Potassium Chromicyanide, K₃[Cr(CN)₆]

Potassium Chromicyanide, K₃[Cr(CN)₆], is the most important salt of chromicyanic acid. It has been prepared by dissolving chromic hydroxide in an aqueous solution of potassium hydroxide which is supersaturated with hydrogen cyanide, and exposing the resulting liquid to air, when it assumes a reddish-brown colour and deposits crystals which may be purified by recrystallisation; by adding excess of aqueous potassium cyanide to a solution of chromous acetate contained in a closed vessel, and allowing to stand for a week, when long yellow needles separate; by the action of excess of potassium cyanide on chromous chloride or carbonate, or by heating powdered chromium with a concentrated solution of potassium cyanide in a sealed tube at 100° C. However, it is best preparedj by the following method: Freshly precipitated moist chromic hydroxide is dissolved in acetic acid, and the solution evaporated nearly to dryness and then diluted considerably with water. This solution is gradually added to a hot solution of potassium cyanide (about 25 per cent.) contained in a flask so as to exclude atmospheric carbon dioxide. After heating for a short time the liquid is filtered, evaporated, and cooled. The crystals obtained are dissolved in water (4 parts) and the solution boiled. Chromic hydroxide is deposited and filtered off, and the filtrate on cooling deposits pale yellow crystals, more of which are obtained by repeatedly boiling the mother-liquor.

Potassium chromicyanide forms yellow, macled crystals, stable in air at ordinary temperatures; density 1.71. At 20° C., 1 c.c. of water dissolves 0.3233 gram of the salt. The aqueous solution on prolonged boiling deposits chromium sesquioxide, with slight evolution of hydrogen cyanide. Heated to dull redness in absence of air, the solid fuses and then undergoes decomposition, giving off nitrogen and leaving chromium carbide and potassium cyanide. With dilute sulphuric acid hydrogen cyanide is evolved, while the concentrated acid liberates carbon monoxide.

When an aqueous solution of potassium chromicyanide is heated with carbon monoxide at 130° C. in a sealed tube, no absorption of the carbon monoxide takes place, but the chromicyanide is completely decomposed with the production of chromium sesquioxide, formic acid, hydrogen cyanide, and ammonia.

The following insoluble chromicyanides are obtained as characteristically coloured precipitates when potassium chromicyanide is added to solutions of metallic salts.