Cadmium Bromide, CdBr₂

Cadmium and bromine do not react in the cold, but if bromine vapour is passed over the metal heated almost to redness, white anhydrous CdBr₂ sublimes. The anhydrous salt is also obtained by heating the tetrahydrate, CdBr₂.4H₂O. It has been prepared pure by dissolving the pure carbonate in pure hydrobromic acid, drying at 200° C., and subliming in a current of pure dry carbon dioxide. The lustrous crystals thus obtained were not hydroscopic. In connection with atomic-weight determinations electrolytic cadmium has been dissolved in dilute aqueous hydrobromic acid and bromine. After treatment with excess of cadmium the filtered solution of cadmium bromide was crystallised. Fractional crystallisation has been found to be very effective in preparing the pure salt. To obtain it in a final anhydrous state it was heated in nitrogen containing hydrogen bromide. Cadmium bromide melts at 567° C. and boils at 863° C. It volatilises without decomposition, and its vapour density corresponds to the formula CdBr₂. The same formula is also indicated by the boiling-points of quinoline solutions. Its density, referred to water at 4° C., is 5.196 at 25° C.

At a red heat it is easily reduced by hydrogen or oxidised by oxygen. Nitrogen peroxide liberates small quantities of bromine from it. Its heat of formation is 84.2 Cal.

The fused salt conducts electricity, and the salt is very soluble in water. Its aqueous solutions are strongly ionised. The solubilities of the tetrahydrate and monohydrate are -

CdBr2.4H2O
Temperature, ° C.	0	18	30	38
CdBr2 in grm. per 100 grm. solution	37.92	48.90	56.9	61.84

CdBr2.H2O
Temperature, ° C.	35	40	45	60	80	100
CdBr2 in grm. per 100 grm. solution	60.29	60.65	60.75	61.10	61.29	61.63

Since the transition-point between these two hydrates is about 36° C., the monohydrate is converted into the tetrahydrate under water at ordinary temperatures, and from solutions of cadmium bromide the tetrahydrate or the monohydrate crystallises according as the temperature is below or above 36° C. There is apparently no CdBr₂.2½H₂O, though CdClBr.2½H₂O has been obtained.

The tetrahydrate crystallises in long efflorescent prisms that are dehydrated by heating to 200° C. The monohydrate crystallises in shining needles that lose their water slowly at 100° C. and rapidly at 145° C.

The electrical conductivity of cadmium bromide has been determined in its solutions in methyl alcohol, ethyl alcohol, ether, acetone, some other ketones, and propionitrile.

[CdBr₂]+400Aq. = CdBr₂.Aq.+0.44 Cal.

[CdBr₂.4H₂O]+610Aq. = CdBr₂.Aq. -7.29 Cal.

Cd₄Br₇ is strictly analogous to Cd₄Cl₇, and is prepared in a similar manner.

The anhydrous salt absorbs ammonia gas freely. The tetrammoniate, CdBr₂.4NH₄, is obtained as a white powder by complete saturation. Colourless octahedral crystals of diammoniate, CdBr₂.2NH₄, are deposited by cooling or evaporating a solution of cadmium bromide in hot ammonia solution. Both substances are decomposed by water and lose ammonia when heated. The latter salt has also been obtained by warming cadmium hydroxide with a solution of ammonium bromide. Tassilly obtained brilliant unstable crystals of a triammoniate, CdBr₂.3NH₄, by passing ammonia gas through the anhydrous salt dissolved in aqueous ammonia.

Some oxybromides of cadmium have been described. Colourless transparent plates of Cd(OH)Br, of density 4.87 at 15° C., were obtained by heating cadmium bromide and marble to 200° C. in sealed tubes.

The following double bromides of cadmium are known: NaBr. CdBr₂.2½H₂O, 2NaBr.3CdBr₂.6H₂O, 4NaBr.CdBr₂; KBr.CdBr₂.½H₂O, KBr.CdBr₂.H₂O, 2KBr.CdBr₂, 4KBr.CdBr₂; RbBr.CdBr₂, 4RbBr.CdBr₂; CsBr.CdBr₂, 2CsBr.CdBr₂, 3CsBr.CdBr₂; NH₄Br.CdBr₂, NH₄Br.CdBr₂.½H₂O, 4NH₄Br.CdBr₂; CuBr.CdBr₂; BaBr₂.CdBr₂.4H₂O.