Cuprous bromide, CuBr

Several methods are available for the preparation of Cuprous bromide, CuBr, examples being the interaction of copper-turnings and an aqueous solution of cupric bromide at its boiling-point, and the direct synthesis from bromine and excess of copper. The most convenient process is that of Sandmeyer. A solution of cupric sulphate (12.5 grams), potassium bromide (36 grams), and concentrated sulphuric acid (6 c.c.) in water (80 c.c.) is boiled under reflux with copper-turnings until the solution has become colourless. After precipitation by filtration through asbestos into a large excess of water covered with a layer of ether, the cuprous bromide is allowed to settle. The mother-liquor is then syphoned off, and the salt is washed on a filter with water, alcohol, and ether, and dried in a vacuum- desiccator over sulphuric acid.

The pure bromide is a white substance, but gradually develops a yellow tint, and on exposure to sunlight it acquires a bluish colour. In phototropic character it resembles cuprous chloride, exposure to light changing its colour through dark green to dark copper. If the duration of the action of the light has been limited to a few minutes, keeping in the dark for 30 hours reverses the colour changes. The melting-point of the bromide is given by Monkemeyer as 480° C., and by Carnelley and Williams as 504° C. The boiling-point is between 861° and 954° C., and the density is given by Bodeker as 4.72.

Cuprous bromide is insoluble in water. Its solutions in hydrochloric acid, hydrobromic acid, and ammonium hydroxide readily absorb carbon monoxide. The maximum absorption for the ammoniacal solution corresponds with one molecule of carbon monoxide to each atom of copper. When prepared in absence of air, the solution in ammonium hydroxide is colourless, but on contact with oxygen it develops a blue colour. The liquid obtained by dissolving cuprous bromide in an aqueous solution of sodium chloride or of sodium thiosulphate does not absorb carbon monoxide.

The heat of formation of the simple molecular compound CuBr from solid copper and liquid bromine is 24.985 Cal.

The complex derivatives of cuprous bromide include CuBr,NH₃ (Richards and Merigold); CuBr,2NH₃ (Saglier); and CuBr,3NH₃ (Richards and Merigold).