Chromium

CHROMIUM, Chromium (from the Greek chroma-color), symbol Cr, chemical element with atomic weight 52.01 (isotopes 50, 52, 53, 54); atomic number 24, occupies a position in the even subgroup of group VI of Mendeleev's table. Compounds of Cr are often found in nature, however in insignificant quantities. The content of Cr in the earth's crust is about 0.03%. The most important chromium ore is chromite iron ore, chromite (FeO-Cr₂O₃), mined in the Urals. Often a negligible impurity of salts or oxides of Cr gives color to many natural compounds (e.g., emerald, blue clays, etc.). In plant and animal organisms, Cr is found in trace amounts. In the ash of viburnum berries, for example, 0.02% Cr has been found. Metallic Cr is obtained by reduction of ores in blast furnaces or by electrolysis, or by aluminothermy; it is a grayish-white metal of leafy-crystalline structure with a specific gravity of 6.9, melting at 1550°. Hardness according to Moss, with a carbon content of 1.5-3%, is 9 (Rydberg). Hydrochloric acid slowly dissolves metallic Cr in the cold, dilute sulfuric acid only upon heating, forming salts of trivalent Cr. Nitric acid and aqua regia do not act on Cr. The properties of alloys of Cr with iron and other metals are extremely important, finding wide application in the manufacture of surgical instruments and thermoelectric apparatus (electric sterilizers, electrocautery, etc.). These alloys have high resistance to corrosion and a small temperature coefficient of resistance. Stainless steels usually contain several percent of Cr. For electric heating appliances, 'nichrome' is used, which is an alloy of iron with Cr and nickel. Cr is used instead of nickel for chromium plating to protect against corrosion. In recent times, Cr has acquired very great importance for the manufacture of chromium-plated silver dental prostheses that replace gold. In its compounds, Cr can be divalent, trivalent, and hexavalent. Compounds of Cr²⁺ are unstable and easily pass into compounds of trivalent Cr. Trivalent Cr resembles iron and aluminum in its ability to form complex compounds and double salts (alums). In the form of a hexavalent ion, Cr forms only anions, in many respects similar to sulfates. Analytical reactions of salts of Cr and chromic acid salts (ions Cr³⁺, CrO₄^2- and Cr₂O₇^2-): salts of trivalent Cr are colored violet or green. Salts of chromic acid are yellow (basic and neutral) or orange-red; in an acidic solution they are converted by reducing agents (e.g., H₂S, SO₂, C₂H₅OH, HCHO, HI, etc.) into salts of trivalent Cr, Cr₂O₇^2- + 3H⁺ + 5H⁺ → Cr³⁺ + 4H₂O. I. Reactions of the trivalent chromium ion (Cr³⁺): a) caustic alkalis precipitate a bluish-green hydrate of Cr(OH)₃, soluble in excess reagent and after dilution with water again precipitating upon boiling; b) carbonic alkalis precipitate a greenish-blue hydroxide of Cr, partially soluble in excess reagent; c) when salts of trivalent Cr are fused with a mixture of 2 parts KNO₃ and 1 part Na₂CO₃, salts of chromic acid are obtained, Cr₂(SO₄)₃; + 5Na₂CO₃ + 3KNO₃ → 2Na₂CrO₄ + 3Na₂SO₄ + 3KNO₂ + 5CO₂; with the chromic salt obtained, the reactions of the chromate ion described below are performed. II. Reactions of the chromate ion (CrO₄^2-) and the dichromate ion (Cr₂O₇^2-): a) silver nitrate gives a red precipitate of silver chromate, soluble in warm HNO₃; b) barium nitrate gives a yellow precipitate of barium chromate, insoluble in caustic alkalis and acetic acid, soluble in hydrochloric acid; c) a solution of chromates acidified with sulfuric acid gives upon shaking with hydrogen peroxide and ether an unstable bright blue coloration of the ether layer. All soluble compounds of Cr are poisonous, causing upon external application phenomena of irritation and inflammation of the skin (see Dermatitis), upon internal application phenomena of local and general poisoning (see Poisoning). Compounds of hexavalent Cr are many times more poisonous than trivalent Cr. Most often, chronic poisoning by compounds of Cr is observed in the production of paints (see Paints), match, textile printing, wallpaper, leather, fur, and photographic industries. When inhaling dust containing chromium compounds, catarrhs of the upper respiratory tract, hyperemia and swelling of the nasal mucosa are observed, which in case of prolonged exposure leads to its necrosis, vomiting, diarrhea, albuminuria. The detection of compounds of Cr in sanitary and forensic-chemical practice is carried out by destroying organic matter if necessary according to Fresenius-Babo and detecting the CrO₄^2- ion by the reactions described above. Compounds of chromium. Oxide of Cr, Cr₂O₃, a more or less light green amorphous powder, dissolving in molten glass to form bright green transparent alloys. Oxide of Cr calcined at high t° passes into more darkly colored modifications insoluble in acids. Oxide of Cr has an amphoteric character. - Chromium chloride, CrCl₃, red-violet crystals, soluble in water in the presence of traces of divalent chromium chloride (CrCl₂), with an intensely green color. - Chromium sulfate, chromic sulfate, is known (like the previous compound) in the form of various hydrates differing from each other in the position of the acid residue in the coordination sphere (see Coordination theory), capable under certain conditions of passing into one another, which can be determined by a change in color. A characteristic property of chromic sulfate is to form alums. Chromium alums, K₂SO₄-Cr₂(SO₄)₃-24H₂O, dark violet crystals soluble in water. The solution turns green upon heating. Widely used as a mordant in dyeing and for tanning leather. - Compounds of hexavalent Cr (chromic anhydride and corresponding hydrates-chromic acid) find application in medical and medical-laboratory practice. - Chromic anhydride, CrO₃, obtained from concentrated solutions of potassium dichromate by the action of concentrated sulfuric acid in the form of shiny red-brown, easily spreading crystals, is used in medicine as a powerful cauterizing agent. A powerful oxidizer, oxidizing (often with explosion and ignition) inorganic and organic substances-2CrO₃ → Cr₂O₃+3O₂. In an aqueous solution of chromic acid, along with a small amount of chromate ions (CrO₄^2-), there are dichromate ions (Cr₂O₇^2-). Salts of CrO₄^2- are yellow, Cr₂O₇^2- are orange-red. In the anhydrous state, chromic acid is unknown; when attempts are made to obtain it, CrO₃ is obtained. By solubility, salts of chromic acid resemble salts of sulfuric acid. Chromium finds only limited application for therapeutic purposes in the form of chromic anhydride, Chromium tri-oxydatum (incorrectly: Acidum chromicum anhydricum). In concentrated solutions (25% and more), chromic anhydride serves as a cauterizing agent (warts, neoplasms, ulcers of mucous membranes), in diluted solutions (1:100) as an astringent, for example, in excessive sweating of the feet, chafes in the groin, armpits, etc. The astringent action of chromic anhydride is explained by its very powerful coagulating action on proteins and albuminoids. At the same time, insoluble compaction products are formed. The cauterizing action of solutions (concentrated) of chromic anhydride is due to the simultaneous oxidation and coagulation of body tissues. Potassium chromate (K₂CrO₄) and potassium dichromate (potassium bichromate, K₂Cr₂O₇). The first is yellow crystals, soluble in 2 parts of water with an intensely yellow color, noticeable upon dilution 1:40,000; when the aqueous solution is acidified and then evaporated, potassium dichromate is isolated in the form of orange-red, water-soluble crystals: 2K₂CrO₄ + 2H⁺ → 2K⁺ + K₂Cr₂O₇ + H₂O. When the latter solution is made alkaline, potassium chromate is obtained. Chromic acid does not form acid salts. In acidified solutions, as well as upon heating, the salts are powerful oxidizers. - Lead chromate, PbCrO₄, a yellow powder, insoluble in water, soluble in hot alkalis. Basic salts, colored in various colors (from orange to red), find application like the previous salt as paints. Calcium chromate and calcium dichromate, CaCrO₄-2H₂O and CaCr₂O₇-3H₂O. The first is rather insoluble in water (1:250), the second are red, easily soluble crystals. Shubin. Toxic effects are exerted by chromic acid, mono- and bichromates of sodium and potassium, as well as chromium alums and the paint-lead chromate.

The industries where workers may be affected by the compounds of Chromium are very numerous, due to the wide application of Chromium. The most important are: the production of chromium compounds from ore at chromium plants, the use of these compounds as tanning, etching, coloring and other substances in the leather, match, textile, chemical, explosives, printing (photomechanics), photography, electroplating and many other industries. Under production conditions, workers are affected by dust released during certain processes (grinding and sifting of salts) and tiny droplets of solutions, which are carried into the air along with water vapor, hydrogen bubbles and others. The effect of Chromium on workers is manifested mainly on the skin and mucous membranes of the nose and respiratory tract: lesions of internal organs in workers in production conditions have not been observed. The most common and characteristic occupational disease are ulcerative and necrotic processes in the nose. These processes develop on the mucous membrane of the nasal septum: particles of Chromium that enter here with the air cause an inflammatory process, which then turns into an ulcerative one, as a result (often after 1-3 months) a necrosis with perforation of the septum forms. The perforation affects only the cartilaginous part of the septum and does not extend to the bony part. In the respiratory tract, inflammations of the pharynx, larynx, trachea, and bronchi of varying severity may develop. The second common consequence of exposure to chromates is skin ulcers. They appear on any part of the body - on places where there is even the slightest violation of the integrity of the skin; most often they are found on areas covered with thin skin. In addition to ulcers, other forms of skin lesions are observed less frequently - dermatitis and eczema. The above-mentioned pathological changes of the skin and mucous membranes occur in all workers in industries where chromates are used; especially many cases have been registered among workers in chromium plants. Legge (1907) found among 176 workers in a chromium plant that 71.6% had perforation of the nasal septum, and 11.3% had its ulceration; Lehmann and Fischer (1911) report almost the same figures for German plants; among workers in the same plants, for a year, skin lesions were registered in 31.5% of workers. In the American literature in recent years, there are many reports of lesions in workers engaged in electroplating; such are the data of Dixon (1929), Blair (1930), Bloomfield and Blum (1928); according to the latter, for example, in one workshop of 14 people, a normal nasal septum was found only in two. Prevention. In chromium plants - mechanization of work processes, sealing of equipment (evaporators, tanks for alkalis); wearing rational special clothing, gloves, thorough washing of hands after work (for the latter, a 5% solution of sodium hyposulfite can be used), monitoring body cleanliness, baths, showers, wearing loose tampons in the nostrils, washing the nose at the end of work with physiological saline solution or McKenzie mixture. For electroplating - properly designed local exhaust ventilation, the use of a "protective pillow" (a layer of liquid - a petroleum distillation product - is poured on top of the bath), which prevents the release of hydrogen bubbles into the air; wearing gloves when immersing and removing. Treatment of ulcerative and other lesions of the nose and skin is carried out according to general principles.