Calcium Bromide, CaBr₂

Calcium will combine directly with bromine vapour at red heat to produce calcium bromide, CaBr₂. The heat of formation is 169.2 Cal. The salt may also be obtained by the action of bromine and a reducing agent, for example iron filings or phosphorus, on calcium carbonate or hydroxide in the presence of water, or by the action of bromine on lime at red heat.

It is most conveniently prepared, however, by the neutralisation of hydrobromic acid by calcium carbonate. In order to obtain the anhydrous salt in a state of purity, it must be fused in an atmosphere of hydrogen and hydrobromic acid to prevent the partial decomposition of the salt by water, and then, after removal of these gases by nitrogen at 400° C., it must be cooled in an atmosphere of nitrogen. The resulting product is a white solid of density 3.353 at 25° C.

Calcium bromide melts at 760° C., and sublimes at 720° C. Like the chloride the bromide is hygroscopic, and may be used as a drying agent. It may even be a little more efficient for this purpose than the former.

Calcium bromide is very soluble in water, the heat of solution being 24.51 Cal. The following values have been found for the solubility at different temperatures: -

Temperature, ° C.	-22	-14	-7	-6	+8	+9	+11	+20	+50
Grams CaBr2 in 100 grams solution	50.3	52.5	52.6	52.6	53.1	55.1	55.7	57.1	62.6

The Hydrates of Calcium Bromide have been studied by no means so carefully as those of the chloride. A hexahydrate, CaBr₂.6H₂O, crystallises at the ordinary temperature and melts at 38° C. Its boiling-point is 149°-150° C. The heat of formation of the hexahydrate from the anhydrous salt is 25.60 Cal., and the heat of solution is -1.090 Cal.

According to Curtmann, the trihydrate, CaBr₂.3H₂O, is the salt most frequently met. It crystallises in bright colourless needles, is very hygroscopic, melts at 80° C., and dissolves in water and alcohol. Its behaviour also points to the possible existence of a compound, 2CaBr₂.5H₂O, at 180°-181° C.

The existence of other hydrates has been reported. Kusnetzoff prepared the tetrahydrate, CaBr₂.4H₂O, transition temperature from the hexahydrate 55° C., and, later, by hot centrifuging, he obtained a dihydrate, CaBr₂.2H₂O.

Guareschi stated that, hydrates, containing 6, 5, 4, 3, 1½ 1, and ½ molecules of water respectively, can be obtained by keeping the bromide under varying conditions of temperature and pressure of water vapour until constant weight is attained.

Calcium bromide unites directly with ammonia. Four compounds are known, CaBr₂.8NH₃, CaBr₂.6NH₃, CaBr₂.2NH₃, and CaBr₂.NH₃. The heats of formation and the dissociation pressures have been studied. Like calcium chloride it also combines with aniline, with carbamide and asparagine, and with phenylhydrazine.

Calcium bromide dissolves readily in absolute alcohol, and from the solution tabular rhombic crystals may be obtained having the composition CaBr₂.3C₂H₅OH. The compound is hygroscopic and is decomposed by heat with the formation of lime, hydrobromic acid, and ethyl bromide.

A double bromide of calcium and manganese, forming pink crystals of composition CaBr₂.MnBr₂.4H₂O, has been described. Calcium bromide also combines with mercuric cyanide to give the compound 2Hg(CN)₂.CaBr₂.7H₂O. The heat of formation from the two solid salts and liquid water is 40.47 Cal. A double bromide of mercury and calcium, HgBr₂.CaBr₂, apparently existing in two forms, has been described.

Freezing-point curves of mixtures of calcium bromide with sodium bromide and with potassium bromide indicate the existence of the compounds 2CaBr₂.NaBr, and CaBr₂.KBr.

According to Herz and Bulla, variations with concentration, of the distribution coefficient of bromine between calcium bromide solution and carbon tetrachloride, indicate the existence of addition compounds of bromine and calcium bromide. Assuming that only the tetrabromide, CaBr₄, is formed, the dissociation constant at = 0.04 approx. for a 1/10 molar solution of calcium bromide when the concentrations are measured in gram-molecules per litre.

It has also been stated that, by warming calcium bromide with one molecular-equivalent of bromine, a reddish-brown liquid, which deposits deep red crystals, is obtained. These crystals lose bromine vapour very rapidly and give white calcium bromide.