Stannic Bromide, SnBr₄

Stannic Bromide, SnBr₄, is formed from its elements, which combine with evolution of light and heat. It is best prepared, according to Lorenz, by dropping bromine from a capillary tube on to strips of tin contained in a distilling flask, removing excess of bromine by warming the flask, and then distilling the bromide, which, so obtained, is a snow-white crystalline mass. The physical constants of this substance are not known with certainty, owing to the different values obtained by different observers. Carnelley and O'Shea determined the vapour density to be 229 (theory requires 219 when H = 1), and the melting-point 30° C., whilst for the latter, Raymann and Preis found 33° C., and Garelli 29.45° C. The boiling-point is 203.3° C. (Raymann and Preis), or 201° C. (Carnelley and O'Shea, Lorenz, Neumann); and the density of the liquid at 35° C. is 3.349 (Raymann and Preis), and at 39° C. 3.322 (Bodeker). Berthelot estimated the heat of formation of stannic bromide to be:

[Sn] + 4Br = [SnBr₄] + 98,000 calories,
and
[Sn] + (4Br) = [SnBr₄] + 112,800 calories.

Because it has a high molecular depression, and a convenient melting- point, this substance has been employed by Garelli in cryoscopic determinations, the molecular depression being estimated to be 280. According to Tolloczko, however, the value should be 305.

Stannic bromide fumes in the air, and is readily soluble in cold water. From this solution the hydroxide separates slowly on standing and quickly on boiling. Nevertheless the hydrate SnBr₄.4H₂O is obtained when an aqueous solution of the anhydrous bromide is evaporated over sulphuric acid (Raymann and Preis).

With ammonia stannic bromide forms the additive compound SnBr₄.2NH₃ (Raymann and Preis).

Bromostannic Acid exists. According to Raymann and Preis it is H₂SnBr₆.8H₂O, but Seubert gives it the formula H₂SnBr₆.7H₂O. Alkali stannibromides are known.