How is chromium designated in chemistry? Oxidation state of chromium. Atomic and molecular weight of chromium
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 bichromate 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 dichromate 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.
In 1766, professor of chemistry and head of the Chemical Laboratory of the St. Petersburg Academy of Sciences I.G. Lehman described a new mineral found in the Urals at the Berezovsky mine, which was called "Siberian red lead", PbCrO 4. The modern name is crocoite. In 1797, the French chemist L. N. Vauquelin isolated a new refractory metal from it.
The element received its name from the Greek. χρῶμα - color, paint - due to the variety of colors of its compounds.
Finding in nature and obtaining:
The most common chromium mineral is chromium iron ore FeCr 2 O 4 (chromite), rich deposits of which are found in the Urals and Kazakhstan; the second most important mineral is crocoite PbCrO 4. The mass fraction of chromium in the earth's crust is 0.03%. Natural chromium consists of a mixture of five isotopes with mass numbers 50, 52, 53, 54 and 56; Other radioactive isotopes have also been artificially obtained.
The main quantities of chromium are obtained and used in the form of an alloy with iron, ferrochrome, by reducing chromite with coke: FeCr 2 O 4 + 4C = Fe + 2Cr + 4CO
Pure chromium is obtained by reducing its oxide with aluminum: Cr 2 O 3 + 2Al = 2Cr + Al 2 O 3
or electrolysis of aqueous solutions of chromium compounds.
Physical properties:
Chrome is a grayish-white shiny metal, similar in appearance to steel, one of the hardest metals, r= 7.19 g/cm 3, Tmelt=2130K, Tboil=2945K. Chrome has all the properties characteristic of metals - it conducts heat and electricity well, and has the luster characteristic of most metals.
Chemical properties:
Chromium is stable in air due to passivation - the formation of a protective oxide film. For the same reason, it does not react with concentrated sulfuric and nitric acids. At 2000°C it burns to form green chromium(III) oxide Cr 2 O 3 .
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.
The most important connections:
Oxidation state +2- basic oxide CrO (black), hydroxide Cr(OH) 2 (yellow). Chromium(II) salts (blue solutions) are obtained by reducing chromium(III) salts with zinc in an acidic environment. Very strong reducing agents, they are slowly oxidized by water, releasing hydrogen.
Oxidation state +3- the most stable oxidation state of chromium, it corresponds to: amphoteric oxide Cr 2 O 3 and hydroxide Cr (OH) 3 (both gray-green), chromium (III) salts - gray-green or purple, chromites MCrO2, which are obtained by fusing chromium oxide with alkalis, tetra- and hexahydroxochromates(III) obtained by dissolving chromium(III) hydroxide in alkali solutions (green), numerous chromium complex compounds.
Oxidation state +6- the second characteristic oxidation state of chromium, it corresponds to the acidic chromium(VI) oxide CrO 3 (red crystals, dissolves in water, forming chromic acids), chromic H 2 CrO 4, dichromic H 2 Cr 2 O 7 and polychromic acids, the corresponding salts : yellow chromates and orange dichromates. Chromium(VI) compounds are strong oxidizing agents, especially in an acidic environment, reduced to chromium(III) compounds
In an aqueous solution, chromates turn into dichromates when the acidity of the medium changes:
2CrO 4 2- + 2H + Cr 2 O 7 2- + H 2 O, which is accompanied by a color change.
Application
Chromium, in the form of ferrochrome, is used in the production of alloyed steels (in particular, stainless steel) and other alloys. Chromium alloys: chromium-30 and chromium-90, indispensable for the production of nozzles for powerful plasma torches and in the aerospace industry, an alloy with nickel (nichrome) - for the production of heating elements. Large quantities of chromium are used as wear-resistant and beautiful electroplating coatings (chrome plating).
Biological role and physiological effect
Chromium is one of the biogenic elements and is constantly included in the tissues of plants and animals. In animals, chromium is involved in the metabolism of lipids, proteins (part of the enzyme trypsin), and carbohydrates. A decrease in chromium content in food and blood leads to a decrease in growth rate and an increase in cholesterol in the blood.
In its pure form, chromium is quite toxic; chromium metal dust irritates lung tissue. Chromium(III) compounds cause dermatitis. Chromium(VI) compounds lead to various human diseases, including cancer. MPC of chromium(VI) in atmospheric air 0.0015 mg/m 3
Kononova A.S., Nakov D.D., Tyumen State University, 501(2) group, 2013
Sources:
Chromium (element) // Wikipedia. URL: http://ru.wikipedia.org/wiki/Chrome (access date: 01/06/2014).
Popular library of chemical elements: Chromium. // URL:
DEFINITION
Chromium- the twenty-fourth element of the Periodic Table. Designation - Cr from the Latin "chromium". Located in the fourth period, VIB group. Refers to metals. The nuclear charge is 24.
Chromium is contained in the earth's crust in an amount of 0.02% (mass.). In nature, it is found mainly in the form of chromium iron ore FeO×Cr 2 O 3.
Chromium is a hard, shiny metal (Fig. 1), melting at 1890 o C; its density is 7.19 g/cm 3 . At room temperature, chromium is resistant to both water and air. Dilute sulfuric and hydrochloric acids dissolve chromium, releasing hydrogen. Chromium is insoluble in cold concentrated nitric acid and after treatment with it becomes passive.
Rice. 1. Chrome. Appearance.
Atomic and molecular weight of chromium
DEFINITION
Relative molecular weight of the substance(M r) is a number showing how many times the mass of a given molecule is greater than 1/12 the mass of a carbon atom, and relative atomic mass of an element(A r) - how many times the average mass of atoms of a chemical element is greater than 1/12 of the mass of a carbon atom.
Since in the free state chromium exists in the form of monatomic Cr molecules, the values of its atomic and molecular masses coincide. They are equal to 51.9962.
Isotopes of chromium
It is known that in nature chromium can be found in the form of four stable isotopes 50 Cr, 52 Cr, 53 Cr and 54 Cr. Their mass numbers are 50, 52, 53 and 54, respectively. The nucleus of an atom of the chromium isotope 50 Cr contains twenty-four protons and twenty-six neutrons, and the remaining isotopes differ from it only in the number of neutrons.
There are artificial isotopes of chromium with mass numbers from 42 to 67, among which the most stable is 59 Cr with a half-life of 42.3 minutes, as well as one nuclear isotope.
Chromium ions
At the outer energy level of the chromium atom there are six electrons, which are valence:
1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 .
As a result of chemical interaction, chromium gives up its valence electrons, i.e. is their donor, and turns into a positively charged ion:
Cr 0 -2e → Cr 2+ ;
Cr 0 -3e → Cr 3+ ;
Cr 0 -6e → Cr 6+ .
Chromium molecule and atom
In the free state, chromium exists in the form of monoatomic Cr molecules. Here are some properties characterizing the chromium atom and molecule:
Chromium alloys
Chromium metal is used for chrome plating and as one of the most important components of alloy steels. The introduction of chromium into steel increases its resistance to corrosion both in aqueous environments at normal temperatures and in gases at elevated temperatures. In addition, chromium steels have increased hardness. Chromium is part of stainless acid-resistant and heat-resistant steels.
Examples of problem solving
EXAMPLE 1
EXAMPLE 2
| Exercise | Chromium (VI) oxide weighing 2 g was dissolved in water weighing 500 g. Calculate the mass fraction of chromic acid H 2 CrO 4 in the resulting solution. |
| Solution | Let us write the reaction equation for the production of chromic acid from chromium (VI) oxide: CrO 3 + H 2 O = H 2 CrO 4. Let's find the mass of the solution: m solution = m(CrO 3) + m (H 2 O) = 2 + 500 = 502 g. n (CrO 3) = m (CrO 3) / M (CrO 3); n (CrO 3) = 2 / 100 = 0.02 mol. According to the reaction equation n(CrO 3) : n(H 2 CrO 4) = 1:1, which means n(CrO 3) = n(H 2 CrO 4) = 0.02 mol. Then the mass of chromic acid will be equal (molar mass - 118 g/mol): m (H 2 CrO 4) = n (H 2 CrO 4) × M (H 2 CrO 4); m (H 2 CrO 4) = 0.02 × 118 = 2.36 g. The mass fraction of chromic acid in the solution is: ω = m solute / m solution × 100%; ω (H 2 CrO 4)=m solute (H 2 CrO 4)/ m solution × 100%; ω (H 2 CrO 4) = 2.36 / 502 × 100% = 0.47%. |
| Answer | The mass fraction of chromic acid is 0.47%. |
Chromium (Cr) is an element with atomic number 24 and atomic mass 51.996 of a secondary subgroup of the sixth group of the fourth period of the periodic system of chemical elements of D. I. Mendeleev. Chrome is a hard metal with a bluish-white color. Has high chemical resistance. At room temperature, Cr is resistant to water and air. This element is one of the most important metals used in industrial alloying of steels. Chromium compounds have bright colors of various colors, which is why it got its name. After all, translated from Greek, “chrome” means “paint”.
There are 24 known isotopes of chromium from 42Cr to 66Cr. Stable natural isotopes are 50Cr (4.31%), 52Cr (87.76%), 53Cr (9.55%) and 54Cr (2.38%). Of the six artificial radioactive isotopes, the most important is 51Cr, with a half-life of 27.8 days. It is used as an isotope indicator.
Unlike the metals of antiquity (gold, silver, copper, iron, tin and lead), chromium has its own “discoverer”. In 1766, a mineral was found in the vicinity of Yekaterinburg, which was called “Siberian red lead” - PbCrO4. In 1797, L. N. Vauquelin discovered element No. 24 in the mineral crocoite, a natural lead chromate. Around the same time (1798), independently of Vauquelin, chromium was discovered by German scientists M. G. Klaproth and Lowitz in a sample of heavy black mineral (it was chromite FeCr2O4), found in the Urals. Later in 1799, F. Tassert discovered a new metal in the same mineral found in southeastern France. It is believed that it was Tassert who first managed to obtain relatively pure metal chromium.
Metal chromium is used for chrome plating, and also as one of the most important components of alloy steels (in particular stainless steels). In addition, chromium has found application in a number of other alloys (acid-resistant and heat-resistant steels). After all, the introduction of this metal into steel increases its resistance to corrosion both in aqueous environments at normal temperatures and in gases at elevated temperatures. Chromium steels are characterized by increased hardness. Chromium is used in thermochrome plating, a process in which the protective effect of Cr is due to the formation of a thin but durable oxide film on the surface of the steel, which prevents the interaction of the metal with the environment.
Chromium compounds are also widely used; chromites are successfully used in the refractory industry: open-hearth furnaces and other metallurgical equipment are lined with magnesite-chromite bricks.
Chromium is one of the biogenic elements that are constantly included in the tissues of plants and animals. Plants contain chromium in their leaves, where it is present in the form of a low-molecular complex not associated with subcellular structures. Until now, scientists have not been able to prove the necessity of this element for plants. However, in animals, Cr is involved in the metabolism of lipids, proteins (part of the enzyme trypsin), and carbohydrates (a structural component of the glucose-resistant factor). It is known that only trivalent chromium is involved in biochemical processes. Like most other important nutrients, chromium enters the animal or human body through food. A decrease in this microelement in the body leads to slower growth, a sharp increase in blood cholesterol levels and a decrease in the sensitivity of peripheral tissues to insulin.
At the same time, chromium in its pure form is very toxic - Cr metal dust irritates lung tissue, chromium (III) compounds cause dermatitis. Chromium (VI) compounds lead to various human diseases, including cancer.
Biological properties
Chromium is an important biogenic element, which is certainly included in the tissues of plants, animals and humans. The average content of this element in plants is 0.0005%, and almost all of it accumulates in the roots (92-95%), the rest is contained in the leaves. Higher plants do not tolerate concentrations of this metal above 3∙10-4 mol/l. In animals, the chromium content ranges from ten thousandths to ten millionths of a percent. But in plankton, the coefficient of chromium accumulation is amazing - 10,000-26,000. In the adult human body, the Cr content ranges from 6 to 12 mg. Moreover, the physiological need for chromium for humans has not been established quite accurately. It largely depends on the diet - when eating food high in sugar, the body's need for chromium increases. It is generally accepted that a person needs approximately 20–300 mcg of this element per day. Like other biogenic elements, chromium can accumulate in body tissues, especially in hair. It is in them that the chromium content indicates the degree of provision of the body with this metal. Unfortunately, with age, the “reserves” of chromium in tissues are depleted, with the exception of the lungs.
Chromium is involved in the metabolism of lipids, proteins (present in the enzyme trypsin), carbohydrates (is a structural component of the glucose-resistant factor). This factor ensures the interaction of cellular receptors with insulin, thereby reducing the body's need for it. Glucose tolerance factor (GTF) enhances the action of insulin in all metabolic processes involving it. In addition, chromium takes part in the regulation of cholesterol metabolism and is an activator of certain enzymes.
The main source of chromium in animals and humans is food. Scientists have found that the concentration of chromium in plant foods is significantly lower than in animal foods. The richest sources of chromium are brewer's yeast, meat, liver, legumes and whole unprocessed grains. A decrease in the content of this metal in food and blood leads to a decrease in growth rate, an increase in cholesterol in the blood, and a decrease in the sensitivity of peripheral tissues to insulin (diabetes-like state). In addition, the risk of developing atherosclerosis and disorders of higher nervous activity increases.
However, even at concentrations of a fraction of a milligram per cubic meter in the atmosphere, all chromium compounds have a toxic effect on the body. Poisoning with chromium and its compounds is common during their production, in mechanical engineering, metallurgy, and in the textile industry. The degree of toxicity of chromium depends on the chemical structure of its compounds - dichromates are more toxic than chromates, Cr+6 compounds are more toxic than Cr+2 and Cr+3 compounds. Signs of poisoning include a feeling of dryness and pain in the nasal cavity, a sore throat, difficulty breathing, coughing and similar symptoms. If there is a slight excess of chromium vapors or dust, the signs of poisoning disappear soon after work in the workshop stops. With prolonged constant contact with chromium compounds, signs of chronic poisoning appear - weakness, constant headaches, weight loss, dyspepsia. Disturbances in the functioning of the gastrointestinal tract, pancreas, and liver begin. Bronchitis, bronchial asthma, and pneumosclerosis develop. Skin diseases appear - dermatitis, eczema. In addition, chromium compounds are dangerous carcinogens that can accumulate in body tissues, causing cancer.
Prevention of poisoning includes periodic medical examinations of personnel working with chromium and its compounds; installation of ventilation, dust suppression and dust collection equipment; use of personal protective equipment (respirators, gloves) by workers.
The root "chrome" in its concept of "color", "paint" is part of many words used in a wide variety of fields: science, technology and even music. So many names of photographic films contain this root: “orthochrome”, “panchrome”, “isopanchrome” and others. The word chromosome is made up of two Greek words: chromo and soma. Literally this can be translated as “painted body” or “body that is painted.” The structural element of a chromosome, formed in the interphase of the cell nucleus as a result of chromosome duplication, is called “chromatid”. “Chromatin” is a substance of chromasomes located in the nuclei of plant and animal cells, which is intensely stained with nuclear dyes. “Chromatophores” are pigment cells in animals and humans. In music, the concept of “chromatic scale” is used. “Khromka” is one of the types of Russian accordion. In optics, there are the concepts of “chromatic aberration” and “chromatic polarization”. “Chromatography” is a physical and chemical method for separating and analyzing mixtures. “Chromoscope” is a device for obtaining a color image by optically combining two or three color-separated photographic images, illuminated through specially selected differently colored filters.
The most toxic is chromium (VI) oxide CrO3; it belongs to hazard class I. Lethal dose for humans (orally) 0.6 g. Ethyl alcohol ignites on contact with freshly prepared CrO3!
The most common grade of stainless steel contains 18% Cr, 8% Ni, about 0.1% C. It has excellent resistance to corrosion and oxidation, and retains strength at high temperatures. It is from this steel that the sheets used in the construction of the sculptural group of V.I. were made. Mukhina "Worker and Collective Farm Woman".
Ferrochrome, used in the metallurgical industry in the production of chromium steels, was of very poor quality at the end of the 19th century. This is due to the low chromium content in it - only 7-8%. Then it was called “Tasmanian cast iron” due to the fact that the original iron-chrome ore was imported from Tasmania.
It was previously mentioned that chrome alum is used in tanning leather. Thanks to this, the concept of “chrome” boots appeared. Leather tanned with chromium compounds acquires shine, gloss and strength.
Many laboratories use a “chromic mixture” - a mixture of a saturated solution of potassium dichromate with concentrated sulfuric acid. It is used in degreasing the surfaces of glass and steel laboratory glassware. It oxidizes fat and removes its remains. Just handle this mixture with caution, because it is a mixture of a strong acid and a strong oxidizing agent!
Nowadays, wood is still used as a building material, because it is inexpensive and easy to process. But it also has many negative properties - susceptibility to fires, fungal diseases that destroy it. To avoid all these troubles, wood is impregnated with special compounds containing chromates and dichromates, plus zinc chloride, copper sulfate, sodium arsenate and some other substances. Thanks to such compositions, wood increases its resistance to fungi and bacteria, as well as to open fire.
Chrome has occupied a special niche in printing. In 1839, it was discovered that paper impregnated with sodium bichromate suddenly turned brown when exposed to bright light. Then it turned out that bichromate coatings on paper, after exposure, do not dissolve in water, but, when wetted, acquire a bluish tint. Printers took advantage of this property. The desired pattern was photographed on a plate with a colloidal coating containing dichromate. The illuminated areas did not dissolve during washing, and the unexposed areas dissolved, and a pattern remained on the plate from which it was possible to print.
Story
The history of the discovery of element No. 24 began in 1761, when an unusual red mineral was found in the Berezovsky mine (the eastern foot of the Ural Mountains) near Yekaterinburg, which, when ground into dust, gave a yellow color. The find belonged to St. Petersburg University professor Johann Gottlob Lehmann. Five years later, the scientist delivered the samples to the city of St. Petersburg, where he conducted a series of experiments on them. In particular, he treated the unusual crystals with hydrochloric acid, resulting in a white precipitate in which lead was found. Based on the results obtained, Lehman named the mineral Siberian red lead. This is the story of the discovery of crocoite (from the Greek “krokos” - saffron) - a natural lead chromate PbCrO4.
Interested in this find, Peter Simon Pallas, a German naturalist and traveler, organized and led an expedition of the St. Petersburg Academy of Sciences to the heart of Russia. In 1770, the expedition reached the Urals and visited the Berezovsky mine, where samples of the mineral being studied were taken. This is how the traveler himself describes it: “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.” German enterprise overcame all the difficulties of mining and delivering crocoite to Europe. Despite the fact that these operations took at least two years, soon the carriages of noble gentlemen of Paris and London were traveling painted with finely ground crocoite. The collections of the mineralogical museums of many universities of the old world have been enriched with the best examples of this mineral from the Russian depths. However, European scientists could not figure out the composition of the mysterious mineral.
This lasted for thirty years, until a sample of Siberian red lead fell into the hands of Nicolas Louis Vauquelin, professor of chemistry at the Paris Mineralogical School, in 1796. After analyzing the crocoite, the scientist found nothing in it except oxides of iron, lead and aluminum. Subsequently, Vauquelin treated crocoite with a solution of potash (K2CO3) and, following the precipitation of a white precipitate of lead carbonate, isolated a yellow solution of an unknown salt. After conducting a series of experiments on treating the mineral with salts of various metals, the professor, using hydrochloric acid, isolated a solution of “red lead acid” - chromium oxide and water (chromic acid exists only in dilute solutions). By evaporating this solution, he obtained ruby-red crystals (chromic anhydride). Further heating of the crystals in a graphite crucible in the presence of coal gave a lot of fused gray needle-shaped crystals - a new, hitherto unknown metal. The next series of experiments showed the high refractoriness of the resulting element and its resistance to acids. The Paris Academy of Sciences immediately witnessed the discovery; the scientist, at the insistence of his friends, gave the name to the new element - chromium (from the Greek “color”, “color”) due to the variety of shades of the compounds it forms. In his further works, Vauquelin confidently stated that the emerald color of some precious stones, as well as natural beryllium and aluminum silicates, is explained by the admixture of chromium compounds in them. An example is emerald, which is a green-colored beryl in which aluminum is partially replaced by chromium.
It is clear that Vauquelin did not obtain pure metal, most likely its carbides, which is confirmed by the needle-shaped shape of light gray crystals. Pure chromium metal was later obtained by F. Tassert, probably in 1800.
Also, independently of Vauquelin, chromium was discovered by Klaproth and Lowitz in 1798.
Being in nature
In the bowels of the earth, chromium is a fairly common element, despite the fact that it is not found in free form. Its clarke (average content in the earth's crust) is 8.3.10-3% or 83 g/t. However, its distribution among breeds is uneven. This element is mainly characteristic of the Earth’s mantle; the fact is that ultramafic rocks (peridotites), which are presumably close in composition to the mantle of our planet, are the richest in chromium: 2 10-1% or 2 kg/t. In such rocks, Cr forms massive and disseminated ores, and the formation of the largest deposits of this element is associated with them. The chromium content is also high in basic rocks (basalts, etc.) 2 10-2% or 200 g/t. Much less Cr is found in acidic rocks: 2.5 10-3%, sedimentary rocks (sandstones) - 3.5 10-3%, shales also contain chromium - 9 10-3%.
It can be concluded that chromium is a typical lithophile element and is almost entirely contained in deep minerals in the Earth’s interior.
There are three main chromium minerals: magnochromite (Mn, Fe)Cr2O4, chromopicotite (Mg, Fe)(Cr, Al)2O4 and aluminochromite (Fe, Mg)(Cr, Al)2O4. These minerals have a single name - chrome spinel and the general formula (Mg, Fe)O (Cr, Al, Fe)2O3. They are indistinguishable in appearance and are inaccurately called “chromites.” Their composition is variable. The content of the most important components varies (weight %): Cr2O3 from 10.5 to 62.0; Al2O3 from 4 to 34.0; Fe2O3 from 1.0 to 18.0; FeO from 7.0 to 24.0; MgO from 10.5 to 33.0; SiO2 from 0.4 to 27.0; TiO2 impurities up to 2; V2O5 up to 0.2; ZnO up to 5; MnO up to 1. Some chromium ores contain 0.1-0.2 g/t of platinum group elements and up to 0.2 g/t of gold.
In addition to various chromites, chromium is part of a number of other minerals - chrome vesuvian, chrome chlorite, chrome tourmaline, chrome mica (fuchsite), chrome garnet (uvarovite), etc., which often accompany ores, but are not of industrial importance. Chromium is a relatively weak aquatic migrant. Under exogenous conditions, chromium, like iron, migrates in the form of suspensions and can precipitate in clays. The most mobile form is chromates.
Of practical importance, perhaps, is only chromite FeCr2O4, which belongs to spinels - isomorphic minerals of the cubic system with the general formula MO Me2O3, where M is a divalent metal ion, and Me is a trivalent metal ion. In addition to spinels, chromium is found in many much less common minerals, for example, melanochroite 3PbO 2Cr2O3, vokelenite 2(Pb,Cu)CrO4(Pb,Cu)3(PO4)2, tarapacaite K2CrO4, ditzeite CaIO3 CaCrO4 and others.
Chromites are usually found in the form of granular masses of black color, less often - in the form of octahedral crystals, have a metallic luster, and occur in the form of continuous masses.
At the end of the 20th century, chromium reserves (identified) in almost fifty countries of the world with deposits of this metal amounted to 1674 million tons. The leading position is occupied by the Republic of South Africa - 1050 million tons, where the main contribution is made by the Bushveld complex (about 1000 million tons ). The second place in chrome resources belongs to Kazakhstan, where very high quality ore is mined in the Aktobe region (Kempirsay massif). Other countries also have reserves of this element. Turkey (in Guleman), Philippines on the island of Luzon, Finland (Kemi), India (Sukinda), etc.
Our country has its own developed chromium deposits in the Urals (Donskoye, Saranovskoye, Khalilovskoye, Alapaevskoye and many others). Moreover, at the beginning of the 19th century, it was the Ural deposits that were the main sources of chrome ores. It was only in 1827 that the American Isaac Tison discovered a large deposit of chrome ore on the border of Maryland and Pennsylvania, seizing the mining monopoly for many years. In 1848, deposits of high-quality chromite were found in Turkey, near Bursa, and soon (after the depletion of the Pennsylvania deposit) it was this country that took over the role of monopolist. This continued until 1906, when rich deposits of chromite were discovered in South Africa and India.
Application
Total consumption of pure chromium metal today is approximately 15 million tons. The production of electrolytic chromium - the purest - accounts for 5 million tons, which is a third of total consumption.
Chromium is widely used to alloy steels and alloys, giving them corrosion and heat resistance. More than 40% of the resulting pure metal is consumed in the production of such “superalloys”. The most well-known resistance alloys are nichrome with a Cr content of 15-20%, heat-resistant alloys - 13-60% Cr, stainless alloys - 18% Cr and ball bearing steels 1% Cr. The addition of chromium to conventional steels improves their physical properties and makes the metal more susceptible to heat treatment.
Metallic chromium is used for chrome plating - applying a thin layer of chromium to the surface of steel alloys in order to increase the corrosion resistance of these alloys. The chrome coating perfectly resists the effects of humid atmospheric air, salty sea air, water, nitric and most organic acids. Such coatings have two purposes: protective and decorative. The thickness of the protective coatings is about 0.1 mm; they are applied directly to the product and give it increased wear resistance. Decorative coatings have an aesthetic value; they are applied to a layer of another metal (copper or nickel), which actually performs a protective function. The thickness of such a coating is only 0.0002–0.0005 mm.
Chromium compounds are also actively used in various fields.
The main chromium ore - chromite FeCr2O4 is used in the production of refractories. Magnesite-chromite bricks are chemically passive and heat-resistant; they can withstand sudden, repeated temperature changes, which is why they are used in the structures of the arches of open-hearth furnaces and the working space of other metallurgical devices and structures.
The hardness of chromium (III) oxide crystals - Cr2O3 is comparable to the hardness of corundum, which ensures its use in the compositions of grinding and lapping pastes used in mechanical engineering, jewelry, optical and watch industries. It is also used as a catalyst for the hydrogenation and dehydrogenation of certain organic compounds. Cr2O3 is used in painting as a green pigment and for coloring glass.
Potassium chromate - K2CrO4 is used in leather tanning, as a mordant in the textile industry, in the production of dyes, and in wax bleaching.
Potassium dichromate (chrompic) - K2Cr2O7 is also used for tanning leather, as a mordant for dyeing fabrics, and is a corrosion inhibitor for metals and alloys. Used in the manufacture of matches and for laboratory purposes.
Chromium (II) chloride CrCl2 is a very strong reducing agent, easily oxidized even by atmospheric oxygen, which is used in gas analysis for the quantitative absorption of O2. In addition, it is used to a limited extent in the production of chromium by electrolysis of molten salts and chromatometry.
Chromium-potassium alum K2SO4.Cr2(SO4)3 24H2O is used mainly in the textile industry - for tanning leather.
Anhydrous chromium chloride CrCl3 is used to apply chromium coatings to the surface of steels by chemical vapor deposition and is a component of some catalysts. CrCl3 hydrates are a mordant for dyeing fabrics.
Various dyes are made from lead chromate PbCrO4.
A solution of sodium dichromate is used to clean and etch the surface of steel wire before galvanizing, and also to brighten brass. Chromic acid is obtained from sodium dichromate, which is used as an electrolyte in chrome plating of metal parts.
Production
In nature, chromium is found mainly in the form of chromium iron ore FeO∙Cr2O3; when it is reduced with coal, an alloy of chromium with iron is obtained - ferrochrome, which is directly used in the metallurgical industry in the production of chromium steels. The chromium content in this composition reaches 80% (by weight).
The reduction of chromium (III) oxide with coal is intended to obtain high-carbon chromium necessary for the production of special alloys. The process is carried out in an electric arc furnace.
To obtain pure chromium, chromium(III) oxide is first prepared and then reduced by an aluminothermic method. In this case, a mixture of powdered or in the form of aluminum shavings (Al) and a charge of chromium oxide (Cr2O3) are first heated to a temperature of 500-600 ° C. Then, reduction is initiated with a mixture of barium peroxide with aluminum powder, or by igniting part of the charge, followed by adding the remaining part . In this process, it is important that the resulting thermal energy is sufficient to melt the chromium and separate it from the slag.
Cr2O3 + 2Al = 2Cr + 2Al2O3
The chromium obtained in this way contains a certain amount of impurities: iron 0.25-0.40%, sulfur 0.02%, carbon 0.015-0.02%. The content of pure substance is 99.1–99.4%. This chromium is fragile and easily ground into powder.
The reality of this method was proven and demonstrated back in 1859 by Friedrich Wöhler. On an industrial scale, aluminothermic reduction of chromium became possible only after a method for producing cheap aluminum became available. Goldschmidt was the first to develop a safe way to regulate the highly exothermic (hence explosive) reduction process.
When it is necessary to obtain high-purity chromium, industry uses electrolytic methods. Electrolysis is carried out using a mixture of chromic anhydride, chromoammonium alum or chromium sulfate with dilute sulfuric acid. Chromium deposited on aluminum or stainless steel cathodes during the electrolysis process contains dissolved gases as impurities. Purity of 99.90–99.995% can be achieved using high-temperature (1500-1700° C) purification in a hydrogen flow and vacuum degassing. Advanced electrolytic chromium refining techniques remove sulfur, nitrogen, oxygen and hydrogen from the raw product.
In addition, it is possible to obtain Cr metal by electrolysis of CrCl3 or CrF3 melts in a mixture with potassium, calcium, and sodium fluorides at a temperature of 900 ° C in an argon environment.
The possibility of an electrolytic method for obtaining pure chromium was proved by Bunsen in 1854 by subjecting an aqueous solution of chromium chloride to electrolysis.
The industry also uses a silicothermic method for producing pure chromium. In this case, chromium is reduced from oxide by silicon:
2Cr2O3 + 3Si + 3CaO = 4Cr + 3CaSiO3
Chromium is silicothermally smelted in arc furnaces. The addition of quicklime allows you to convert refractory silicon dioxide into low-melting calcium silicate slag. The purity of silicothermic chromium is approximately the same as aluminothermic chromium, however, naturally, the silicon content in it is slightly higher and the aluminum content is slightly lower.
Cr can also be obtained by the reduction of Cr2O3 with hydrogen at 1500° C, the reduction of anhydrous CrCl3 with hydrogen, alkali or alkaline earth metals, magnesium and zinc.
To obtain chromium, they also tried to use other reducing agents - carbon, hydrogen, magnesium. However, these methods are not widely used.
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.
Physical properties
Chrome is a hard, very heavy, refractory, malleable metal of a steel-gray color. Pure chromium is quite plastic, crystallizes in a body-centered lattice, a = 2.885 Å (at a temperature of 20 ° C). At a temperature of about 1830° C, there is a high probability of transformation into a modification with a face-centered lattice, a = 3.69 Å. Atomic radius 1.27 Å; ionic radii of Cr2+ 0.83 Å, Cr3+ 0.64 Å, Cr6+ 0.52 Å.
The melting point of chromium directly depends on its purity. Therefore, determining this indicator for pure chromium is a very difficult task - after all, even a small content of nitrogen or oxygen impurities can significantly change the value of the melting point. Many researchers have been studying this issue for decades and received results that are far from each other: from 1513 to 1920 ° C. Previously, it was generally accepted that this metal melts at a temperature of 1890 ° C, but modern research indicates a temperature of 1907 ° C, chromium boils at temperatures above 2500° C - the data also varies: from 2199° C to 2671° C. The density of chromium is less than that of iron; it is 7.19 g/cm3 (at a temperature of 200° C).
Chrome has all the basic characteristics of metals - it conducts heat well, its resistance to electric current is very low, like most metals, chrome has a characteristic shine. In addition, this element has one very interesting feature: the fact is that at a temperature of 37 ° C its behavior cannot be explained - a sharp change in many physical properties occurs, this change has an abrupt nature. Chrome, like a sick person at a temperature of 37° C, begins to act up: the internal friction of chromium reaches a maximum, the elastic modulus drops to minimum values. The value of electrical conductivity jumps, the thermoelectromotive force and the coefficient of linear expansion constantly change. Scientists cannot yet explain this phenomenon.
The specific heat capacity of chromium is 0.461 kJ/(kg.K) or 0.11 cal/(g °C) (at a temperature of 25 °C); thermal conductivity coefficient 67 W/(m K) or 0.16 cal/(cm sec °C) (at a temperature of 20 °C). Thermal coefficient of linear expansion 8.24 10-6 (at 20 °C). Chromium at a temperature of 20 ° C has a specific electrical resistivity of 0.414 μΩ m, and its thermal coefficient of electrical resistance in the range of 20-600 ° C is 3.01 10-3.
It is known that chromium is very sensitive to impurities - the smallest fractions of other elements (oxygen, nitrogen, carbon) can make chromium very brittle. It is extremely difficult to obtain chromium without these impurities. For this reason, this metal is not used for structural purposes. But in metallurgy it is actively used as an alloying material, since its addition to the alloy makes the steel hard and wear-resistant, because chromium is the hardest of all metals - it cuts glass like diamond! The Brinell hardness of high-purity chromium is 7-9 Mn/m2 (70-90 kgf/cm2). Spring, spring, tool, stamp and ball bearing steels are alloyed with chromium. In them (except for ball bearing steels) chromium is present along with manganese, molybdenum, nickel, and vanadium. The addition of chromium to conventional steels (up to 5% Cr) improves their physical properties and makes the metal more susceptible to heat treatment.
Chromium is antiferromagnetic, specific magnetic susceptibility 3.6 10-6. Electrical resistivity 12.710-8 Ohm. The temperature coefficient of linear expansion of chromium is 6.210-6. The heat of vaporization of this metal is 344.4 kJ/mol.
Chrome is resistant to corrosion in air and water.
Chemical properties
Chemically, chromium is quite inert, this is explained by the presence of a durable thin oxide film on its surface. Cr does not oxidize in air, even in the presence of moisture. When heated, oxidation occurs exclusively on the metal surface. At 1200°C the film is destroyed and oxidation occurs much faster. At 2000° C, chromium burns to form green chromium (III) oxide Cr2O3, which has amphoteric properties. By fusing Cr2O3 with alkalis, chromites are obtained:
Cr2O3 + 2NaOH = 2NaCrO2 + H2O
Uncalcined chromium(III) oxide easily dissolves in alkaline solutions and acids:
Cr2O3 + 6HCl = 2CrCl3 + 3H2O
In compounds, chromium mainly exhibits oxidation states Cr+2, Cr+3, Cr+6. The most stable are Cr+3 and Cr+6. There are also some compounds where chromium has oxidation states Cr+1, Cr+4, Cr+5. Chromium compounds are very diverse in color: white, blue, green, red, purple, black and many others.
Chromium easily reacts with dilute solutions of hydrochloric and sulfuric acids to form chromium chloride and sulfate and release hydrogen:
Cr + 2HCl = CrCl2 + H2
Aqua regia and nitric acid passivate chromium. Moreover, chromium passivated by nitric acid does not dissolve in dilute sulfuric and hydrochloric acids even after prolonged boiling in their solutions, but at some point dissolution does occur, accompanied by violent foaming from the liberated hydrogen. This process is explained by the fact that chromium goes from a passive state to an active one, in which the metal is not protected by a protective film. Moreover, if nitric acid is added again during the dissolution process, the reaction will stop, since chromium is again passivated.
Under normal conditions, chromium reacts with fluorine to form CrF3. At temperatures above 600° C, interaction with water vapor occurs, the result of this interaction is chromium (III) oxide Cr2O3:
4Cr + 3O2 = 2Cr2O3
Cr2O3 is green microcrystals with a density of 5220 kg/m3 and a high melting point (2437° C). Chromium(III) oxide exhibits amphoteric properties, but is very inert and difficult to dissolve in aqueous acids and alkalis. Chromium(III) oxide is quite toxic. When it comes into contact with the skin, it can cause eczema and other skin diseases. Therefore, when working with chromium (III) oxide, it is imperative to use personal protective equipment.
In addition to the oxide, other compounds with oxygen are known: CrO, CrO3, obtained indirectly. The greatest danger is from inhaled oxide aerosol, which causes severe diseases of the upper respiratory tract and lungs.
Chromium forms a large number of salts with oxygen-containing components.
Chromium is a trace element that is used in different forms. In dietary supplements, this is usually its chloride or picolinate (salt that is better absorbed by the intestines). The complex present in yeast, known as the glucose tolerance factor and including chromium and three amino acids - glutamic, glycine and cysteine, is well absorbed.
Beneficial properties of chromium and role in the body
Chromium is necessary for insulin to work. This hormone is responsible for transporting glucose from the blood into cells, where it is “burned” to release energy. Insulin is effective and helps maintain normal blood sugar levels only if the body has enough chromium. This metal increases the number of insulin receptors on the cell membrane. By increasing our glucose tolerance (the ability to tolerate its consumption without negative health consequences) by increasing the effectiveness of insulin, chromium inhibits its production, and as a result suppresses the conversion of sugar into fats. This results in a decrease in blood cholesterol levels (especially “bad” cholesterol, i.e. low-density lipoproteins) and triglycerides.
Prevention
Chromium supplements reduce the risk of diabetes in insulin-resistant people. They produce enough insulin, but the cells' sensitivity to it is reduced. As a result, to maintain normal blood glucose levels, the pancreas must secrete increased amounts of this enzyme. However, even these may not be enough, and then type II diabetes (non-insulin-dependent) develops with excess sugar in the blood, which is usually accompanied by obesity and hypercholesterolemia (high cholesterol) with all the ensuing consequences. This risk is reduced by prophylactic intake of chromium, which weakens insulin resistance and therefore increases glucose tolerance.
The benefits of chromium
Stress, infection, and increased physical activity accelerate the “burning” of glucose, and as a result, the mobilization of chromium, which is excreted more intensively in the urine. The same is observed with hyperglycemic exacerbations in diabetic patients. Chromium intake from food is usually barely adequate, so in such situations it is advisable to take chromium supplements.
Indications and methods of use of chromium, recommended daily intake, contraindications, food sources of chromium
There are no recommended daily allowances for chromium, but it is believed that chromium deficiency in adults can be prevented with doses of 50 to 200 mcg per day. It should be noted that even with a varied, healthy diet, it is almost impossible to get 200 mcg of chromium per day from food. The standard menu usually gives us 40-50 mcg/day, and a fasting diet (for example, when losing weight) is naturally less.
- Flaw. Chromium deficiency is fraught with irritability, weight gain and impaired sensitivity of the limbs, as well as exacerbation of non-insulin-dependent diabetes.
Excess. Chromium supplements appear to be harmless. However, their high doses make it difficult to absorb and.
Indications for the use of chromium
Difficulty digesting proteins, fats or carbohydrates.
Increased blood glucose levels (insulin resistance, type II diabetes).
Increased levels of “bad” cholesterol (low-density lipoprotein) and triglycerides in the blood.
Contraindications
Diabetic patients should take chromium only after consulting a doctor. They may need to adjust the doses of insulin and/or other medications already prescribed for their illness.
Methods of application
Doses
Usually, chromium in supplements is combined with other minerals, so you need to check the label on the package to find out the amount in the preparation. One tablet or capsule should contain from 25 to 200 mcg (more is dangerous). Such dietary supplements are taken as general tonics, as well as when losing weight through a fasting diet and to increase the effectiveness of insulin.
