Bismuth Cyanides

When potassium cyanide is added to a solution of bismuth nitrate a brown precipitate is obtained, the composition of which is doubtful. Early investigators considered it to be an oxide, probably a peroxide, of bismuth, but it was found to contain sulphur, which may have been due to impurities in the potassium cyanide.

By triturating calculated amounts of bismuth bromide and potassium cyanide with small quantities of xylene a reaction occurs from which a complex salt, potassium bismuthobromocyanide, K₃[BiBr₃(CN)₃], is obtained. This compound is decomposed by cold water, but is soluble in dilute hydrochloric acid, the solution however decomposing rapidly. When heated out of contact with air, potassium bromide and metallic bismuth are obtained. In a similar manner other complex salts have been obtained, such as the orange-yellow silver salt, Ag₃[BiBr₃(CN)₃], the greenish-grey cuprous salt, Cu₃[BiBr₃(CN)₃], and a mercury salt, Hg₃[BiBr₃(CN)₃]₂, which is at first sulphur-yellow, but changes to white prismatic needles. These salts are all decomposed by water.

Other complex cyanides that have been obtained are bismuth ferrocyanide, Bi₄[Fe(CN)₆]₃, which is formed by precipitation with potassium ferrocyanide; bismuth ferricyanide, bismuth cobalticyanide, BiCo(CN)₆.5H₂O, a greenish-white crystalline substance which turns blue on drying, and when dried over sulphuric acid has the formula 2BiCo(CN)₆.7H₂O.