The hexammine salts are formed by the oxidizing action of air on dilute ammoniacal solutions of cobaltous salts, especially in presence of a large excess of ammonium chloride. They form yellow or bronze-coloured crystals, which decompose on boiling their aqueous solution. On boiling their solution in caustic alkalis, ammonia is liberated. The pentammine purpureo-salts are formed from the luteo-salts by loss of ammonia, or from an air slowly oxidized ammoniacal cobalt salt solution, the precipitated luteo-salt being filtered off and the filtrate boiled with concentrated acids. They are violet-red in colour, and on boiling or long standing with dilute acids they pass into the corresponding roseo-salts.
The pentammine nitrito salts are known as the xanthocobalt salts and have the general formula [NO2·Co·(NH3)5]X2. They are formed by the action of nitrous fumes on ammoniacal solutions of cobaltous salts, or purpureo-salts, or by the mutual reaction of chlorpurpureo-salts and alkaline nitrites. They are soluble in water and give characteristic precipitates with platinic and auric chlorides, and with potassium ferrocyanide. The pentammine roseo-salts can be obtained from the action of concentrated acids, in the cold, on air-oxidized solutions of cobaltous salts. They are of a reddish colour and usually crystallize well; on heating with concentrated acids are usually transformed into the purpureo-salts. Their alkaline solutions liberate ammonia on boiling. They give a characteristic pale red precipitate with sodium pyrophosphate, soluble in an excess of the precipitant; they also form precipitates on the addition of platinic chloride and potassium ferrocyanide. For methods of preparation of the tetrammine and triammine salts, see O. Dammer’s Handbuch der anorganischen Chemie, vol. 3 (containing a complete account of the preparation of the cobaltammine salts). The diammine salts are prepared by the action of alkaline nitrites on cobaltous salts in the presence of much ammonium chloride or nitrate; they are yellow or brown crystalline solids, not very soluble in cold water.
The above series of salts show striking differences in their behaviour towards reagents; thus, aqueous solutions of the luteo chlorides are strongly ionized, as is shown by their high electric conductivity; and all their chlorine is precipitated on the addition of silver nitrate solution. The aqueous solution, however, does not show the ordinary reactions of cobalt or of ammonia, and so it is to be presumed that the salt ionizes into [Co(NH3)6] and 3Cl′. The purpureo chloride has only two-thirds of its chlorine precipitated on the addition of silver nitrate, and the electric conductivity is much less than that of the luteo chloride; again in the praseo-salts only one-third of the chlorine is precipitated by silver nitrate, the conductivity again falling; while in the triammine salts all ionization has disappeared. For the constitution of these salts and of the “metal ammonia” compounds generally, see A. Werner, Zeit. für anorg. Chemie, 1893 et seq., and Berichte, 1895, et seq.; and S. Jörgensen, Zeit. für anorg. Chemie, 1892 et seq.
The oxycobaltammines are a series of compounds of the general type [Co2O3·H2(NH3)10]X4 first observed by L. Gmelin, and subsequently examined by E. Frémy, W. Gibbs and G. Vortmann (Monatshefte für Chemie, 1885, 6, p. 404). They result from the cobaltammines by the direct taking up of oxygen and water. On heating, they decompose, forming basic tetrammine salts.
The atomic weight of cobalt has been frequently determined, the earlier results not being very concordant (see R. Schneider, Pog. Ann., 1857, 101, p. 387; C. Marignac, Arch. Phys. Nat. [2], 1, p. 373; W. Gibbs, Amer. Jour. Sci. [2], 25, p. 483; J. B. Dumas, Ann. Chim. Phys., 1859 [3], 55, p. 129; W. J. Russell, Jour. Chem. Soc., 1863, 16, p. 51). C. Winkler, by the analysis of the chloride, and by the action of iodine on the metal, obtained the values 59.37 and 59.07, whilst W. Hempel and H. Thiele (Zeit. f. anorg. Chem., 1896, II, p. 73), by reducing cobalto-cobaltic oxide, and by the analysis of the chloride, have obtained the values 58.56 and 58.48. G. P. Baxter and others deduced the value 58.995 (O = 16).
Cobalt salts may be readily detected by the formation of the black sulphide, in alkaline solution, and by the blue colour they produce when fused with borax. For the quantitative determination of cobalt, it is either weighed as the oxide, Co3O4, obtained by ignition of the precipitated monoxide, or it is reduced in a current of hydrogen and weighed as metal. For the quantitative separation of cobalt and nickel, see E. Hintz (Zeit. f. anal. Chem., 1891, 30, p. 227), and also [Nickel].
COBALTITE, a mineral with the composition CoAsS, cobalt sulpharsenide. It is found as granular to compact masses, and frequently as beautifully developed crystals, which have the same symmetry as the isomorphous mineral pyrites, being cubic with parallel hemihedrism. The usual forms are the cube, octahedron and pentagonal dodecahedron {210}. The colour is silver-white with a reddish tinge, and the lustre brilliant and metallic, hence the old name cobalt-glance; the streak is greyish-black. The mineral is brittle, and possesses distinct cleavages parallel to the faces of the cube; hardness 5½; specific gravity 6.2. The brilliant crystals from Tunaberg in Sodermanland and Håkansboda in Vestmanland, Sweden, and from Skutterud near Drammen in Norway are well known in mineral collections. The cobalt ores at these localities occur with pyrites and chalcopyrite as bands in gneiss. Crystals have also been found at Khetri in Rajputana, and under the name sehta the mineral is used by Indian jewellers for producing a blue enamel on gold and silver ornaments. Massive cobaltite has been found in small amount in the Botallack mine, Cornwall. A variety containing much iron replacing cobalt, and known as ferrocobaltite (Ger. Stahlkobalt), occurs at Siegen in Westphalia.
(L. J. S.)