Osmocyanic Acid, H₄Os(CN)₆

Hydrogen Osmocyanide, Osmocyanic Acid, H₄Os(CN)₆, is best prepared by adding concentrated hydrochloric acid to a cold, saturated solution of potassium osmocyanide. On standing, osmocyanic acid separates out in small white scales. These are washed with concentrated hydrochloric acid, dissolved in alcohol, and finally precipitated out with ether, as brilliant crystals.

Obtained in this way osmocyanic acid is anhydrous, and stable in dry air. In moist air it gives a blue colour, probably owing to the formation of osmium cyanide. It readily dissolves in water, giving an acid reaction. It is also soluble in alcohol, but not in ether. It has an astringent, metallic taste. Analysis agrees with the formula given above. It unites with bases to form salts known as osmocyanides, which closely resemble the corresponding ferro- and rutheno-cyanides.