Chromium(III) Sulfate Composition and Molar Mass. Chromium - a general characteristic of the element, the chemical properties of chromium and its compounds Finding in nature

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CHROMIUM– (Chromium) Cr, chemical element 6(VIb) groups 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 Siberian red lead replenished 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 this 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 pure metal, but its carbides, as evidenced by the needle-like 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

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. Average content of Chromium in earth's crust(clarke) 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; associated with education largest deposits Chrome. 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 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 big number salts with oxygenated 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.

A hard bluish-white metal. Chromium is sometimes referred to as a ferrous metal. This metal is capable of painting compounds in different colors, which is why it was called "chromium", which means "paint". Chromium is a microelement necessary for the normal development and functioning of the human body. Its most important biological role consists in the regulation of carbohydrate metabolism and blood glucose levels.

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STRUCTURE

Depending on the types chemical bond- like all metals, chromium has a metallic type of crystal lattice, that is, there are metal atoms at the lattice nodes.
Depending on the spatial symmetry - cubic, body-centered a = 0.28839 nm. A feature of chromium is a sharp change in its physical properties at a temperature of about 37°C. Crystal cell A metal is made up of its ions and mobile electrons. Similarly, the chromium atom in the ground state has an electronic configuration. At 1830°C, transformation into a modification with a face-centered lattice is possible, a = 3.69Å.

PROPERTIES

Chromium has a Mohs hardness of 9, one of the hardest pure metals (second only to iridium, beryllium, tungsten and uranium). Very pure chrome can be machined fairly well. 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. When heated, it reacts with many non-metals, often forming compounds of non-stoichiometric composition - carbides, borides, silicides, nitrides, etc. Chromium forms numerous compounds in various oxidation states, mainly +2, +3, +6. Chromium has all the characteristic properties of metals - it conducts heat well, electricity, has a luster inherent in most metals. It is an antiferromagnet and a paramagnet, that is, at a temperature of 39 ° C it changes from a paramagnetic state to an antiferromagnetic state (Néel point).

RESERVES AND PRODUCTION

The largest deposits of chromium are in South Africa (1st place in the world), Kazakhstan, Russia, Zimbabwe, Madagascar. There are also deposits in Turkey, India, Armenia, Brazil, and the Philippines. The main deposits of chromium ores in the Russian Federation are known in the Urals (Donskoye and Saranovskoye). Explored reserves in Kazakhstan are over 350 million tons (2nd place in the world).Chromium occurs in nature mainly in the form of chromium iron ore Fe(CrO 2) 2 (iron chromite). Ferrochromium is obtained from it by reduction in electric furnaces with coke (carbon). To obtain pure chromium, the reaction is carried out as follows:
1) iron chromite is fused with sodium carbonate (soda ash) in air;
2) dissolve sodium chromate and separate it from iron oxide;
3) convert chromate to dichromate by acidifying the solution and crystallizing the dichromate;
4) pure chromium oxide is obtained by reduction of sodium dichromate with charcoal;
5) with the help of aluminothermy, metallic chromium is obtained;
6) using electrolysis, electrolytic chromium is obtained from a solution of chromic anhydride in water containing the addition of sulfuric acid.

ORIGIN

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 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 relatively weak water migrant; Chromium content in sea water is 0.00005 mg/l.
In general, Chromium is the 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).
There are three main chromium minerals: magnochromite (Mg, Fe)Cr 2 O 4 , chrompicotite (Mg, Fe) (Cr, Al) 2 O 4 and aluminochromite (Fe, Mg) (Cr, Al) 2 O 4 . By appearance they are indistinguishable and are inaccurately referred to as "chromites".

APPLICATION

Chromium is an important component in many alloyed steels (in particular, stainless steels), as well as in a number of other alloys. The addition of chromium significantly increases the hardness and corrosion resistance of the alloys. 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.
It is used as wear-resistant and beautiful galvanic coatings (chrome plating).
Chromium is used for the production of alloys: chromium-30 and chromium-90, indispensable for the production of high-power plasma torch nozzles and in the aerospace industry.

Chromium - Cr

The discovery of chromium belongs to the period of rapid development of chemical-analytical studies of salts and minerals. In Russia, chemists showed a particular 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 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 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.

1. We strongly heat in the flame of a burner on a porcelain cup such an amount of potassium bichromate 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 .
2. 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 bichromate. If we now add a 50% solution of sulfuric acid in portions, then the red-yellow color of the bichromate will appear again.
3. 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.
4. 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.
5. 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.

1. 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?
2. 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 ?
3. 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?
4. 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.

1. 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.
2. When CrO 4 2- and Ba 2+ ions interact, a yellow precipitate of barium chromate BaCrO 4 precipitates.
3. Silver nitrate forms brick red precipitate of silver chromate with CrO 4 2 ions.
4. 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

1. 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 an acid is added, and turns green in a neutral or alkaline medium.
2. 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.
3. 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 compressed 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.
4. 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.
5. 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). The variability of the composition of the mineral found in the Urals, in the Perm and Kirov regions determines its diverse coloration - from the color of a darkened winter fir to the bright green color of a swamp 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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Chromium, - a chemical element, a solid silvery metal with atomic number 24. For the bright colors characteristic of salts, chromium received the name - χρώμα (Greek color, paint).

Biological action

Chromium plays a leading role in glucose metabolism:

• it is necessary for the processing of glucose (it is an active component of GTF - glucose tolerance factor);
• improves insulin sensitivity;
• reduces cravings for sweets;
• lowers blood sugar levels in people with type 1 and type 2 diabetes;
• is a catalyst for the synthesis of certain proteins, necessary for muscle growth;
• participating in the metabolism of fats, regulates the level of "bad" cholesterol in the blood;
• promotes weight loss.

Table 1. Physiological requirements for chromium depending on age

Higher doses of chromium are needed for increased metabolism, such as in athletes.

Chromium Sources

Yeast, liver, meat, brown rice, whole grains, corn, eggs, tomatoes, oatmeal, iceberg lettuce, mushrooms, cheese. These foods are the richest in chromium (in descending order), but it should be borne in mind that it is contained in microdoses and the average diet barely reaches the minimum requirements for this mineral.

Absorption of chromium reduces iron levels.

Chromium deficiency

Chromium deficiency is observed when eating foods depleted of this trace element, and the level of chromium in the body decreases in old age.

It should also be taken into account that the absorption of chromium in the intestine is low, even from modern complexes with chromium, where it is in the most assimilable form (chromium picolinate, an amino acid complex with chromium), absorption is 1.5-3%.

Chromium deficiency results to decrease glucose tolerance, decrease in growth rate increase the risk of developing diabetes, coronary disease heart, hypercholesterolemia (increased blood cholesterol levels), hyperglycemia and hypoglycemia (changes in sugar levels).

Most suitable for deficiency elimination chromium picolinate, at the same time, it is necessary to reduce the amount of light carbohydrates consumed (sugar, etc.). Chromium chloride (CrCl2) is practically useless for this purpose, due to the very low absorption of chromium from this form.

Long-term use of chromium-containing drugs for prophylactic purposes (in the absence of deficiency) increases the load on the body, which is fraught with activation of mutagenesis.

Excess chromium in the body

An excess of chromium among Russians is a fairly common phenomenon, but it is caused by hexavalent chromium, a known carcinogen used in the metallurgical and textile industries. Hexavalent chromium compounds cause allergic reactions (dermatitis), increase the risk of lung cancer.

Chromium in food has a trivalent form, which is safe for the body.

mineral-details

Trivalent chromium is part of alum, which has long been used in dressing hides, dyeing fabrics, in our time, alum has found application as a cauterizing agent "alum pencil", as a deodorant-antiperspirant, is part of cosmetics, etc.

Getting a normal amount of chromium (in accordance with physiological needs) by dieters allows you to reduce "fat deposits" while maintaining muscle.