Chromium color compounds. Elective course "Chromium and Its Compounds". Indications and methods of use of chromium, recommended daily intake, contraindications, food sources of chromium
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The discovery of chromium dates back to a period of rapid development of chemical and 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 Siberian red lead ore (crocoite), described by Lomonosov. The mineral was examined, but nothing but oxides of lead, iron and aluminum were found in it. However, in 1797, Vaukelin, boiling a finely ground sample of the mineral with potash and precipitating lead carbonate, obtained a solution colored orange-red. From this solution he crystallized a ruby-red salt, from which the oxide and free metal, different from all known metals, were isolated. Vauquelin called him Chromium ( Chrome ) from the Greek word- coloring, color; True, what was meant here was not the property of the metal, but its brightly colored salts.
Being 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, the Urals, North America, and 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 has been found in the Sun, stars, and meteorites.
Physical properties.
Chrome is a white, hard and brittle metal, extremely chemically resistant to acids and alkalis. In air it oxidizes and has a thin transparent film of oxide on the surface. Chromium has a density of 7.1 g/cm3, its melting point is +1875 0 C.
Receipt.
When chromium iron ore is heated strongly with coal, chromium and iron are reduced:
FeO * Cr 2 O 3 + 4C = 2Cr + Fe + 4CO
As a result of this reaction, a chromium-iron alloy 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 = Al 2 O 3 + 2Cr
In this process, two oxides are usually used - Cr 2 O 3 and CrO 3
Chemical properties.
Thanks to the thin protective film of oxide covering the surface of chrome, 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 = Cr 2 (SO 4) 3 + 3H 2
2Cr + 6HCl = 2CrCl3 + 3H2
At high temperatures, chromium burns in oxygen, forming oxide(III).
Hot chromium reacts with water vapor:
2Cr + 3H 2 O = Cr 2 O 3 + 3H 2
At high temperatures, chromium also reacts with halogens, halogen with hydrogen, sulfur, nitrogen, phosphorus, carbon, silicon, boron, for example:
Cr + 2HF = CrF 2 + H 2
2Cr + N2 = 2CrN
2Cr + 3S = Cr 2 S 3
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 produce high-strength, corrosion-resistant coatings in mechanical engineering. Alloys in the form of ferrochrome are used as metal-cutting tools. Chrome alloys have found application in medical technology and in the manufacture of chemical technological equipment.
Position of chromium in the periodic table of chemical elements:
Chromium heads the secondary subgroup of group VI of the periodic table 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 in the chromium atom, the pattern according to which the 4S orbital should first be filled to the 4S 2 state is violated. However, due to the fact that the 3d orbital occupies a more favorable energy position in the chromium atom, it is filled to the value 4d 5 . This phenomenon is observed in atoms of some other elements of secondary subgroups. Chromium can exhibit oxidation states from +1 to +6. The most stable are chromium compounds with oxidation states +2, +3, +6.
Compounds of divalent chromium.
Chromium (II) oxide CrO is a pyrophoric black powder (pyrophoricity - the ability to ignite in air in a finely crushed 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 when chromium metal is dissolved in acids. These reactions take place in an atmosphere of low-active gas (for example H 2), because in the presence of air, oxidation of Cr(II) to Cr(III) easily occurs.
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 and is a reducing agent. The hydrated Cr2+ ion is pale blue. An aqueous solution of CrCl 2 is blue in color. In air in aqueous solutions, Cr(II) compounds transform into Cr(III) compounds. This is especially pronounced in Cr(II) hydroxide:
4Cr(OH) 2 + 2H 2 O + O 2 = 4Cr(OH) 3
Trivalent chromium compounds.
Chromium (III) oxide Cr 2 O 3 is a refractory green powder. Its hardness is close to corundum. In the laboratory it can be obtained by heating ammonium dichromate:
(NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 + 4H 2
Cr 2 O 3 is an amphoteric oxide, when fused with alkalis it forms chromites: Cr 2 O 3 + 2NaOH = 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 (III) sulfate Cr 2 (SO 4) 3 is pink in color and is 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 containing less water are also known. Chromium alum KCr(SO 4) 2 *12H 2 O crystallizes from solutions containing violet chromium sulfate and potassium sulfate. A solution of chrome 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 with a high degree of probability conclude that chromium is absent.
- Let us 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. The salt will not release water of crystallization, but will melt at a temperature of about 400 0 C to form a dark liquid. Let's heat it for a few more minutes over high heat. After cooling, a green precipitate forms on the shard. Let's dissolve part of it in water (it turns yellow), and leave the other part 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 bichromate in 50ml of water. Add a little potassium carbonate to one part. 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 you now add a 50% sulfuric acid solution in portions, the red-yellow color of the dichromate will appear again.
- Pour 5 ml into a test tube. potassium bichromate solution, boil with 3 ml of concentrated hydrochloric acid under pressure. Yellow-green toxic chlorine gas is released from the solution because the 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, fused with soda and saltpeter, converted into 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 the potassium dichromate solution and acidify the solution with sulfuric acid. The solution acquires a deep blue color due to the formation of chromium peroxide. When shaken with a certain amount of ether, the peroxide will transform into an organic solvent and color 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, you need to dissolve the metal. During prolonged boiling with 30% sulfuric acid (you can also add hydrochloric acid), chromium and many steels are partially dissolved. The resulting solution contains chromium (III) sulfate. To be able to carry out a detection reaction, we first neutralize it with caustic soda. Gray-green chromium(III) hydroxide precipitates, which dissolves in excess NaOH to form green sodium chromite. Filter the solution and add 30% hydrogen peroxide. When heated, the solution will turn yellow as chromite oxidizes to chromate. Acidification will cause the solution to appear blue. The colored compound can be extracted by shaking with ether.
Analytical reactions for chromium ions.
- Add a 2M NaOH solution to 3-4 drops of chromium chloride solution CrCl 3 until the initial precipitate dissolves. Note the color of the sodium chromite formed. Heat the resulting solution in a water bath. What happens?
- To 2-3 drops of CrCl 3 solution, add an equal volume of 8 M NaOH solution and 3-4 drops of 3% H 2 O 2 solution. Heat the reaction mixture in a water bath. What happens? 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 the test tube. Heat the reaction mixture for several minutes in 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 emerging blue color of the solution 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 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- interacts with Ba 2+ ions, a yellow precipitate of barium chromate BaCrO 4 precipitates.
- Silver nitrate forms a brick-red silver chromate precipitate with CrO 4 2 ions.
- Take three test tubes. Place 5-6 drops of K 2 Cr 2 O 7 solution into one of them, the same volume of K 2 CrO 4 solution into the second, and three drops of both solutions into the third. Then add three drops of potassium iodide solution to each test tube. Explain your result. Acidify the solution in the second test tube. What happens? 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, after filtration a bright green solution is obtained, which turns yellow when acid is added, and turns green in a neutral or alkaline environment.
- Place a “ruby mixture” in the center of a tin can with thermite - carefully ground and placed in aluminum foil Al 2 O 3 (4.75 g) with the addition of Cr 2 O 3 (0.25 g). To prevent the jar from cooling down longer, it is necessary to bury it under the top edge in sand, and after the thermite is set on fire and the reaction begins, cover it with an iron sheet and cover it with sand. Dig out the jar in a day. The result is a red ruby powder.
- 10 g of potassium dichromate is ground 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 at the end, 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 extends 10 times.
- If you mix solutions of copper sulfate and potassium dichromate and add a little ammonia solution, an amorphous brown precipitate of the composition 4СuCrO 4 * 3NH 3 * 5H 2 O will form, which dissolves in hydrochloric acid to form a yellow solution, and in excess of ammonia a green solution is obtained. If you further add alcohol 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 remaining after the “volcano” or “pharaoh’s snakes” experiments can be regenerated. To do this, you need 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. The result is a soluble chromate, which can 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 = 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
Cr 2+
a) CrO + 2HCl = CrCl 2 + H 2 O
b) CrO + H 2 O = 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) 4Сr(NO 3) 3 = 2Cr 2 O 3 + 12NO 2 + O 2
e) Cr 2 O 3 + 2 NaOH = 2NaCrO 2 + H 2 O
Chromium 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 producing many painting 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 hydrous chromium(III) oxide. It began to be produced under the name “emerald green” in 1837. Later, L. Vauquelin proposed several new paints: barite, zinc and chrome yellow. Over time, they were replaced by more persistent yellow and orange cadmium-based pigments.
Green chrome is the most durable and light-resistant paint that is not susceptible to atmospheric gases. Chromium green ground in oil has great covering power and is capable of drying quickly, which is why it has been used 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 lightly fired product, which is then covered with a layer of glaze. This is followed by the main, high-temperature firing: to sinter the porcelain mass and melt 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 chrome. Black cobalt oxide applied to the surface of a porcelain product fuses with the glaze during firing, chemically interacting with it. As a result, bright blue cobalt silicates are formed. Everyone knows this cobalt-decorated blue porcelain tableware well. Chromium (III) oxide does not react chemically with the components of the glaze and simply lies between the porcelain shards and the transparent glaze as a “blind” layer.
In addition to chrome green, artists use paints obtained 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 in honor of 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 aluminum and iron (III) oxides. The composition of the mineral, found in the Urals, Perm and Kirov regions, is inconsistent. determines its varied color - from the color of winter darkened 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 produces paint of a uniquely fresh tone. Currently, a method for producing artificial volkonskoite has been developed. It is interesting to note that, according to modern research, Russian icon painters used paints from this material back in the Middle Ages, long before its “official” discovery. Guinier greens (created in 1837), the chromoform of which is chromium oxide hydrate Cr 2 O 3 * (2-3) H 2 O, where part of the water is chemically bound and part is adsorbed, was also famously popular among artists. This pigment gives the paint an emerald hue.
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Chromium is a chemical element with atomic number 24. It is a hard, shiny, steel-gray metal that polishes well and does not tarnish. Used in alloys such as stainless steel and as a coating. The human body requires small amounts of trivalent chromium to metabolize sugar, but Cr(VI) is highly toxic.
Various chromium compounds, such as chromium(III) oxide and lead chromate, are brightly colored and used in paints and pigments. The red color of ruby is due to the presence of this chemical element. Some substances, especially sodium, are oxidizing agents used to oxidize organic compounds and (together with sulfuric acid) to clean laboratory glassware. In addition, chromium (VI) oxide is used in the production of magnetic tape.
Discovery and etymology
The history of the discovery of the chemical element chromium is as follows. In 1761, Johann Gottlob Lehmann found an orange-red mineral in the Ural Mountains and named it “Siberian red lead.” Although it was erroneously identified as a compound of lead with selenium and iron, the material was actually lead chromate with the chemical formula PbCrO 4 . Today it is known as the mineral croconte.
In 1770, Peter Simon Pallas visited the site where Lehmann found the red lead mineral, which had very useful properties as a pigment in paints. The use of Siberian red lead as paint developed rapidly. In addition, the bright yellow color of crocont has become fashionable.
In 1797, Nicolas-Louis Vauquelin obtained samples of red. By mixing croconte with hydrochloric acid, he obtained CrO 3 oxide. Chromium was isolated as a chemical element in 1798. Vauquelin obtained it by heating the oxide with charcoal. He was also able to detect traces of chromium in gemstones such as ruby and emerald.
In the 1800s, Cr was primarily used in dyes and tanning salts. Today, 85% of the metal is used in alloys. The remainder is used in the chemical, refractory and foundry industries.
The pronunciation of the chemical element chromium corresponds to the Greek χρῶμα, meaning "color", due to the variety of colored compounds that can be obtained from it.
Mining and production
The element is produced from chromite (FeCr 2 O 4). About half of the world's ore is mined in South Africa. In addition, Kazakhstan, India and Türkiye are its major producers. There are enough explored deposits of chromite, but geographically they are concentrated in Kazakhstan and southern Africa.
Deposits of native chromium metal are rare, but they do exist. For example, it is mined at the Udachnaya mine in Russia. It is rich in diamonds, and the reducing environment helped produce pure chromium and diamonds.
For industrial metal production, chromite ores are treated with molten alkali (caustic soda, NaOH). In this case, sodium chromate (Na 2 CrO 4) is formed, which is reduced by carbon to the oxide Cr 2 O 3. The metal is produced by heating the oxide in the presence of aluminum or silicon.
In 2000, approximately 15 million tons of chromite ore were mined and processed into 4 million tons of ferrochrome, a 70% chromium-iron alloy, with an approximate market value of US$2.5 billion.
Main characteristics
The characteristics of the chemical element chromium are due to the fact that it is a transition metal of the fourth period of the periodic table and is located between vanadium and manganese. Included in group VI. Melts at a temperature of 1907 °C. In the presence of oxygen, chromium quickly forms a thin layer of oxide, which protects the metal from further interaction with oxygen.
As a transition element, it reacts with substances in different proportions. Thus, it forms compounds in which it has different oxidation states. Chromium is a chemical element with the basic states +2, +3 and +6, of which +3 is the most stable. In addition, in rare cases conditions +1, +4 and +5 are observed. Chromium compounds in the +6 oxidation state are strong oxidizing agents.
What color is chrome? The chemical element gives the ruby hue. The Cr 2 O 3 used for is also used as a pigment called chrome green. Its salts color glass emerald green. Chromium is the chemical element whose presence makes rubies red. Therefore, it is used in the production of synthetic rubies.
Isotopes
Isotopes of chromium have atomic weights ranging from 43 to 67. Typically, this chemical element consists of three stable forms: 52 Cr, 53 Cr and 54 Cr. Of these, 52 Cr is the most common (83.8% of all natural chromium). In addition, 19 radioisotopes have been described, of which the most stable is 50 Cr with a half-life exceeding 1.8x10 17 years. 51 Cr has a half-life of 27.7 days, and for all other radioactive isotopes it does not exceed 24 hours, and for most of them it lasts less than one minute. The element also has two meta states.
Isotopes of chromium in the earth's crust, as a rule, accompany isotopes of manganese, which is used in geology. 53 Cr is formed during the radioactive decay of 53 Mn. The Mn/Cr isotope ratio reinforces other clues about the early history of the Solar System. Changes in the 53 Cr/ 52 Cr and Mn/Cr ratios from different meteorites prove that new atomic nuclei were created just before the formation of the Solar System.
Chemical element chromium: properties, formula of compounds
Chromium(III) oxide Cr 2 O 3, also known as sesquioxide, is one of the four oxides of this chemical element. It is obtained from chromite. The green color compound is commonly called "chrome green" when used as a pigment for enamel and glass painting. The oxide can dissolve in acids, forming salts, and in molten alkali - chromites.
Potassium dichromate
K 2 Cr 2 O 7 is a powerful oxidizing agent and is preferred as a means for cleaning laboratory glassware from organic matter. For this purpose, its saturated solution is used. Sometimes, however, it is replaced with sodium bichromate, based on the higher solubility of the latter. In addition, it can regulate the oxidation process of organic compounds, converting primary alcohol into aldehyde and then into carbon dioxide.
Potassium dichromate can cause chrome dermatitis. Chromium is likely to cause sensitization leading to the development of dermatitis, especially of the hands and forearms, which is chronic and difficult to cure. Like other Cr(VI) compounds, potassium dichromate is carcinogenic. It must be handled with gloves and appropriate protective equipment.
Chromic acid
The compound has the hypothetical structure H 2 CrO 4 . Neither chromic nor dichromic acids occur in nature, but their anions are found in various substances. The “chromic acid” that can be found on sale is actually its acid anhydride - CrO 3 trioxide.
Lead(II) chromate
PbCrO 4 has a bright yellow color and is practically insoluble in water. For this reason, it has found use as a coloring pigment called crown yellow.
Cr and pentavalent bond
Chromium is distinguished by its ability to form pentavalent bonds. The compound is created by Cr(I) and a hydrocarbon radical. A pentavalent bond is formed between two chromium atoms. Its formula can be written as Ar-Cr-Cr-Ar, where Ar represents a specific aromatic group.
Application
Chromium is a chemical element whose properties have given it many different uses, some of which are listed below.
It gives metals corrosion resistance and a glossy surface. Therefore, chromium is included in alloys such as stainless steel, used, for example, in cutlery. It is also used for chrome plating.
Chromium is a catalyst for various reactions. It is used to make molds for firing bricks. Its salts are used to tan leather. Potassium bichromate is used for the oxidation of organic compounds such as alcohols and aldehydes, as well as for cleaning laboratory glassware. It serves as a fixing agent for fabric dyeing and is also used in photography and photo printing.
CrO 3 is used to make magnetic tapes (for example, for audio recording), which have better characteristics than films with iron oxide.
Role in biology
Trivalent chromium is a chemical element necessary for the metabolism of sugar in the human body. In contrast, hexavalent Cr is highly toxic.
Precautionary measures
Chromium metal and Cr(III) compounds are generally not considered a health hazard, but substances containing Cr(VI) can be toxic if ingested or inhaled. Most of these substances are irritating to the eyes, skin and mucous membranes. With chronic exposure, chromium(VI) compounds can cause eye damage if not treated properly. In addition, it is a recognized carcinogen. The lethal dose of this chemical element is about half a teaspoon. According to the recommendations of the World Health Organization, the maximum permissible concentration of Cr (VI) in drinking water is 0.05 mg per liter.
Because chromium compounds are used in dyes and to tan leather, they are often found in soil and groundwater from abandoned industrial sites requiring environmental cleanup and remediation. Primer containing Cr(VI) is still widely used in the aerospace and automotive industries.
Element properties
The main physical properties of chromium are as follows:
- Atomic number: 24.
- Atomic weight: 51.996.
- Melting point: 1890 °C.
- Boiling point: 2482 °C.
- Oxidation state: +2, +3, +6.
- Electron configuration: 3d 5 4s 1.
The discovery of chromium dates back to a period of rapid development of chemical and 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 Siberian red lead ore (crocoite), described by Lomonosov. The mineral was examined, but nothing but oxides of lead, iron and aluminum were found in it. However, in 1797, Vaukelin, boiling a finely ground sample of the mineral with potash and precipitating lead carbonate, obtained a solution colored orange-red. From this solution he crystallized a ruby-red salt, from which the oxide and free metal, different from all known metals, were isolated. Vauquelin called him Chromium ( Chrome ) from the Greek word- coloring, color; True, what was meant here was not the property of the metal, but its brightly colored salts.
Being 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, the Urals, North America, and 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 has been found in the Sun, stars, and meteorites.
Physical properties.
Chrome is a white, hard and brittle metal, extremely chemically resistant to acids and alkalis. In air it oxidizes and has a thin transparent film of oxide on the surface. Chromium has a density of 7.1 g/cm3, its melting point is +1875 0 C.
Receipt.
When chromium iron ore is heated strongly with coal, chromium and iron are reduced:
FeO * Cr 2 O 3 + 4C = 2Cr + Fe + 4CO
As a result of this reaction, a chromium-iron alloy 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 = Al 2 O 3 + 2Cr
In this process, two oxides are usually used - Cr 2 O 3 and CrO 3
Chemical properties.
Thanks to the thin protective film of oxide covering the surface of chrome, 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 = Cr 2 (SO 4) 3 + 3H 2
2Cr + 6HCl = 2CrCl3 + 3H2
At high temperatures, chromium burns in oxygen, forming oxide(III).
Hot chromium reacts with water vapor:
2Cr + 3H 2 O = Cr 2 O 3 + 3H 2
At high temperatures, chromium also reacts with halogens, halogen with hydrogen, sulfur, nitrogen, phosphorus, carbon, silicon, boron, for example:
Cr + 2HF = CrF 2 + H 2
2Cr + N2 = 2CrN
2Cr + 3S = Cr 2 S 3
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 produce high-strength, corrosion-resistant coatings in mechanical engineering. Alloys in the form of ferrochrome are used as metal-cutting tools. Chrome alloys have found application in medical technology and in the manufacture of chemical technological equipment.
Position of chromium in the periodic table of chemical elements:
Chromium heads the secondary subgroup of group VI of the periodic table 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 in the chromium atom, the pattern according to which the 4S orbital should first be filled to the 4S 2 state is violated. However, due to the fact that the 3d orbital occupies a more favorable energy position in the chromium atom, it is filled to the value 4d 5 . This phenomenon is observed in atoms of some other elements of secondary subgroups. Chromium can exhibit oxidation states from +1 to +6. The most stable are chromium compounds with oxidation states +2, +3, +6.
Compounds of divalent chromium.
Chromium (II) oxide CrO is a pyrophoric black powder (pyrophoricity - the ability to ignite in air in a finely crushed 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 when chromium metal is dissolved in acids. These reactions take place in an atmosphere of low-active gas (for example H 2), because in the presence of air, oxidation of Cr(II) to Cr(III) easily occurs.
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 and is a reducing agent. The hydrated Cr2+ ion is pale blue. An aqueous solution of CrCl 2 is blue in color. In air in aqueous solutions, Cr(II) compounds transform into Cr(III) compounds. This is especially pronounced in Cr(II) hydroxide:
4Cr(OH) 2 + 2H 2 O + O 2 = 4Cr(OH) 3
Trivalent chromium compounds.
Chromium (III) oxide Cr 2 O 3 is a refractory green powder. Its hardness is close to corundum. In the laboratory it can be obtained by heating ammonium dichromate:
(NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 + 4H 2
Cr 2 O 3 is an amphoteric oxide, when fused with alkalis it forms chromites: Cr 2 O 3 + 2NaOH = 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 (III) sulfate Cr 2 (SO 4) 3 is pink in color and is 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 containing less water are also known. Chromium alum KCr(SO 4) 2 *12H 2 O crystallizes from solutions containing violet chromium sulfate and potassium sulfate. A solution of chrome 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 with a high degree of probability conclude that chromium is absent.
- Let us 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. The salt will not release water of crystallization, but will melt at a temperature of about 400 0 C to form a dark liquid. Let's heat it for a few more minutes over high heat. After cooling, a green precipitate forms on the shard. Let's dissolve part of it in water (it turns yellow), and leave the other part 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 bichromate in 50ml of water. Add a little potassium carbonate to one part. 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 you now add a 50% sulfuric acid solution in portions, the red-yellow color of the dichromate will appear again.
- Pour 5 ml into a test tube. potassium bichromate solution, boil with 3 ml of concentrated hydrochloric acid under pressure. Yellow-green toxic chlorine gas is released from the solution because the 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, fused with soda and saltpeter, converted into 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 the potassium dichromate solution and acidify the solution with sulfuric acid. The solution acquires a deep blue color due to the formation of chromium peroxide. When shaken with a certain amount of ether, the peroxide will transform into an organic solvent and color 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, you need to dissolve the metal. During prolonged boiling with 30% sulfuric acid (you can also add hydrochloric acid), chromium and many steels are partially dissolved. The resulting solution contains chromium (III) sulfate. To be able to carry out a detection reaction, we first neutralize it with caustic soda. Gray-green chromium(III) hydroxide precipitates, which dissolves in excess NaOH to form green sodium chromite. Filter the solution and add 30% hydrogen peroxide. When heated, the solution will turn yellow as chromite oxidizes to chromate. Acidification will cause the solution to appear blue. The colored compound can be extracted by shaking with ether.
Analytical reactions for chromium ions.
- Add a 2M NaOH solution to 3-4 drops of chromium chloride solution CrCl 3 until the initial precipitate dissolves. Note the color of the sodium chromite formed. Heat the resulting solution in a water bath. What happens?
- To 2-3 drops of CrCl 3 solution, add an equal volume of 8 M NaOH solution and 3-4 drops of 3% H 2 O 2 solution. Heat the reaction mixture in a water bath. What happens? 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 the test tube. Heat the reaction mixture for several minutes in 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 emerging blue color of the solution 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 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- interacts with Ba 2+ ions, a yellow precipitate of barium chromate BaCrO 4 precipitates.
- Silver nitrate forms a brick-red silver chromate precipitate with CrO 4 2 ions.
- Take three test tubes. Place 5-6 drops of K 2 Cr 2 O 7 solution into one of them, the same volume of K 2 CrO 4 solution into the second, and three drops of both solutions into the third. Then add three drops of potassium iodide solution to each test tube. Explain your result. Acidify the solution in the second test tube. What happens? 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, after filtration a bright green solution is obtained, which turns yellow when acid is added, and turns green in a neutral or alkaline environment.
- Place a “ruby mixture” in the center of a tin can with thermite - carefully ground and placed in aluminum foil Al 2 O 3 (4.75 g) with the addition of Cr 2 O 3 (0.25 g). To prevent the jar from cooling down longer, it is necessary to bury it under the top edge in sand, and after the thermite is set on fire and the reaction begins, cover it with an iron sheet and cover it with sand. Dig out the jar in a day. The result is a red ruby powder.
- 10 g of potassium dichromate is ground 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 at the end, 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 extends 10 times.
- If you mix solutions of copper sulfate and potassium dichromate and add a little ammonia solution, an amorphous brown precipitate of the composition 4СuCrO 4 * 3NH 3 * 5H 2 O will form, which dissolves in hydrochloric acid to form a yellow solution, and in excess of ammonia a green solution is obtained. If you further add alcohol 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 remaining after the “volcano” or “pharaoh’s snakes” experiments can be regenerated. To do this, you need 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. The result is a soluble chromate, which can 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 = 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
Cr 2+
a) CrO + 2HCl = CrCl 2 + H 2 O
b) CrO + H 2 O = 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) 4Сr(NO 3) 3 = 2Cr 2 O 3 + 12NO 2 + O 2
e) Cr 2 O 3 + 2 NaOH = 2NaCrO 2 + H 2 O
Chromium 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 producing many painting 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 hydrous chromium(III) oxide. It began to be produced under the name “emerald green” in 1837. Later, L. Vauquelin proposed several new paints: barite, zinc and chrome yellow. Over time, they were replaced by more persistent yellow and orange cadmium-based pigments.
Green chrome is the most durable and light-resistant paint that is not susceptible to atmospheric gases. Chromium green ground in oil has great covering power and is capable of drying quickly, which is why it has been used 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 lightly fired product, which is then covered with a layer of glaze. This is followed by the main, high-temperature firing: to sinter the porcelain mass and melt 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 chrome. Black cobalt oxide applied to the surface of a porcelain product fuses with the glaze during firing, chemically interacting with it. As a result, bright blue cobalt silicates are formed. Everyone knows this cobalt-decorated blue porcelain tableware well. Chromium (III) oxide does not react chemically with the components of the glaze and simply lies between the porcelain shards and the transparent glaze as a “blind” layer.
In addition to chrome green, artists use paints obtained 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 in honor of 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 aluminum and iron (III) oxides. The composition of the mineral, found in the Urals, Perm and Kirov regions, is inconsistent. determines its varied color - from the color of winter darkened 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 produces paint of a uniquely fresh tone. Currently, a method for producing artificial volkonskoite has been developed. It is interesting to note that, according to modern research, Russian icon painters used paints from this material back in the Middle Ages, long before its “official” discovery. Guinier greens (created in 1837), the chromoform of which is chromium oxide hydrate Cr 2 O 3 * (2-3) H 2 O, where part of the water is chemically bound and part is adsorbed, was also famously popular among artists. This pigment gives the paint an emerald hue.
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- Designation - Cr (Chromium);
- Period - IV;
- Group - 6 (VIb);
- Atomic mass - 51.9961;
- Atomic number - 24;
- Atomic radius = 130 pm;
- Covalent radius = 118 pm;
- Electron distribution - 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 ;
- melting temperature = 1857°C;
- boiling point = 2672°C;
- Electronegativity (according to Pauling/according to Alpred and Rochow) = 1.66/1.56;
- Oxidation state: +6, +3, +2, 0;
- Density (no.) = 7.19 g/cm3;
- 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. 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 on 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 when n. y.;
- plastic (in its pure form).
Chemical properties of chromium
- at t=300°C reacts with oxygen:
4Cr + 3O 2 = 2Cr 2 O 3; - at t>300°C reacts with halogens, forming mixtures of halides;
- at t>400°C reacts with sulfur to form sulfides:
Cr + S = CrS; - at t=1000°C finely ground chromium reacts with nitrogen, forming chromium nitride (a semiconductor with high chemical stability):
2Cr + N 2 = 2CrN; - reacts with dilute hydrochloric and sulfuric acids to release hydrogen:
Cr + 2HCl = CrCl 2 + H 2;
Cr + H 2 SO 4 = CrSO 4 + H 2; - warm concentrated nitric and sulfuric acids dissolve chromium.
With concentrated sulfuric and nitric acid at no. chromium does not react, and 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 becomes covered with a very dense oxide film (passivates) and stops reacting with dilute acids.
Chromium compounds
It was already 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 oxidation state +3, chromium gives a purple-red or green color (ruby, spinel, emerald, garnet); in the oxidation state +6 - yellow-orange color (crocoite).
In addition to chromium, chromophores also include 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 - acidic 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 article is devoted to element No. 24 of the periodic table - chromium, the history of its discovery and distribution in nature, the structure of its atom, chemical properties and compounds, how it is obtained and why we need it. The average chromium content in the earth's crust is not high: 0.0083%. This element is probably more characteristic of the Earth's mantle.
Chromium forms massive and disseminated ores in ultramafic rocks; The formation of the largest chromium deposits is associated with them. In basic rocks, the Chromium content reaches only 2·10-2%, in acidic rocks - 2.5·10-3%, in sedimentary rocks (sandstones) - 3.5·10-3%, in clay shales - 9·10-3 %. Chromium is a relatively weak water migrant: the Chromium content in sea water is 0.00005 mg/l, in surface water -0.0015 mg/l.
In general, chromium is a metal in the deep zones of the Earth.
Today, the total consumption of pure chromium (at least 99% Cr) is about 15 thousand tons, of which about a third is electrolytic chromium. The world leader in the production of high-purity chromium is the English company Bell Metals. The first place in terms of consumption volumes is occupied by the United States (50%), European countries second (25%), and Japan third. The market for chromium metal is quite volatile, and prices for the metal fluctuate widely.
1. CHROME AS A CHEMICAL ELEMENT
Chromium– (Chromium) Cr, chemical element 6(VIb) of group of the Periodic table. Atomic number 24, atomic mass 51.996. There are 24 known isotopes of chromium from 42 Cr to 66 Cr. The isotopes 52 Cr, 53 Cr, 54 Cr are stable. 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, chemistry professor at St. Petersburg University Johann Gottlob Lehmann, at the eastern foot of the Ural Mountains at the Berezovsky mine, discovered a wonderful red mineral, which, when crushed into powder, gave a bright yellow color. In 1766 Lehman brought samples of the mineral to St. Petersburg. Having treated the crystals with hydrochloric acid, he obtained a white precipitate, in which he discovered lead. Lehmann called the mineral Siberian red lead (plomb rouge de Sibérie); it is now known that it was crocoite (from the Greek “krokos” - saffron) - a natural lead chromate PbCrO 4.
The German traveler and naturalist Peter Simon Pallas (1741–1811) led an 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 Lehmann, became interested in crocoite. Pallas wrote: “This amazing red lead mineral is not found in any other deposit. When ground into powder it turns yellow and can be used in artistic miniatures.” 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 agent 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 examples of Siberian red lead replenished the collections of many mineralogical cabinets in Europe.
In 1796, a sample of crocoite came to the professor of chemistry at the Paris Mineralogical School, Nicolas-Louis Vauquelin (1763–1829), who analyzed the mineral, but found nothing in it except oxides of lead, iron and aluminum. Continuing his research on Siberian red lead, Vaukelin 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 treated with lead salt, a yellow precipitate was formed, with mercury salt, a red one, and when tin chloride was added, the solution became green. By 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 I discovered many fused gray needle-shaped crystals of a metal unknown to that time. Vaukelin noted the high refractoriness of the metal and its resistance to acids.
Vaukelin named the new element chromium (from the Greek - color, color) due to the many multi-colored compounds it forms. Based on his research, Vauquelin was the first to state that the emerald color of some precious stones is explained by 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-shaped shape of the resulting crystals, 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.
In 1798, Lowitz and Klaproth, independently of Vaukelin, discovered chromium in a sample of a heavy black mineral (it was chromite FeCr 2 O 4), found in the Urals, but much north of the Berezovsky deposit. In 1799, F. Tassaert discovered a new element in the same mineral found in southeastern France. It is believed that it was Tassert who first managed to obtain relatively pure metal chromium.
2. CHROME IN NATURE AND ITS INDUSTRIAL EXTRACTION
Chromium is a fairly common element on Earth. Its clarke (average content in the earth’s crust) is 8.3·10–3%. Chromium is never found in a free state. In chromium ores, only chromite FeCr 2 O 4 is of practical importance, which belongs to spinels - isomorphic minerals of the cubic system with the general formula MO·Me 2 O 3, where M is a divalent metal ion, and Me is a trivalent metal ion. Spinels can form solid solutions with each other, therefore, in nature, separately or as impurities to chromite, magnochromite (Mg,Fe)Cr 2 O 4, aluminum chromite Fe(Cr,Al) 2 O 4, chromopicotite (Mg,Fe) are also found. Cr,Al) 2 O 4 - all of them belong to the class of chrome spinels. In addition to spinels, chromium is found in many much less common minerals, for example, melanochroite 3PbO 2Cr 2 O 3, vokelenite 2(Pb,Cu)CrO 4 (Pb,Cu) 3 (PO 4) 2, tarapacaite K 2 CrO 4, ditzeite CaIO 3 ·CaCrO 4 and others.
Chromites are dark or almost black in color, have a metallic luster and usually occur in the form of continuous masses. Chromite deposits are of igneous origin. Its identified resources are estimated in 47 countries and amount to 15 billion tons. The first place in terms of chromite reserves is occupied by South Africa (76% of proven world reserves), where the group of Bushveld deposits is of greatest importance, the content of chrome ore is 1 billion tons. Kazakhstan ranks second in the world in terms of chromite resources (9% of world reserves); chromium ores there are of very high quality. All chromite resources in Kazakhstan are concentrated in the Aktobe region (Kempirsay massif with reserves of 300 million tons); the deposits have been developed since the late 1930s. Zimbabwe ranks third (6% of world reserves). In addition, the USA, India, the Philippines, Turkey, Madagascar, and Brazil have significant chromite resources. In Russia, quite large deposits of chromite are found in the Urals (Saranovskoye, Verblyuzhyegorskoye, Alapaevskoye, Monetnaya Dacha, Khalilovskoye and other deposits).
At the beginning of the 19th century. The main source of chromite was the Ural deposits, but in 1827 the American Isaac Tyson discovered a large deposit of chromium ore on the border of Maryland and Pennsylvania, becoming a monopolist in mining for many years. In 1848, deposits of high quality chromite were found in Turkey, near Bursa. After the depletion of reserves in Maryland, Turkey was the leader in chromite mining, until India and South Africa took over the baton in 1906.
Currently, 11–14 million tons of chromite are mined annually in the world. South Africa occupies the leading place in the production of chrome ore (about 6 million tons annually), followed by Kazakhstan, providing 20% of world needs. Due to the great depth of chrome ore, it is usually mined by open-pit mining (85%), but open pit mining is also sometimes practiced, for example, in Finland and Madagascar. Typically, the mined ores are of fairly high quality and only require mechanical sorting. It is often impractical to enrich chromites, since this can only increase the Cr 2 O 3 content, and the Fe ratio :
Cr remains unchanged. The price of chromite on the world market ranges from 40–120 US dollars per ton.
Chrome is a silvery metal with a density of 7200 kg/m3. Determining the melting point of pure chromium is an extremely difficult task, since the slightest impurities of oxygen or nitrogen significantly affect the value of this temperature. According to the results of modern measurements, it is equal to 1907° C. The boiling point of chromium is 2671° C. Absolutely pure (without gas impurities and carbon) chromium is quite viscous, malleable and malleable. At the slightest contamination with carbon, hydrogen, nitrogen, etc. becomes brittle, brittle and hard. At ordinary temperatures it exists in the form of an a-modification and has a body-centered cubic lattice. Chemically, chromium is quite inert due to the formation of a strong thin oxide film on its surface. It does not oxidize in air even in the presence of moisture, and when heated, oxidation occurs only on the surface. Chromium is passivated by dilute and concentrated nitric acid, aqua regia, and even when the metal is boiled with these reagents, it dissolves only slightly. Chromium passivated by nitric acid, unlike metal without a protective layer, does not dissolve in dilute sulfuric and hydrochloric acids, even after prolonged boiling in solutions of these acids; however, at a certain moment, rapid dissolution begins, accompanied by foaming from the liberated hydrogen - from the passive form chromium becomes activated, not protected by an oxide film:
Cr + 2HCl = CrCl 2 + H 2
If nitric acid is added during the dissolution process, the reaction immediately stops - the chromium is again passivated.
When heated, chromium metal combines with halogens, sulfur, silicon, boron, carbon and some other elements:
Cr + 2F 2 = CrF 4 (with an admixture of CrF 5)
2Cr + 3Cl2 = 2CrCl3
Cr + C = mixture of Cr 23 C 6 + Cr 7 C 3.
When chromium is heated with molten soda in air, nitrates or chlorates of alkali metals, the corresponding chromates (VI) are obtained:
2Cr + 2Na 2 CO 3 + 3O 2 = 2Na 2 CrO 4 + 2CO 2.
Depending on the required degree of metal purity, there are several industrial methods for producing chromium.
Opportunity aluminothermic reduction of chromium(III) oxide was demonstrated by Friedrich Wöhler in 1859; however, this method became available on an industrial scale as soon as it became possible to obtain cheap aluminum. The industrial aluminothermic production of chromium began with the work of Goldschmidt, who was the first to develop a reliable method for regulating the highly exothermic (and therefore explosive) reduction process:
Cr 2 O 3 + 2Al = 2Cr + 2Al 2 O 3.
Previously, the mixture is uniformly heated to 500-600 ° C. Reduction can be initiated either by a mixture of barium peroxide with aluminum powder, or by igniting a small portion of the mixture, followed by adding the rest of the mixture. It is important that the heat released during the reaction is sufficient to melt the resulting chromium and separate it from the slag. Chromium produced by the aluminothermic process usually contains 0.015–0.02% C, 0.02% S and 0.25–0.40% Fe, and the mass fraction of the main substance in it is 99.1–99.4% Cr. It is very fragile and easily ground into powder.
To obtain high-purity chromium, electrolytic methods are used; the possibility of this was demonstrated in 1854 by Bunsen, who subjected an aqueous solution of chromium chloride to electrolysis. Now electrolysis is carried out using a mixture of chromic anhydride or chromoammonium alum with dilute sulfuric acid. The chromium released during electrolysis contains dissolved gases as impurities. Modern technologies make it possible to obtain metal with a purity of 99.90–99.995% on an industrial scale using high-temperature purification in a hydrogen flow and vacuum degassing. Unique methods for refining electrolytic chromium allow you to get rid of oxygen, sulfur, nitrogen and hydrogen contained in the “raw” product.
There are several other less significant ways to obtain chromium metal. Silicothermic reduction is based on the reaction:
2Cr 2 O 3 + 3Si + 3CaO = 4Cr + 3CaSiO 3.
Silicon reduction, although exothermic in nature, requires the process to be carried out in an arc furnace. The addition of quicklime allows you to convert refractory silicon dioxide into low-melting calcium silicate slag.
The reduction of chromium(III) oxide with coal is used to obtain high-carbon chromium intended for the production of special alloys. The process is also carried out in an electric arc furnace.
The Van Arkel-Kuchman-De Boer process uses the decomposition of chromium(III) iodide on a wire heated to 1100° C with the deposition of pure metal on it.
Chromium can also be obtained by the reduction of Cr 2 O 3 with hydrogen at 1500 ° C, the reduction of anhydrous CrCl 3 with hydrogen, alkali or alkaline earth metals, magnesium and zinc.
3. APPLICATION OF CHROME IN INDUSTRY
For many decades since the discovery of the metal chromium, only crocoite and some of its other compounds were used as pigments in the manufacture of paints. In 1820, Cochlen proposed the use of potassium dichromate as a mordant for dyeing fabrics. In 1884, the active use of soluble chromium compounds as tannins in the leather industry began. Chromite was first used in France in 1879 as a refractory substance, but its main use began in the 1880s in England and Sweden, when the industrial smelting of ferrochrome began to increase in speed. They were able to obtain ferrochrome in small quantities already at the beginning of the 19th century, so Berthier, back in 1821, proposed reducing a mixture of iron and chromium oxides with charcoal in a crucible. The first patent for the manufacture of chromium steel was issued in 1865. Industrial production of high-carbon ferrochrome began using blast furnaces to reduce chromite with coke. Ferrochrome late 19th century. was of very low quality, as it usually contained 7–8% chromium, and was known as “Tasmanian pig iron” due to the fact that the original iron-chromium ore was imported from Tasmania. The turning point in ferrochrome production came in 1893, when Henri Moissan first smelted high-carbon ferrochrome containing 60% Cr. The main achievement in this industry was the replacement of the blast furnace with an electric arc furnace, created by Moissan, which made it possible to increase the temperature of the process, reduce energy consumption and significantly improve the quality of the smelted ferrochrome, which began to contain 67–71% Cr and 4–6% C. Moissan’s method is still por lies at the basis of modern industrial production of ferrochrome. Chromite reduction is usually carried out in open electric arc furnaces, and the charge is loaded from above. An arc is formed between electrodes immersed in the charge.
Chromium occurs in nature mainly in the form of chromium iron ore Fe(CrO 2) 2 (iron chromite). Ferrochrome is obtained from it by reduction in electric furnaces with coke (carbon):
FeO Cr 2 O 3 + 4C → Fe + 2Cr + 4CO
6) using electrolysis, electrolytic chromium is obtained from a solution of chromic anhydride in water containing the addition of sulfuric acid. In this case, mainly 3 processes take place at the cathodes:
– reduction of hexavalent chromium to trivalent chromium with its transition into solution;
– discharge of hydrogen ions with the release of hydrogen gas;
– discharge of ions containing hexavalent chromium with precipitation of metallic chromium;
Cr 2 O 7 2− + 14Н + + 12е − = 2Сr + 7H 2 O
In its free form, it is a bluish-white metal with a body-centered cubic lattice, a = 0.28845 nm. At a temperature of 39 °C it changes from a paramagnetic state to an antiferromagnetic state (Néel point).
Stable in air. At 300 °C it burns to form green chromium(III) oxide Cr 2 O 3, which has amphoteric properties. By fusing Cr 2 O 3 with alkalis, chromites are obtained
Despite the great importance of high-carbon ferrochrome for the production of many types of stainless steels, it is not suitable for smelting some high-chromium steels, since the presence of carbon (in the form of Cr 23 C 6 carbide, crystallizing along the grain boundaries) makes them brittle and easily susceptible to corrosion. The production of low-carbon ferrochrome began to develop with the beginning of the use of industrial aluminothermic reduction of chromite. Nowadays, the aluminothermic process has been replaced by the silicothermic process (Perrin process) and the simplex process, which consists of mixing high-carbon ferrochrome with partially oxidized ferrochrome powder, subsequent briquetting and heating to 1360 ° C in a vacuum. Ferrochrome prepared by the simplex process typically contains only 0.008% carbon, and briquettes made from it are easily dissolved in molten steel.
The ferrochrome market is cyclical. World production of ferrochrome in 2000 was 4.8 million tons, and in 2001, due to low demand, 3.4 million tons. In 2002, the demand for ferrochrome intensified again. The first place in the world in ferrochrome smelting is occupied by the South African “Big Two” – Xstrata South Africa (Pty) Ltd. (a subsidiary of Xstrata AG) and Samancor Chrome Division (a subsidiary of Samancor Ltd.). They account for up to 40% of the world's ferrochrome smelting. In South Africa and Finland, mainly charge chrome is produced (from the English charge - load coal), containing 52–55% Cr, and in China, Russia, Zimbabwe, Kazakhstan, ferrochrome containing more than 60% Cr. Ferrochrome is used as an alloying additive for low alloy steels. With a chromium content of more than 12%, steel almost does not rust.
The corrosion resistance of iron alloys can be significantly increased by applying a thin layer of chromium to their surface. This procedure is called chrome plating. Chrome-plated layers are well resistant to exposure to humid atmosphere, sea air, tap water, nitric and many organic acids. All chromium plating methods can be divided into two types - diffusion and electrolytic. The Becker-Davies-Steinberg diffusion method involves heating a chrome-plated product to 1050–1100° C in a hydrogen atmosphere, filled with a mixture of ferrochrome and refractory, pre-treated with hydrogen chloride at 1050° C. The CrCl 2 located in the pores of the refractory evaporates and chromes the product. During the electrolytic chromium plating process, metal is deposited on the surface of the workpiece, which acts as a cathode. The electrolyte is often a hexavalent chromium compound (usually CrO 3 ) dissolved in aqueous H 2 SO 4 . Chrome coatings are either protective or decorative. The thickness of protective coatings reaches 0.1 mm; they are applied directly to the product and give it increased wear resistance. Decorative coatings have an aesthetic value and are applied to a sublayer of another metal (nickel or copper), which performs the actual protective function. The thickness of such a coating is only 0.0002–0.0005 mm.
4. BIOLOGICAL ROLE OF CHROME
Chromium is a trace element necessary for the normal development and functioning of the human body. It has been established that only trivalent chromium takes part in biochemical processes. Its most important biological role is the regulation of carbohydrate metabolism and blood glucose levels. Chromium is an integral part of a low-molecular complex - glucose tolerance factor (GTF), which facilitates the interaction of cellular receptors with insulin, thereby reducing the body's need for it. The tolerance factor enhances the action of insulin in all metabolic processes with its participation. In addition, chromium takes part in the regulation of cholesterol metabolism and is an activator of certain enzymes.
The chromium content in the human body is 6–12 mg. There is no exact information about a person’s physiological need for this element; in addition, it strongly depends on the nature of the diet (for example, it increases greatly with an excess of sugar in the diet). According to various estimates, the daily intake of chromium in the body is 20–300 mcg. An indicator of the body's supply of chromium is its content in the hair (the norm is 0.15–0.5 mcg/g). Unlike many microelements, the chromium content in body tissues (with the exception of the lungs) decreases as a person ages.
The concentration of the element in plant foods is an order of magnitude lower than its concentration in mammalian tissues. The chromium content in brewer's yeast is especially high; in addition, it is found in noticeable quantities in meat, liver, legumes, and whole grains. Chromium deficiency in the body can cause a diabetes-like condition, contribute to the development of atherosclerosis and disruption of higher nervous activity.
Already in relatively small concentrations (fractions of a milligram per m 3 for the atmosphere), all chromium compounds have a toxic effect on the body. Particularly dangerous in this regard are soluble compounds of hexavalent chromium, which have allergic, mutagenic and carcinogenic effects.
Poisoning with chromium and its compounds occurs during their production; in mechanical engineering (galvanic 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, cough, etc.; they can go away when contact with Chromium is stopped. With prolonged contact with chromium compounds, signs of chronic poisoning develop: headache, weakness, dyspepsia, weight loss and others. The functions of the stomach, liver and pancreas are impaired. Possible bronchitis, bronchial asthma, diffuse pneumosclerosis. When exposed to Chromium on the skin, dermatitis and eczema can develop. According to some data, chromium compounds, mainly Cr(III), have a carcinogenic effect.
chrome plating A decrease in chromium content in food and blood leads to a decrease in growth rate, an increase
Ripan R., Ceteanu I. Inorganic chemistry, vol. 2. – M.: Mir, 1972.