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 200° C. In the hydrated condition it is 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 CoSO4·H2O are also known. The heptahydrated salt combines with the alkaline sulphates to form double sulphates of composition CoSO4·M2SO4·6H2O (M = K, NH4, &c.).
The cobaltic salts corresponding to the oxide Co2O3 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)6·Co2] 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 hydrochloric 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 corresponding single salts, and of these potassium cobaltinitrite, Co2(NO2)6·6KNO2·3H2O, is best known. It may be prepared by the addition of potassium nitrite to an acetic acid solution of cobalt chloride. The yellow precipitate obtained is washed with a solution of potassium acetate and finally with dilute alcohol. The reaction proceeds according to the following equation: 2CoCl2 + 10KNO2 + 4HNO2 = Co2(NO2)6·6KNO2 + 4KCl + 2NO + 2H2O (A. Stromeyer, Annalen, 1855, 96, p. 220). This salt may be used for the separation of cobalt and nickel, since the latter metal does not form a similar double nitrite, but it is necessary that the alkaline earth metals should be absent, for in their presence nickel forms complex nitrites containing the alkaline earth metal and the alkali metal. A sodium cobaltinitrite is also known.
Cobalt nitrate, Co(NO3)2·6H2O, is obtained in dark-red monoclinic tables by the slow evaporation of a solution of the metal, its hydroxide or carbonate, in nitric acid. It deliquesces in the air and melts readily on heating. By the addition of excess of ammonia to its aqueous solution, in the complete absence of air, a blue precipitate of a basic nitrate of the composition 6CoO·N2O5·5H2O is obtained.
By boiling a solution of cobalt carbonate in phosphoric acid, the acid phosphate CoHPO4·3H2O is obtained, which when heated with water to 250° C. is converted into the neutral phosphate Co3(PO4)2·2H2O (H. Debray, Ann. de chimie, 1861, [3] 61, p. 438). Cobalt ammonium phosphate, CoNH4PO4·12H2O, is formed when a soluble cobalt salt is digested for some time with excess of a warm solution of ammonium phosphate. It separates in the form of small rose-red crystals, which decompose on boiling with water.
Cobaltous cyanide, Co(CN)2·3H2O, is obtained when the carbonate is dissolved in hydrocyanic acid or when the acetate is precipitated by potassium cyanide. It is insoluble in dilute acids, but is readily soluble in excess of potassium cyanide. The double cyanides of cobalt are analogous to those of iron. Hydrocobaltocyanic acid is not known, but its potassium salt, K4Co(CN)6, is formed when freshly precipitated cobalt cyanide is dissolved in an ice-cold solution of potassium cyanide. The liquid is precipitated by alcohol, and the washed and dried precipitate is then dissolved in water and allowed to stand, when the salt separates in dark-coloured crystals. In alkaline solution it readily takes up oxygen and is converted into potassium cobalticyanide, K3Co(CN)6, which may also be obtained by evaporating a solution of cobalt cyanide, in excess of potassium cyanide, in the presence of air, 8KCN + 2Co(CN)2 + H2O + O = 2K3Co(CN)6 + 2KHO. It forms monoclinic crystals which are very soluble in water. From its aqueous solution, concentrated hydrochloric acid precipitates hydrocobalticyanic acid, H3Co(CN)6, as a colourless solid which is very deliquescent, and is not attacked by concentrated hydrochloric and nitric acids. For a description of the various salts of this acid, see P. Wesselsky, Berichte, 1869, 2, p. 588.
Cobaltammines. A large number of cobalt compounds are known, of which the empirical composition represents them as salts of cobalt to which one or more molecules of ammonia have been added. These salts have been divided into the following series:—
Diammine Series, [Co(NH3)2]X4M. In these salts X = NO2 and M = one atomic proportion of a monovalent metal, or the equivalent quantity of a divalent metal.
Triammine Series, [Co(NH3)3]X3. Here X = Cl, NO3, NO2, ½SO4, &c.