Cobalt Iodate, Co(IO₃)₂

Cobalt Iodate, Co(IO₃)₂, was known to Rammelsberg in 1838, and was further studied by Fullerton and Ditte, the last-named investigator claiming to have prepared the anhydrous salt, and salts with 2, 3, 4, and 5 molecules of water, whilst Fullerton describes a hexahydrate.

Meusser confirms the existence of the anhydrous salt, and of the di- and tetra-hydrates, but throws doubt on the existence of the tri-, penta-, and hexa-hydrates.

The anhydrous salt may be obtained by maintaining for a prolonged period at 80° to 90° C. a mixed solution of cobalt nitrate and alkali iodate in the presence of free nitric acid. It is also formed at ordinary temperatures by mixing saturated solutions of cobalt nitrate and alkali iodate and agitating with a glass rod the flocculent, rose-coloured, hydrated precipitate, when it yields the blue-violet anhydrous compound. The salt is also produced by heating the same solutions from two to three hours in a glass tube at 120° C.

Anhydrous cobalt iodate yields microscopic needles, blue-violet in colour, and which on heating become dark blue in colour, but regain their original hue on cooling. Below 200° C. decomposition sets in, iodine and oxygen being evolved.

The dihydrate, Co(IO₃)₂.2H₂O, is obtained by mixing equivalent amounts of cobalt nitrate and alkali iodate (or free iodic acid) in ten times their weight of water and allowing to crystallise at 30° C. Microscopic crystals of lilac hue are obtained, which are the stable form of the salt up to 68° C.

The tetrahydrate, Co(IO₃)₂.4H₂O, is obtained as red crystals by evaporating a 5 per cent, solution containing equivalent proportions of cobalt nitrate and iodic acid at 0° to 20° C. When agitated for a prolonged period with water at 30° C. the tetrahydrate yields the dihydrate.

The various solubilities of the hydrates of cobalt iodate at different temperatures are as follow:

Tetrahydrate, Co(IO3)2.4H2O
Temperature ° C	0	18	30	50	60	65
Grams Co(IO3)2 per 100 grams solution	0.54	0.83	1.03	1.46	1.86	2.17
Dihydrate, Co(IO3)2.2H2O
Temperature ° С	0	18	30	50	75	100
Grams Co(IO3)2 per 100 grams solution	0.32	0.45	0.52	0.67	0.84	1.02
Anhydrous Salt, Co(IO3)2
Temperature °C.		18	30	50	75	100
Grams Co(IO3)2 per 100 grams solution		1.03	0.89	0.85	0.75	0.69

..

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. Solubility curves of the hydrates of cobalt iodate.

These results are shown graphically in Fig., and are interesting as showing the rapid rise in solubility of the tetrahydrate with increase of temperature, and the simultaneous fall in solubility of the anhydrous salt.