С); electrical resistivity 0.414 μm m (20 °C); the thermal coefficient of electrical resistance in the range of 20-600 °C is 3.01·10 -3 . Chromium is antiferromagnetic, specific magnetic susceptibility is 3.6·10 -6 . The hardness of high-purity Chromium according to Brinell is 7-9 MN / m 2 (70-90 kgf / cm 2).

Chemical properties of Chromium. The external electron configuration of the Chromium atom is 3d 5 4s 1 . In compounds, it usually exhibits oxidation states +2, +3, +6, among which Cr 3+ is the most stable; individual compounds are known in which Chromium has oxidation states +1, +4, +5. Chromium is chemically inactive. Under normal conditions, it is resistant to oxygen and moisture, but combines with fluorine, forming CrF 3 . Above 600 °C, it interacts with water vapor, giving Cr 2 O 3; nitrogen - Cr 2 N, CrN; carbon - Cr 23 C 6, Cr 7 C 3, Cr 3 C 2; gray - Cr 2 S 3. When fused with boron, it forms CrB boride; with silicon, it forms silicides Cr 3 Si, Cr 2 Si 3, CrSi 2. Chromium forms alloys with many metals. The interaction with oxygen proceeds at first quite actively, then it slows down sharply due to the formation of an oxide film on the metal surface. At 1200°C, the film breaks down and oxidation proceeds rapidly again. Chromium ignites in oxygen at 2000°C to form dark green chromium (III) oxide Cr 2 O 3 . In addition to the oxide (III), there are other compounds with oxygen, such as CrO, CrO 3 obtained indirectly. Chromium easily reacts with dilute solutions of hydrochloric and sulfuric acids to form chloride and chromium sulfate and release hydrogen; aqua regia and nitric acid passivate Chromium.

With an increase in the degree of oxidation, the acidic and oxidizing properties of Chromium increase. Cr 2+ derivatives are very strong reducing agents. The Cr 2+ ion is formed at the first stage of dissolution of Chromium in acids or during the reduction of Cr 3+ in an acidic solution with zinc. Nitrous hydrate Cr(OH) 2 during dehydration passes into Cr 2 O 3 . Cr 3+ compounds are stable in air. They can be both reducing and oxidizing agents. Cr 3+ can be reduced in an acidic solution with zinc to Cr 2+ or oxidized in an alkaline solution to CrO 4 2- with bromine and other oxidizing agents. Hydroxide Cr (OH) 3 (more precisely, Cr 2 O 3 nH 2 O) is an amphoteric compound that forms salts with the Cr 3+ cation or salts of chromic acid HCrO 2 - chromites (for example, KC-O 2, NaCrO 2). Cr 6+ compounds: CrO 3 chromic anhydride, chromic acids and their salts, among which the most important are chromates and dichromates - strong oxidizing agents. Chromium forms a large number of salts with oxygen-containing acids. Chromium complex compounds are known; complex compounds of Cr 3+ are especially numerous, in which Chromium has a coordination number of 6. There is a significant number of Chromium peroxide compounds

Get Chrome. Depending on the purpose of use, chromium is obtained in various degrees of purity. The raw material is usually chrome spinels, which are enriched and then fused with potash (or soda) in the presence of atmospheric oxygen. With regard to the main component of ores containing Cr 3 +, the reaction is as follows:

2FeCr 2 O 4 + 4K 2 CO 3 + 3.5O 2 \u003d 4K 2 CrO 4 + Fe 2 O 3 + 4CO 2.

The resulting potassium chromate K 2 CrO 4 is leached with hot water and the action of H 2 SO 4 converts it into dichromate K 2 Cr 2 O 7 . Further, by the action of a concentrated solution of H 2 SO 4 on K 2 Cr 2 O 7, chromic anhydride C 2 O 3 is obtained or by heating K 2 Cr 2 O 7 with sulfur - Chromium oxide (III) C 2 O 3.

The purest Chromium is obtained under industrial conditions either by electrolysis of concentrated aqueous solutions of CrO 3 or Cr 2 O 3 containing H 2 SO 4 , or by electrolysis of Chromium sulfate Cr 2 (SO 4) 3 . In this case, chromium is precipitated on an aluminum or stainless steel cathode. Complete purification from impurities is achieved by treating Chromium with highly pure hydrogen at high temperature (1500-1700 °C).

It is also possible to obtain pure Chromium by electrolysis of CrF 3 or CrCl 3 melts mixed with sodium, potassium, calcium fluorides at a temperature of about 900 °C in an argon atmosphere.

Chromium is obtained in small quantities by reduction of Cr 2 O 3 with aluminum or silicon. In the aluminothermic method, a preheated mixture of Cr 2 O 3 and Al powder or shavings with the addition of an oxidizing agent is loaded into a crucible, where the reaction is initiated by igniting a mixture of Na 2 O 2 and Al until the crucible is filled with Chromium and slag. Chromium is smelted silicothermally in arc furnaces. The purity of the resulting Chromium is determined by the content of impurities in Cr 2 O 3 and in Al or Si used for recovery.

In industry, chromium alloys are produced on a large scale - ferrochrome and silicochrome.

Chromium application. The use of Chromium is based on its heat resistance, hardness and corrosion resistance. Most of all Chromium is used for smelting chromium steels. Alumino- and silicothermic chromium is used for smelting nichrome, nimonic, other nickel alloys, and stellite.

A significant amount of Chromium is used for decorative corrosion-resistant coatings. Chromium powder has been widely used in the production of metal-ceramic products and materials for welding electrodes. Chromium in the form of the Cr 3+ ion is an impurity in ruby, which is used as a gemstone and laser material. Chromium compounds are used to etch fabrics during dyeing. Some Chromium salts are used as an ingredient in tanning solutions in the leather industry; PbCrO 4 , ZnCrO 4 , SrCrO 4 - as art paints. Chromite-magnesite refractory products are made from a mixture of chromite and magnesite.

Chromium compounds (especially Cr 6 + derivatives) are toxic.

Chromium in the body. Chromium is one of the biogenic elements that is constantly included in the tissues of plants and animals. The average content of Chromium in plants is 0.0005% (92-95% of Chromium accumulates in the roots), in animals - from ten thousandths to ten millionths of a percent. In planktonic organisms, the accumulation coefficient of Chromium is enormous - 10,000-26,000. Higher plants do not tolerate Chromium concentrations above 3-10 -4 mol/l. In leaves, it is present as a low molecular weight complex not associated with subcellular structures. In animals, chromium is involved in the metabolism of lipids, proteins (part of the trypsin enzyme), carbohydrates (a structural component of the glucose-resistant factor). The main source of Chromium in the body of animals and humans is food. A decrease in the content of Chromium in food and blood leads to a decrease in growth rate, an increase in blood cholesterol and a decrease in the sensitivity of peripheral tissues to insulin.

Chromium poisoning and its compounds occur during their production; in mechanical engineering (electroplated coatings); metallurgy (alloying additives, alloys, refractories); in the manufacture of leather, paints, etc. The toxicity of chromium compounds depends on their chemical structure: dichromates are more toxic than chromates, Cr (VI) compounds are more toxic than Cr (II), Cr (III) compounds. The initial forms of the disease are manifested by a feeling of dryness and pain in the nose, sore throat, difficulty breathing, coughing, etc.; they may disappear when contact with Chrome is discontinued. With prolonged contact with Chromium compounds, signs of chronic poisoning develop: headache, weakness, dyspepsia, weight loss, and others. Functions of a stomach, a liver and a pancreas are broken. Bronchitis, bronchial asthma, diffuse pneumosclerosis are possible. When exposed to Chromium, dermatitis and eczema may develop on the skin. According to some reports, Chromium compounds, mainly Cr(III), have a carcinogenic effect.

Chromium is a trace mineral that is used in various forms. In bioadditives, this is usually its chloride or picolinate (salt better absorbed by the intestines). The complex present in yeast, known as the glucose tolerance factor and including chromium, and three amino acids - glutamine, glycine and cysteine, is well absorbed.

Useful properties of chromium and role in the body

Chromium is essential for insulin to work. This hormone is responsible for transporting glucose from the blood to the cells, where it is “burned” to release energy. Insulin is effective and helps maintain normal blood sugar levels only if the body has enough chromium. This metal increases the number of insulin receptors on the cell membrane. By increasing our glucose tolerance (the ability to tolerate its consumption without negative health consequences) by increasing the effectiveness of insulin, chromium inhibits its production, and as a result inhibits the conversion of sugar into fats. This leads to a decrease in blood levels of cholesterol (especially "bad", i.e., low-density lipoproteins) and triglycerides.

Prevention

Chromium supplements reduce the risk of diabetes in insulin resistant people. They produce enough insulin, but the sensitivity of the cells to it is reduced. As a result, to maintain normal blood glucose levels, the pancreas has to secrete increased amounts of this enzyme. However, even they may not be enough, and then type II diabetes (non-insulin dependent) develops with excess blood sugar, which is usually accompanied by obesity and hypercholesterolemia (high cholesterol) with all the ensuing consequences. This risk is reduced by the prophylactic use of chromium, which reduces insulin resistance and thus increases glucose tolerance.

The benefits of chromium

Stress, infection, increased physical activity accelerate the “burning” of glucose, and as a result, the mobilization of chromium, which is more intensively excreted in the urine. The same is observed in hyperglycemic exacerbations in diabetic patients. The intake of chromium from food is usually barely up to standard, so in such situations it is advisable to take its supplements.

Indications and uses of chromium, recommended daily allowance, contraindications, food sources of chromium

There are no recommended daily allowances for chromium, but it is believed that chromium deficiency in adults can be prevented by doses of 50 to 200 micrograms per day. It should be noted that even with a varied, healthy diet, getting 200 micrograms of chromium per day from food is almost impossible. The standard menu usually gives us 40-50 mcg / day, and a starvation diet (for example, when losing weight), of course, less.

- Flaw. Chromium deficiency is fraught with irritability, weight gain and impaired sensitivity of the limbs, as well as exacerbation of non-insulin dependent diabetes.

Excess. Chromium supplements appear to be harmless. However, their high doses make it difficult to digest and.

Indications for the use of chromium

Difficulty digesting proteins, fats or carbohydrates.

Elevated blood glucose levels (insulin resistance, type II diabetes).

Elevated blood levels of "bad" cholesterol (low-density lipoprotein) and triglycerides.

Contraindications

Diabetic patients should only take chromium after consulting their doctor. They may need to adjust their doses of insulin and/or other medications already prescribed for their illness.

Application methods

Doses

Usually, chromium in additives is combined with other minerals, so it is necessary to specify its amount in the preparation according to the inscription on the package. In one tablet or capsule it should be from 25 to 200 mcg (more is dangerous). Such dietary supplements are taken as a general tonic, as well as when losing weight with a starvation diet and to increase the effectiveness of insulin.

"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 \u003d 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.

Chromium compounds have been synthesized 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.

• Designation - Cr (Chromium);
• Period - IV;
• Group - 6 (VIb);
• Atomic mass - 51.9961;
• Atomic number - 24;
• Radius of an atom = 130 pm;
• Covalent radius = 118 pm;
• Electron distribution - 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 ;
• melting point = 1857°C;
• boiling point = 2672°C;
• Electronegativity (according to Pauling / according to Alpred and Rochov) = 1.66 / 1.56;
• Oxidation state: +6, +3, +2, 0;
• Density (n.a.) \u003d 7.19 g / cm 3;
• Molar volume = 7.23 cm 3 / mol.

Chromium (color, paint) was first found at the Berezovsky gold deposit (Middle Urals), the first mentions date back to 1763, in his work "The First Foundations of Metallurgy" M.V. Lomonosov calls it "red lead ore".

Rice. The structure of the chromium atom.

The electronic configuration of the chromium atom is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 (see Electronic structure of atoms). In the formation of chemical bonds with other elements, 1 electron located at the outer 4s level + 5 electrons of the 3d sublevel (6 electrons in total) can participate, therefore, in compounds, chromium can take oxidation states from +6 to +1 (the most common are +6 , +3, +2). Chromium is a chemically inactive metal, it reacts with simple substances only at high temperatures.

Physical properties of chromium:

• bluish-white metal;
• very hard metal (in the presence of impurities);
• fragile at n. y.;
• plastic (in its pure form).

Chemical properties of chromium

• at t=300°C it reacts with oxygen:
  4Cr + 3O 2 \u003d 2Cr 2 O 3;
• at t>300°C it reacts with halogens, forming mixtures of halides;
• at t>400°C it reacts with sulfur to form sulfides:
  Cr + S = CrS;
• at t=1000°C, finely ground chromium reacts with nitrogen to form chromium nitride (a semiconductor with high chemical resistance):
  2Cr + N 2 = 2CrN;
• reacts with dilute hydrochloric and sulfuric acids to release hydrogen:
  Cr + 2HCl \u003d CrCl 2 + H 2;
  Cr + H 2 SO 4 \u003d CrSO 4 + H 2;
• warm concentrated nitric and sulfuric acids dissolve chromium.

With concentrated sulfuric and nitric acid at n.o. chromium does not interact, chromium also does not dissolve in aqua regia, it is noteworthy that pure chromium does not react even with dilute sulfuric acid, the reason for this phenomenon has not yet been established. During long-term storage in concentrated nitric acid, chromium is covered with a very dense oxide film (passivated), and ceases to react with dilute acids.

Chromium compounds

It has already been said above that the "favorite" oxidation states of chromium are +2 (CrO, Cr (OH) 2), +3 (Cr 2 O 3, Cr (OH) 3), +6 (CrO 3, H 2 CrO 4 ).

Chrome is chromophore, i.e., an element that gives color to the substance in which it is contained. For example, in the +3 oxidation state, chromium gives a lilac-red or green color (ruby, spinel, emerald, garnet); in oxidation state +6 - yellow-orange color (crocoite).

Chromophores, in addition to chromium, are also iron, nickel, titanium, vanadium, manganese, cobalt, copper - all these are d-elements.

The color of common compounds that include chromium:

• chromium in oxidation state +2:
  • chromium oxide CrO - red;
  • chromium fluoride CrF 2 - blue-green;
  • chromium chloride CrCl 2 - has no color;
  • chromium bromide CrBr 2 - has no color;
  • chromium iodide CrI 2 - red-brown.
• chromium in oxidation state +3:
  • Cr 2 O 3 - green;
  • CrF 3 - light green;
  • CrCl 3 - violet-red;
  • CrBr 3 - dark green;
  • CrI 3 - black.
• chromium in oxidation state +6:
  • CrO 3 - red;
  • potassium chromate K 2 CrO 4 - lemon yellow;
  • ammonium chromate (NH 4) 2 CrO 4 - golden yellow;
  • calcium chromate CaCrO 4 - yellow;
  • lead chromate PbCrO 4 - light brown-yellow.

Chromium oxides:

• Cr +2 O - basic oxide;
• Cr 2 +3 O 3 - amphoteric oxide;
• Cr +6 O 3 - acid oxide.

Chromium hydroxides:

• ".

  Application of chromium

  • as a alloying additive in the smelting of heat-resistant and corrosion-resistant alloys;
  • for chrome plating of metal products in order to give them high corrosion resistance, abrasion resistance and a beautiful appearance;
  • chromium-30 and chromium-90 alloys are used in plasma torch nozzles and in the aviation industry.

The content of the article

CHROMIUM– (Chromium) Cr, chemical element 6(VIb) of group of the Periodic system. Atomic number 24, atomic mass 51.996. There are 24 known isotopes of chromium from 42 Cr to 66 Cr. Isotopes 52 Cr, 53 Cr, 54 Cr are stable. The isotopic composition of natural chromium: 50 Cr (half-life 1.8 10 17 years) - 4.345%, 52 Cr - 83.489%, 53 Cr - 9.501%, 54 Cr - 2.365%. The main oxidation states are +3 and +6.

In 1761, a professor of chemistry at St. Petersburg University, Johann Gottlob Lehmann, at the eastern foot of the Ural Mountains at the Berezovsky mine, discovered a remarkable red mineral, which, when crushed into powder, gave a bright yellow color. In 1766 Leman brought samples of the mineral to St. Petersburg. After treating the crystals with hydrochloric acid, he obtained a white precipitate, in which he found lead. Leman called the mineral Siberian red lead (plomb rouge de Sibérie), now it is known that it was crocoite (from the Greek "krokos" - saffron) - natural lead chromate PbCrO 4.

The German traveler and naturalist Peter Simon Pallas (1741-1811) led the expedition of the St. Petersburg Academy of Sciences to the central regions of Russia and in 1770 visited the Southern and Middle Urals, including the Berezovsky mine and, like Lehman, became interested in crocoite. Pallas wrote: “This amazing red lead mineral is not found in any other deposit. Turns yellow when ground into powder and can be used in miniature art. Despite the rarity and difficulty of delivering crocoite from the Berezovsky mine to Europe (it took almost two years), the use of the mineral as a coloring matter was appreciated. In London and Paris at the end of the 17th century. all noble persons rode in carriages painted with finely ground crocoite, in addition, the best samples of Siberian red lead were added to the collections of many mineralogical cabinets in Europe.

In 1796, a sample of crocoite came to Nicolas-Louis Vauquelin (1763–1829), professor of chemistry at the Paris Mineralogical School, who analyzed the mineral, but found nothing in it except oxides of lead, iron, and aluminum. Continuing the study of Siberian red lead, Vauquelin boiled the mineral with a solution of potash and, after separating the white precipitate of lead carbonate, obtained a yellow solution of an unknown salt. When it was treated with a lead salt, a yellow precipitate formed, with a mercury salt, a red one, and when tin chloride was added, the solution turned green. Decomposing crocoite with mineral acids, he obtained a solution of "red lead acid", the evaporation of which gave ruby-red crystals (it is now clear that it was chromic anhydride). Having calcined them with coal in a graphite crucible, after the reaction, he discovered a lot of intergrown gray needle-shaped crystals of a metal unknown until that time. Vauquelin stated the high refractoriness of the metal and its resistance to acids.

Vauquelin called the new element chromium (from the Greek crwma - color, color) in view of the many multi-colored compounds formed by it. Based on his research, Vauquelin stated for the first time that the emerald color of some precious stones is due to the admixture of chromium compounds in them. For example, natural emerald is a deep green colored beryl in which aluminum is partially replaced by chromium.

Most likely, Vauquelin obtained not a pure metal, but its carbides, as evidenced by the acicular shape of the crystals obtained, but the Paris Academy of Sciences nevertheless registered the discovery of a new element, and now Vauquelin is rightly considered the discoverer of element No. 24.

Yuri Krutyakov