Lead Dibromide, PbBr₂

Lead Dibromide, PbBr₂, resembles the corresponding chloride, and is prepared by dissolving lead oxide in hydrobromic acid and crystallising the solution, or by precipitating a lead salt with potassium bromide solution. It crystallises from hot water in white needles, which belong to the rhombic system, and have a density of 6.63.

If hydrobromic acid solution is added in excess to a 0.5 per cent, solution of lead bromide in hot water, needle-shaped crystals separate on cooling, which have the composition PbBr₂.3H₂O.

Various observations have been made upon the melting-point of this salt, which lies between 480° C. and 499° C. When fused it is a red liquid, solidifying to a white, horny mass; if, however, it is melted in moist air it emits white fumes and forms an oxybromide. The boiling-point of lead bromide is about 920° C., and its molecular heat of formation from the metal and liquid bromine is 75,000 calories (Berthelot), or 64,450 calories (Thomsen). This salt slowly darkens in the light owing to the separation of metal.

The solubility of lead bromide in water increases rapidly with rising temperature as the following figures show:

Temp. ° C.	0	15	25	35	45	55	65	80	100
Grams PbBr2 in 100 grams H2O	0.4554	0.7305	0.9744	1.3220	1.7457	2.1376	2.574	3.343	4.751

In general, the solubility of lead bromide in hydrochloric, nitric, and acetic acids, as well as in ammonium chloride solution, is greater than in water; but the solubility of this compound in hydrogen bromide solution is analogous to that of lead chloride in hydrochloric acid; that is to say it is less soluble in dilute hydrogen bromide solution than in pure water - 1 litre of 0.5 normal solution of HBr dissolves only 1.25 gram PbBr₂ at 11° C., whilst a litre of water at the same temperature dissolves 6 grams - but more soluble in a concentrated solution than in pure water, owing to the formation of complex ions.