Potassium Chromocyanide, K₄Cr(CN)₆

Potassium Chromocyanide, K₄Cr(CN)₆, can be obtained pure by the addition of alcohol to a solution of chromous acetate in concentrated potassium cyanide solution, or by shaking up chromous acetate with aqueous carbon dioxide, pouring the mixture into a 25 per cent, solution of potassium cyanide, the whole then being gently warmed in an atmosphere of hydrogen, cooled in snow, and precipitated by adding solid potassium cyanide. The salt appears in the form of dark blue crystals, which are readily oxidised to the yellow chromicyanide - a fact which caused Moissan to mistake the latter salt for the chromocyanide. Salts of the following metals, barium, iron (Fe^••), lead, mercury (Hg^••), and zinc, when added to a solution of potassium chromocyanide, produce characteristically coloured precipitates.