Potassium Ruthenocyanide, K₄Ru(CN)₆

Potassium Ruthenocyanide, K₄Ru(CN)₆.3H₂O, is obtained by heating to the point of fusion ammonium chlor-ruthenate with a slight excess of potassium cyanide for ten to fifteen minutes. Extraction of the melt with boiling water and subsequent evaporation yields crystals of potassium ruthenocyanide.

Howe has prepared the salt in a variety of ways, the three yielding the best results were as follows:

1. Fusion of potassium ruthenium nitrosoehloride with potassium cyanide. This gave a somewhat better yield than the method of Claus.
2. Ruthenium trichloride fused with potassium cyanide.
3. The melt formed by fusion of ruthenium in potassium hydroxide and nitrate, and thus containing potassium ruthenate, was dissolved in water and boiled with potassium cyanide. The orange colour was quickly bleached, the ruthenocyanide produced being subsequently isolated by crystallisation.

Potassium ruthenocyanide crystallises in square pseudo-rhombic plates, isomorphous with the corresponding ferro- and osmo-cyanides. Its solution in water yields no precipitate with salts of the alkaline earth metals as their ruthenocyanides are soluble. Ferric chloride gives a rich purple precipitate resembling Prussian blue in its chemical properties. Soluble in pure water it is precipitated by salts or alcohol. When precipitated in cotton fibre it adheres well, imparting its own beautiful colour. Alkalies decompose the salt, which, however, is re-formed on addition of dilute acid. This constitutes a useful test for ruthenocyanides.

Ferrous sulphate yields, with potassium ruthenocyanide, a pale blue precipitate which gradually oxidises to the purple one on standing, but instantly on addition of bromine.

With hydrochloric acid hydrogen cyanide is evolved on warming, . and after a time a deep violet-blue precipitate of ruthenium cyanide with a little potassium cyanide is obtained. Chlorine colours the solution brownish yellow, possibly in consequence of the formation of a ruthenicyanide, although no crystalline salt can be isolated from it.