Zinc Bromide, ZnBr₂

The pure salt of Zinc Bromide, ZnBr₂, has been prepared by dissolving pure zinc in pure hydrobromic acid or by dissolving pure zinc in pure bromine. The salt was purified by crystallisation, sublimation in an atmosphere of carbon dioxide, and heating in nitrogen or carbon dioxide containing some hydrogen bromide. Distilled zinc has also been heated to just above its melting-point in hydrogen bromide. The zinc bromide which distilled over was redistilled.

Bromine water converts zinc into the bromide with the evolution of heat, and the anhydrous salt also results from the action of bromine vapour on zinc heated to redness. ZnBr₂ is very deliquescent, melts to a clear liquid, sublimes in white vapours at a red heat, and its taste is both sweet and astringent. The anhydrous substance crystallises in white rhombic needles, with a density of 4.22 at 20° C. (compared with water at 4° C.), which melt at 394° C. and boil at 650° C. Its heat of formation at 390° C. is 80.100 Cal. The fused salt is a good electrolyte.

Zinc bromide is very soluble in water. When cooled to -25° C. an 80 per cent, solution deposits needle-shaped crystals of the trihydrate. Octahedral crystals of the dihydrate separate when an 86 per cent, solution is cooled to 0° C. They melt at 37° C. The following solubilities, in grm. ZnBr₂ per 100 grm. solution, have been determined: -

ZnBr2.
Temperature, ° C.	35	40	60	80	100
Solubility	83.45	85.53	86.08	86.57	87.05

ZnBr2.2H2O.
Temperature, ° C.	-8	0	+13	+25	+30	+37
Solubility	79.06	79.55	80.76	82.46	84.08	86.20

ZnBr2.3H2O.
Temperature, ° C.	-15	-10	-5
Solubility	77.13	78.45	80.64

Its aqueous solutions are strongly ionised, and their conductivities are lowered by the addition of non-electrolytes.

Zinc bromide is also soluble in alcohol or ether, and its molecular weight in boiling quinoline corresponds to the formula ZnBr₂.

[ZnBr₂]+400Aq. = ZnBr₂.Aq. + 15.030 Cal.

Andre has described some oxybromides of zinc.

Rammelsberg obtained octahedral crystals of the diammoniate ZnBr₂.2NH₄ by evaporating a solution of zinc bromide in warm ammonia. According to Andre, their formula is ZnBr₂.2NH₄.H₂O. Andre also obtained 3ZnBr₂.8NH₄.2H₂O in fine needles by evaporating a solution of zinc bromide in warm ammonia, fine needles of 3ZnBr₂.10NH₄.H₂O by passing ammonia gas into a concentrated solution of the salt till the precipitate first formed dissolved, and evaporating, and the pentammoniate ZnBr₂.5NH₄ in brilliant crystals by a method corresponding to the preparation of ZnCl₂.5NH₄.2H₂O by Divers.

All these compounds are decomposed by water. Zinc bromide hexammoniate, ZnBr₂.6NH₄, results when zinc bromide absorbs ammonia gas at -18° C. It decomposes at 62° C., and about 4° higher forms the tetrammoniate, that begins to decompose at 143° C. The composition of the residue at 148° C. corresponds to the diammoniate.

The following double bromides of zinc have been prepared: NaBr.ZnBr₂.H₂O; 2NaBr.ZnBr₂.5H₂O; KBr.ZnBr₂.2H₂O; 2KBr.ZnBr₂.2H₂O; 2CsBr.ZnBr₂; 3CsBr.ZnBr₂; NH₄Br.ZnBr₂.xH₂O; 2NH₄Br.ZnBr₂; 2NH₄Br.ZnBr₂.H₂O; 3NH₄Br.ZnBr₂; 3NH₄Br.ZnBr₂.H₂O; ZnBr₂.PtBr₄.12H₂O.