Cobaltous Iodide, CoI₂

Cobaltous Iodide, CoI₂, may be obtained in the anhydrous condition by heating cobalt in a current of iodine vapour, or by desiccation of the hydrated salt over concentrated sulphuric acid in vacuo. It is then obtained as a black metallic-looking mass. It dissolves in water, yielding interesting colour changes possessing in concentrated solution a green colour, which become brown and finally red on dilution.

Cobalt iodide is most readily obtained in solution by warming the finely divided metal with water and iodine. Its formation is accompanied by evolution of heat. It may also be obtained by allowing cobalt to remain in prolonged contact with water and iodine.

Several hydrates have been isolated. The hexahydrate, CoI₂.6H₂O, is obtained by slightly evaporating and then strongly cooling the aqueous solution. It yields dark red, hexagonal prisms. Further concentration yields the nonahydrate, CoI₂.9H₂O, which crystallises in thin, red, rhombic plates, which lose three molecules of water at 6.4° C. The tetrahydrate, CoI₂.4H₂O, and dihydrate, CoI₂.2H₂O, are green.

The solubility of cobalt iodide in water is as follows:

Temperature ° C.	- 2	+ 9	14	25	34	46	60	82	111	156
Grams of CoI2 per 100 grams solution	58.7	61.4	61.6	66.4	73.8	79.0	79.2	80.7	80.9	83.1

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. The solubility curves of cobalt iodide.

The solution changes in colour as the temperature rises, being red below 20° C., olive between 20° and 40° C., and green at higher temperatures. These changes correspond to breaks in the solubility curve, as shown in Fig., and are usually regarded as indicating the existence in solution of the hexa-, tetra-, and di-hydrates respectively. Both the hexammoniate, CoI₂.6NH₃, and tetrammoniate, CoI₂.4NH₃, have been prepared.