Cobalt blue double salts

Nickel sulfate forms double salts with ammonium or alkali metal sulfates. For example, blue-green hydrated ammonium nickel sulfate, (NH4)2S04 NiS04 6H20, crystallizes from a mixed solution of nickel sulfate and ammonium sulfate. Such double sulfates are isomorphous to corresponding alkali metal or ammonium double sulfates of iron, cobalt, magnesium, zinc, and other bivalent metals. 

General Properties of Compounds of Cobalt.—Cobalt, in its salts, behaves both as a trivalent and a divalent element, although the trivalent salts are unstable except in the case of double salts. The colour of cobalt salts varies greatly according to the degree of hydration, but it is usually pink or blue. Cobalt salts absorb nitric oxide,1 and with ammonia readily yield complex ammino derivatives. 

This simple hydration theory cannot explain all the known phenomena, as, for example, the opposite effects of calcium chloride and zinc chloride on the colours. Engel2 therefore assumed that the observed colours were due to certain double salts present in the solutions. In the case of pure cobalt chloride, hydrolysis was supposed to occur on heating the solution, the hydrochloric acid liberated uniting with unchanged cobalt chloride and as an explanation of the colour change this is almost certainly incorrect. Ostwald 3 suggested a simple ionic explanation, namely, that the red colour is that of the cobalt cation, and the blue that of the undissociated salt. This is certainly not a complete explanation, and seems to necessitate a very marked decrease in ionisation with rise of temperature, which experiment, so far, does not support.4... 

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