Cobaltous Bromide, CoBr₂

Cobaltous Bromide, CoBr₂, may be obtained as a green mass by passing bromine vapour or a mixture of bromine and hydrogen bromide over heated cobalt; by heating the hydrated salt to 130° C.; and also by addition of the calculated quantity of dry bromine to finely divided cobalt under ether. The green salt, CoBr₂.(C₂H₅)₂O, is produced which, on heating, yields CoBr₂. The salt may be purified by sublimation in a current of hydrogen bromide. The product is green, crystalline, of density 4.91. It begins to be reduced by dry hydrogen at about 350° C. The reaction is more rapid at higher temperatures, but is accompanied by partial sublimation. With moist hydrogen the reduction begins at 250° C.

The salt deliquesces to a dark red liquid upon exposure to air. The solution, which also results when metallic cobalt is exposed to the prolonged action of bromine and water, yields, on concentration over sulphuric acid, splendid purple-red prismatic crystals of the hexahydrate, CoBr.6H₂O. These melt at 47-48° C. At 100° C. they fuse to a deep blue liquid, evolving water, and leaving on cooling purplish blue crystals of the dihydrate, CoBr₂.2H₂O. When heated to 130° C., the salt dries up to an opaque, amorphous, vivid green mass of anhydrous bromide.

In addition to the foregoing, several other hydrates are known, their formulae and appearance, together with those above cited, being as follow:

CoBr2.6H2O	red prisms m.pt. 47-48° C.
CoBr2.5.5H2O	rose-coloured
CoBr2.5H2O	pink
CoBr2.4H2O	reddish-violet m.pt. 70-71° C.
CoBr2.2H2O	purple
CoBr2.H2O	blue
CoBr2.0.5H2O	?
CoBr2	green

On exposing the anhydrous salt to ammonia gas, as also by passing ammonia into a solution of cobalt bromide in methyl acetate, the hexammoniate, CoBr₂.6NH₃, is obtained as pale rose-red crystals. These lose ammonia on heating, becoming light blue at 120° C. and having the composition CoBr₂.2NH₃.

The solubility of cobalt bromide in water is as follows:

Temperature ° С	59	75	97
Grams CoBr2 in 100 grams solution	66.7	66.8	68.1

When dissolved in organic liquids the molecular weight, as determined by ebullioscopic methods, appears to correspond to the single formula, CoBr₂.