Cobaltous Nitrate, Co(NO₃)₂

Cobaltous Nitrate, Co(NO₃)₂, may be obtained in the anhydrous condition by the action of nitric anhydride, or a solution of this in nitric acid upon the hydrated salt.

It is a slightly pink powder which decomposes at 100° to 105° C., evolving nitrous fumes.

A solution of cobaltous nitrate in water is readily prepared by dissolving the oxides or carbonate in dilute nitric acid. Slow evaporation yields red, monoclinic prisms of the hexahydrate, Co(NO₃)₂.6H₂O, of density 1.83. The crystals are very slightly deliquescent in moist air; over concentrated sulphuric acid they effloresce. They melt at 56° C. to a red liquid which, at higher temperatures, thickens, becoming green, and decomposes evolving brown fumes leaving a residue of oxide. The solubility of the hexahydrate in water is as follows:

Temperature ° C	-21	-10	-4	0	18	41	56
Grams Co(NO3)2 per 100 grams solution	41.55	43.69	44.85	45.66	49.73	55.96	62.88

A solution of cobaltous nitrate saturated at 18° C. has a density of 1.575. Between.-21° and 41° C. the composition of the saturated solution at any temperature is given by the expression:

Co(NO₃)₂ + (12.183 - 0.10177t)H₂O.

There is no evidence of the existence of a tetrahydrate. When the hexahydrate is kept at 20° to 60° C. in vacuo, or when melted and kept at 70° to 74° C., three molecules of water are lost, and the trihydrate, Co(NO₃)₂.3H₂O, crystallises out in large, rhombic prisms. These melt at 91° C., but do not lose any more water without decomposition. The solubilities of the trihydrate in water at various temperatures are as follow:

Temperature ° C	55	62	70	84	91
Grams Co(NO3)2 per 100 grams solution	61.74	62.88	64.89	68.84	77.21

There is a break in the solubility curve of the salt at.-22° C. which is attributed by Funk to the formation of a nonahydrate at temperatures below this point. This hydrate has not been isolated, although the corresponding nickel salt has been prepared and analyzed. Its solubility in water is as follows:

Temperature ° C.	-26	-23.5	-20.5
Grams Co(NO3)2 per 100 grams solution	39.45	40.40	42.77

Addition of ammonia in excess to a solution of cobaltous nitrate in the absence of air results in the deposition of rose-coloured crystals of the hexammoniate, Co(NO₃)₂.6NH₃.3H₂O.

A double nitrate of cobalt and bismuth, 3Co(NO₃)₂.2Bi(NO₃)₃.24H₂O, has been prepared as red crystals, melting at 58° C., and of density 2.48 at 16° C.

Basic nitrates have been described.