CHROMIUM, one of the metallic chemical elements, so called from the Greek xp°Va> colour, in allusion to the fine colours of its compounds; symbol Cr, atomic weight 52-4. It does not occur in the free state or very abundantly in nature. It is a constituent of the minerals chrome iron-stone, Cr2Fe04; chrome-ochre, Cr203 ; ouvarovite, or chrome garnet, Si3CaCr?0 10; crocoisite, PbCr04, in which it was discovered by Vauquelin in 1797 ; of Vauquelinite, a chromate of lead and copper ; and of some iron-ores and meteoric irons; and it is the cause of the colour of green serpentine, pyrope, and the emerald. The properties of chromium vary much according to the method of its pre-paration. By Wohler's process of reducing the sesquioxide with zinc, it is obtained as a shining green powder, of specific gravity 6-81, which tarnishes in the air, and dissolves in hydrochloric and warm dilute sulphuric acid, but is not acted on by strong nitric acid. Deville, by intensely igniting chromic oxide with sugar-charcoal in a lime crucible, procured chromium of a bright steel-grey colour, very hard, capable of a high polish, less fusible than platinum, and of specific gravity 6. Crystallized chromium obtained by F'remy was unaffected by the strongest acids.
Chromium forms three series of compounds: the chromous, typified by the chloride CrCL, in which chromium is a dyad ; the chromic or sesgm'-compounds, such as the oxide, Cr203, and chromic chloride, Cr2Cl6, in which the metal plays the part of a tetrad, or pseudo-triad; and chromate compounds, in which it is a hexad, exemplified by the hexafluoride, CrF6, the oxychloride, Cr02Cl2, the anhy-dride, Cr03, and potassium chromate, K2Cr04. In the hypothetical perchromic anhydride, Cr207, chromium may be regarded as an octad.
Chromous salts resemble the ferrous, in forming a dark-brown compound when nitric oxide is added to their solutions. The anhydrous protoxide, CrO, has not been obtained; and the hydrate 2CrO.H20, precipitated of a brown colour from solutions by potassium hydrate, is a very unstable body. The dichloride CrCl2 is a powerful deoxidizer ; it forms with water a blue solution, which turns green on exposure to the air.
The salts of the sesquioxide, or chromic salts, have an acid reaction. They are green, or from red to violet in colour. Ammonia precipitates from solutions of the violet salts a grey-blue hydrate, and from solutions of the green salts, a grey-green hydrate,the former precipitate giving a red, the latter a green solution with acids. Potash or soda throws down from solutions of the violet or green salts a bluish-green hydrate, soluble in excess of the precipitant, but reprecipitated on boiling. Accord-ing to Lowel there are four modifications of the hydrate of chrome, two green, one violet-carmine, and a violet-blue. The hydrate can be economically made by boiling two parts by weight of potassium chromate with one of sulphur ; the presence of a little potash assists the opera-tion. Sesquioxide of chromium, Cr203, occurs native in argillaceous deposits. It may be obtained in the amorphous state by the ignition of the hydrate, and crystallized by decomposing potassium chromate with chlorine at a red heat. It can be melted at a high temperature into a greenish-black mass, and is with difficulty reduced by charcoal at an intense heat. A mixture of nitric acid and potassium chlorate converts it into chromic acid. The chromites are a class of bodies in which chromic oxide Cr203 is united with protoxides, as in the compound Cr304, and in chrome ironstone, Cr2Fe04. The latter, the most abundant ore of chromium, contains besides iron and chromium variable proportions of magnesium andaluminium, and is isomorphous with magnetic iron ore, Fe304, and spinel, MgAl204. It is a massive and compact, granular, rarely crystallized, black-coloured mineral, of specific gravity about 4'4, and hardness 5'5. It occurs usually in serpentine, and is found in the Shetland Isles, the depart-ment of Var in France, Volterra in Tuscany, Silesia, Bohemia, Boraas in Norway, the Urals, near Baltimore in the United States, and in many other localities. In 1869 the exports of chrome ore from Norway amounted to 210 tons. Chrome ironstone can be decomposed in the dry way for analysis by fusion with potassium bisulphate, to which sodium carbonate and then a little nitre are subse-quently added. Chromic chloride, Cr2Cl6, can be obtained either in the anhydrous insoluble condition, or as the green hydrated salt, soluble in water. Similar compounds are the fluoride, Cr,F0. the bromide, Cr2Br6, and the iodide, Cr2I6. The sesquisulphates of chromium are the anhydrous salt, Cr2(S04)3, the hydrated salts with 5 and 15 molecules of water, a basic sulphate, Cr903.2S03, and the chrome alums, of the general constitution Cr2M2'(S04)4, 24H20, isomorphous with common alum. Certain oxides inter-mediate between the sesquioxide and trioxide have been termed chromâtes of chromium.
The trioxide or anhydride of chromium, Cr03, com-parable with sulphuric anhydride, S03, can be prepared by the action of strong sulphuric acid upon potassium bichromate ; as the liquid cools, the anhydride separates from it in crimson acicular crystals. It can also be obtained by decomposing barium chromate with nitric acid. Chromic anhydride deliquesces in air, and dissolves in water, forming a solution which contains chromic acid ; it is a powerful oxidizer, and acts violently on organic substances, such as alcohol and ether, becoming reduced to the sesquioxide. Dry ammonia gas is converted by it into nitrogen and aqueous vapour. When peroxide of hydrogen, H202, is added to a solution of chromium trioxide, an unstable blue compound of probable constitution H2Cr2Og, or perchromic acid, is formed, which dissolves in ether, and is decomposed by potash and soda, but forms stable compounds with ammonia, quinine, and other bodies. Chromic acid, H2Cr04, has not been isolated, and hydrated salts of the type HM'Cr04 are unknown. The chromâtes are a large class of bodies, isomorphous with the sulphates. They are bitter and poisonous salts, mostly of a yellow or red colour, and except those of the alkali metals, and of calcium, strontium, and magnesium, are more or less insoluble in water. Most of the insoluble chromâtes are basic. The chromâtes of the alkali metals are the neutral or normal yellow salts, M'2Cr04, or M'2O.Cr03, and the orange-red bichromates, or acid salts, M'2O.2Cr03. Potas-sium trichromate, K,0.3CrOs, represents a third series of chromâtes. With salts of lead, bismuth, and barium, the alkaline chromâtes give yellow precipitates ; with silver salts, a purplish red ; and with mercurous salts, a brick-red precipitate. Boiled with hydrochloric acid they give a green solution of sesquichloride of chromium ; and with sulphurous acid, or with sulphuric acid and alcohol, sugar, and other reducing agents, they yield chrome alum. When a chromate is heated with sulphuric acid and common salt, vapours of oxychloride of chromium, or chlorochromic acid, are produced. Insoluble chromates fused with nitre yield soluble yellow potassium chromate, K2Cr04. This salt is prepared on a large scale by oxidizing chrome-iron ore in a furnace, in the presence of carbonate, sulphate, or chloride of potassium, and chalk or lime. The red bichromate or acid chromate of potassium, K2Cr207, is made by acidifying a solution of the neutral salt, or by Jacquelin's process, in which chrome-iron ore is ignited with chalk to obtain the neutral chromate of calcium ; this is then converted by sulphuric acid into calcium bichromate, which by double decomposition with potassium carbonate yields the potas-sium bichromate. It melts at a red heat, and at a white heat evolves oxygen, as also when warmed with sulphuric acid. Heated with sulphur or charcoal it deflagrates. The solution gives with sulphydric acid a precipitate of mixed chromic oxide and sulphur. Ammonium bichromate is decomposed on the application of heat into nitrogen, water, and chromic oxide. The oxides and salts of chromium give, in both the inner and outer blowpipe flames, a green bead with borax. Chromium unites with iron and aluminium; and can be obtained combined with mercury by treating a solution of a chromic salt with sodium amalgam. Chromium is estimated gravimetrically in the form of the sesquioxide, or of a lead or barium salt ; volumetrically, by the oxidizing effect of a chromate on oxalic acid, hydriodic acid, or potassium ferrocyanide.
The alloy termed chromeisen, containing about three parts by weightof chromium to one of iron, is hard enoughto serve for cutting glass. An extremely soft steel can be made by em-ploying it instead of spiegeleisen in Siemens's steel process. Chromium compounds are in request for a great diversity of purposes. Free chromic acid and potassium bichromate are used in calico-printing, and in bleaching tallow and palm oil. The bichromate is also employed for the volumetric estimation of ferrous salts, in the printing of photographs, and in galvanic batteries ; it has even been used with lead chromate for the adulteration of snuff. It is itself sometimes adulterated with a mixture of sulphate and chloride of sodium, coloured with a strong solution of bichromate. Potassium bichromate in contact with the skin produces dangerous ulcers, and internally it acts as a violent poison. Fused lead chromate is of value in organic chemistry as an oxidizer, and the unfused salt is the well-known pigment chrome-yellow. Chrome-red is a basic lead chromate, Pb2Cr05. Other pigments are the sesquioxide of chromium, or chrome-green, used in glass-staining, porcelain-painting, and in the printing of bank-notes; Pannetier's emerald green, a hydrate of the composition H4Cr205; Leune and Castelhaz's green, another hydrate; Guignet's pigment vert, a basic chromic borate; and Plessy's green, which is a phosphate of chromium. Casali (Gazzetta Chimica Itallana, 1874) recommends for the preparation of an intense green pigment to calcine an intimate mixture of 1 part of potassium bichromate and 3 parts of baked gypsum; the mass obtained is boiled with water, or treated with hydrochloric acid. The pigment used to pro-duce a pink colour on earthenware is made by heating to redness a mixture of 30 parts of peroxide of tin, 10 of chalk, and 1 of potassium chromate,the product being powdered and washed with hydrochloric acid. (F. H. B.)