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Encyclopedia Britannica - Main :: CLI-COM |
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COBALT (symbol Co, atomic weight 59) , one of the metallic chemical elements. The term " cobalt
Paracelsus
mineral
cobalt
chief
oxide
porcelain
Metallic cobalt may be obtained by reduction of the oxide
fair
Three characteristic oxides of cobalt are known, the monoxide, CoO, the sesquioxide, Co203, and tricobalt tetroxide, Co304; besides these there are probably oxides of composition CoO2, Co3Q6, Co60s and Co4Oi. Cobalt monoxide, CoO, is prepared by heating the hydroxide or carbonate in a current of air, or by heating the oxide Co304 in a current of carbon dioxides It is a brown coloured powder which is stable in air, but gives a higher oxide when heated. On heating in hydrogen, ammonia or carbon monoxide, or with carbon or sodium, it is reduced to the metallic state. It is readily soluble in warm dilute mineral
By heating a mixture of cobalt oxalate and sal-ammoniac in air, it is obtained in the form of minute hard octahedra, which are not magnetic, and are only soluble in concentrated sulphuric acid. The cobaltous salts are formed when the metal, cobaltous oxide, hydroxide or carbonate, are dissolved in 'acids, or, in the case of the insoluble salts, by precipitation. The insoluble salts are rose-red or violet in colour. The soluble salts are, when in the hydrated condition, also red, but in the anhydrous condition are blue. They are precipitated from their alkaline solutions as cobalt sulphide by sulphuretted hydrogen, but this precipitation is prevented by the presence of citric and tartaric acids; similarly the presence of ammonium salts hinders their precipitation by caustic alkalis. Alkaline carbonates give precipitates of basic carbonates, the formation of which is also retarded by the presence of ammonium salts. For the action of ammonia on the cobaltous salts in the presence of air see Cobaltammines (below). On the addition of potassium cyanide they give a brown precipitate of cobalt cyanide, Co(CN)2, which dissolves in excess of potassium cyanide to a green solution. Cobalt chloride, CoC12, in the anhydrous state, is formed by burning the metal in chlorine or by heating the sulphide in a current of the same gas. It is blue in colour and sublimes readily. It dissolves easily in water, forming the hydrated chloride, CoC12.6H2O, which may also be prepared by dissolving the hydroxide or carbonate in hydrochloric acid. The hydrated salt forms rose-red prisms, readily soluble in water to a red solution, and in alcohol to a blue solution. Other hydrated forms of the chloride, of composition CoC12.2H2O and CoC.I2.4H2O have been described (P. Sabatier, Bull . Soc. Chim. 51, p. 88; Bersch, Jahresb. d. Chemie, 1867, p. 291). Double
standing
Cobalt fluoride, CoF2.2H2O, is formed when cobalt carbonate is evaporated with an excess of aqueous hydrofluoric acid, separating in rose-red crystalline crusts. Electrolysis of a solution in hydrofluoric acid gives cobaltic fluoride, CoF3. Sulphides of cobalt of composition Co4S3, CoS, Co3S4, Co2S3 and CoS2 are known. The most common of these sulphides is cobaltous sulphide, CoS, which occurs naturally as syepoorite, and can be artificially prepared by heating cobaltous oxide with sulphur, or by fusing anhydrous cobalt sulphate with barium sulphide and common salt. By either of these methods, it is obtained in the form of bronze-coloured crystals. It may be prepared in the amorphous form by heating cobalt with sulphur dioxide, in a sealed tube, at Zoo C. In the hydrated condition js formed by the action of alkaline sulphides on cobaltous salts, or by precipitating cobalt acetate with sulphuretted hydrogen (in the absence of free acetic acid). It is a black amorphous powder soluble in concentrated sulphuric and hydrochloric acids, and when in the moist state readily oxidizes on exposure. Cobaltous sulphate, CoSO4.7H2O, is found naturally as the mineral bieberite, and is formed when cobalt, cobaltous oxide or carbonate are dissolved in dilute sulphuric acid. It forms dark red crystals isomorphous with ferrous sulphate, and readily soluble in water. By dissolving it in concentrated sulphuric acid and warming the solution, the anhydrous salt is obtained. Hydrated sulphates of composition CoSO4.6H2O, CoSO4.4H2O and CoSO4H2O are also known. The heptahydrated salt combines with the alkaline sulphates to form double
The cobaltic salts corresponding to the oxide Co203 are generally unstable compounds which exist only in solution. H. Marshall (Proc. Roy. Soc. Edin. 59, p. 760) has prepared cobaltic sulphate Co2(SO4)3.18H2O, in the form of small needles, by the electrolysis of cobalt sulphate. In a similar way potassium and ammonium cobalt alums have been obtained. A cobaltisulphurous acid, probably H6 [(SO3)6Co2] has been obtained by E. Berglund (Berichte, 1874, 7, p. 469), in aqueous solution, by dissolving ammonium cobalto- cobaltisulphite (NH4)2Co2 [(SO3)6.Co2] 14H2O in dilute hydrochloric or nitric acids, or by decomposition of its silver salt with hydro- chloric acid. The ammonium cobalto-cobaltisulphite is prepared by saturating an air-oxidized ammoniacal solution of cobaltous chloride with sulphur dioxide. The double salts containing the metal in the cobaltic form are more stable than the corre- sponding single salts, and of these potassium cobaltinitrite, End of Article: COBALT (symbol Co, atomic weight 59) If you wish, you can link directly to this article.
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