Potassium Osmocyanide, K₄Os(CN)₆

Potassium Osmocyanide, K₄Os(CN)₆.3H₂O, is obtained on heating to fusion ammonium chlorosmate with one and a half times its weight of potassium cyanide1 for ten to fifteen minutes. Extraction of the melt with boiling water, and subsequent evaporation, yields crystals of potassium osmocyanide containing three molecules of water.

Martius recommended the following method: Osmium tetroxide is dissolved in potassium hydroxide solution, the latter being present in sufficient quantity to render the whole distinctly alkaline. The dark, reddish brown solution is now mixed with the requisite quantity of potassium cyanide, whereby the colour becomes increasingly darker, until finally it is greenish black. On complete evaporation and gentle warming, the dry residue gradually changes in colour through yellow to white. This product is redissolved, and from the solution crystals are obtainable in the usual way.

Potassium osmocyanide is easily soluble in hot water, and the boiling saturated solution deposits, on cooling, the salt as a fine yellow powder. It is almost insoluble in alcohol, and quite insoluble in ether. On slow evaporation of its aqueous solution, the salt yields crystalline leaflets isomorphous with the ferro- and rutheno-cyanides of potassium. It is a useful reagent for detecting iron, giving with ferrous salts a violet precipitate, and with ferric salts a dark violet-blue precipitate.

Anhydrous potassium osmocyanide is white, and when heated to redness in the absence of air yields some free osmium, whilst in air osmic acid is formed. With concentrated hydrochloric acid, hydrogen osmocyanide is formed (vide supra). Soon, however, a further reaction sets in whereby hydrogen cyanide is evolved and osmium cyanide is precipitated out. Other mineral acids behave similarly.

When chlorine is passed through a solution of potassium osmocyanide, the latter assumes a dark colour like that of potassium ferri-cyanide. Martius could not obtain any osmicyanide, however, on evaporation, but found that, on standing, potassium chlorosmate was formed.