CHROMIUM is a metallic chemical element, called so from the Greek xpW,ua, colour, on account of the varied colours of its compounds. In 1798, L. N. Vauquelin and M. H. Klaproth simultaneously and independently discovered a new element in addition to lead in the mineral crocoisite or crocoite (lead chro mate, first described by Lehmann in 1762. The metal was not isolated, however, until 1859, when F. Wi hler obtained small quantities of it by reduction of the trichloride with potas sium. Owing to the difficulty of reduction and of electro-deposi tion, the metal remained a chemical curiosity for many decades, though it is now produced on a large scale for use in stainless and alloy steels. Its symbol is Cr, and it has an atomic number of 24 and an atomic weight of 52•01. Owing to the possession of six valency electrons it has a maximum of valency of six.
Occurrence and Production.—The metal is not found free in nature, and, though the element has a minimum valency of two, all of the ores contain trivalent or sexavalent chromium. The chief mineral from which the metal and commercial chromium compounds are obtained is chromite, or chrome iron ore, which consists largely of ferrous chromite, Fe(Cr02)2 or FeO,Cr203. This ore is widely distributed, but Rhodesia, the United States and India are the chief producers. In addition to chromite and crocoisite, chromium is also found as sesqui-oxide in chrome ochre, as potassium chromate, as redingtonite (a hydrated sulphate), as phospho-chromates, as basic chromates, as ferrous thiochromite, and in various other com binations. The green colour of emerald, serpentine, chrome-mica and chrome-garnet is due to chromium, as is also the red colour of ruby, and some sapphires and spinels.
The chromium compounds of commerce are practically all derived from the alkali chromates and dichromates (q.v.), ob tained by roasting chromite with lime in an oxidizing atmosphere and subsequent treatment with an aqueous solution of alkali sulphate. Metallic chromium is obtained by H. Goldschmidt's alumino-thermic process, in which chromium sesquioxide or po tassium dichromate is reduced by heating with aluminium powder. It may also be obtained by electro-deposition from solutions of chromic sulphate mixed with much "chromic acid," this method being used for plating iron and steel. Pure chromium may be obtained by reduction of the sesquioxide at C with pure hydrogen, while pure chromium amalgam is obtained on treating concentrated solutions of chromic chloride with sodium amalgam. If the amalgam be distilled at 35o° C in hydrogen, or 300° C in vacuo, a residue of pyrophoric (spontaneously inflam mable) chromium powder remains. Ferro-chrome alloys may be obtained by direct smelting of chromite, with or without addition of chromium sesquioxide, by means of carbon in the electric furnace. The natural or enriched ferro-chrome alloy is utilized in the manufacture of stainless steels and chrome alloy steels.
Properties.—Chromium is a steel-white metal, harder than iron, cobalt or nickel, and capable of taking a brilliant polish. It is non-magnetic at ordinary temperatures, but becomes mag netic at —15° C. The melting point is I,540° C and the boiling point 2,200° C, the melting point being higher than that of manga nese, iron, cobalt or nickel, though the boiling point is less than that of any of these. The density of the metal varies from 6.7 to 7.1. The electrolytically deposited metal occludes about 25o times its volume of hydrogen. It crystallizes either as rhombo hedra or tetragonal pyramids, and X-ray examination discloses that it can exist in two allotropic modifications.
Though pyrophoric in the finely divided condition, chromium in the massive form is not affected by moist air, and only very slowly by dilute acids. Neither fuming nitric acid nor aqua regia attacks it, but with hot concentrated sulphuric acid it gives sulphur dioxide and a dark solution. Warm dilute hydrochloric or sul phuric acid slowly dissolves the metal, hydrogen being evolved with the formation of chromous salts. Heated in hydrogen chloride, the metal yields crystalline colourless chromous chloride, while chlorine yields violet chromic choride. Chromium is not attacked even by fused alkalis, but fused potassium nitrate or chlorate oxidizes it rapidly to potassium chromate. Heated with sulphur or in hydrogen sulphide, chromium yields chromous sul phide, CrS, whereas with carbon, silicon or boron, various carbides, silicides or borides are formed. Though a chromium carbonyl, is known, it is not formed by heating chromium in car bon monoxide, but by treating chromic chloride with carbon monoxide in the presence of magnesium phenyl bromide. Pyro phoric chromium combines directly with nitrogen to form a magnetic nitride. Chromium amalgam also combines with nitro gen to form chromic nitride (CrN), chromous nitride being formed by heating chromium to 85o° in ammonia.
Chromium is known in five conditions of oxidation, having all valencies from two to six. The salts of bivalent chromium (chro mous) are all powerful reducing agents, thus resembling the bivalent salts of vanadium and titanium. Chromous salts of nearly all the commoner acids have been prepared and are gen erally colourless in the anhydrous state and blue when hydrated or in solution. Chromous fluoride is green; potassium chromous carbonate, is yellow; and chromous acetate, is red. Chromous sulphate, which is iso morphous with copperas, also yields double sul phates of the type where R is a monovalent metal.
Chromic Compounds.—Chromic salts, derived from the triva lent form, are very stable and are neither reducing nor oxidizing agents. They can be reduced or oxidized only with difficulty, to the chromous stage by zinc and acid, and to the chromate stage by alkaline peroxides, hypochlorites or hypobromites. Chromates, like permanganates, are reduced by hydrogen peroxide on warm ing in acid solution, but the reduction is not carried to the biva lent stage, chromic salts only being formed. Chromic oxide, Cr203, is a much feebler base than chromous oxide, CrO, and chromic salts are consequently always hydrolysed in solution and have an acidic reaction. Chromic oxide, in fact, has decided acidic properties and yields salts with alkalis, of which barium chromite, is a characteristic example. This acidic nature of chromic oxide gives rise to an extended series of natural and artificial gem-like minerals called chrome-spinels, for example, nickel spinel, brilliant green crystals belonging to the cubic system. All chromic salts are highly coloured, most be ing green or violet, though the chromicyanides are yellow or orange, (CN) and the chromithiocyanates are ruby-red, Chromium yields purple double-sulphates of the type R2SO4,Cr2(SO4)3,24H20, which are alums (q.v.).
Chromic salts closely resemble cobaltic salts in their capacity for forming double and complex salts. In nearly all these salts it is possible to discern six non-ionising groups attached or co ordinated to the chromium atom, which is accordingly said to possess the co-ordination number six like cobalt. In the case of the ammonia-addition compounds (see AMMINES) whole series are known which are indistinguishable in appearance and general properties from the corresponding cobaltammines. These com prise such types as luteo-chromic ammines, pur pureo-ammines, and among many others, praseo ammines, Complex salts are also formed by replacing ammonia with pyridine, ethylenediamine, and similar organic bases. Some of these complex salts have been shown to be capable of existing in optically active forms, thus prov ing that the groups co-ordinated to the chromium atom are not planar but have a spatial distribution which is octahedral. The use of chrome alum and chromic fluoride as mordants in fixing soluble dyestuffs on fabrics has been shown to be due to the capacity of the chromium atom to enter into complex forma tion with the dyestuffs, yielding highly insoluble compounds, often of different colour from the combined dyestuff, firmly fixed on or in the fibre. It is probable that the use of chromium com pounds in tanning is due to complex reaction with the organic substances present in hides.
F. Hein has recently proved that chromium can possess the valency four, a fact long suspected from the existence of an oxide having the empirical formula hitherto regarded as chromous chromate, or as basic chromic chromate, (Cr0)2Cr04. Hein prepared triphenyl chromium hydroxide a powerfully alkaline base, yielding a series of neutral salts such as the perchlorate, He also prepared compounds containing chromium with valency five, for example, tetraphenyl chromium hydroxide, and salts such as the iodide, were isolated. The only inorganic compounds of chro mium known with the valency of five are the oxytrichloride, and its derivatives.
Without exception all inorganic compounds containing chro mium with valency six contain oxygen, and are derivatives of the trioxide, Cr03, commonly but improperly called chromic acid. Chromium trioxide is properly the anhydride of the hypothetical acids, and analogous to sulphuric acid and pyro sulphuric acid. The salts of these two acids, chromates and dichro mates respectively, are known for the majority of the stronger bases. (See CHROMATES and DICHROMATES.) Chromic anhydride was originally obtained by the hydrolysis of chromyl fluoride, but is nowadays made commercially by the action of concentrated sulphuric acid on solutions of chromates or dichro mates, usually potassium dichromate. It forms dark red rhombic needles, which decompose at 33o° evolving oxygen and leaving green chromium sesquioxide, It is extremely soluble in water, giving acid solutions from which chromates and dichro mates can be produced.
Analysis and Uses.—Chromium can be detected in compounds by the formation of a green borax bead, by the yellow colour of chromates formed on fusion with potassium nitrate, by the for mation of red chromyl chloride, and by the blue colour of per chromic acid. Dichromates may be detected in the presence of chromates by the brown precipitate formed on addition of a hot solution of sodium thiosulphate, while free chromic acid may be detected in the presence of dichromates by the liberation of io dine from a solution of potassium iodide. Chromium is sepa rated in group analysis from aluminium and ferric hydroxides by oxidation to soluble chromate with sodium peroxide. Gravimet rically, chromium is usually precipitated with ammonia as chromic hydroxide, and ignited to the sesquioxide. Volumetrically, as chromate or dichromate, it may be estimated by reduction with ferrous salts or acidified potassium iodide.
Metallic chromium is used industrially in the manufacture of steels, and is a common constituent of many ferrous and non ferrous alloys. Stainless steel, commonly used for rustlers cut lery, tools and machinery, is essentially a ferro-chrome alloy, the percentages of chromium and iron usually being about 13 and 86 respectively. As a protective surface for metals, chromium is also used in the form of plate which may be applied electrolyt ically or by welding. Chromium compounds, usually dichromates or chrome alum, are used in the manufacture and application of dyes and pigments, in tanning hides, and in photography. Chromic salts are practically non-toxic, but the soluble chromates and dichromates are therapeutically dangerous.