Barium Osmocyanide, Ba₂Os(CN)₆

Barium Osmocyanide, Ba₂Os(CN)₆.6H₂O, is obtained by adding an excess of baryta water to the violet precipitate resulting from interaction of a ferric salt with potassium osmocyanide. The ferric osmocyanide is at once decomposed, yielding a brown precipitate of ferric hydroxide, whilst barium osmocyanide passes into solution, imparting to it a yellow colour. Any excess of barium hydroxide is removed by passing carbon dioxide through, and on slow evaporation of the filtered liquid the salt crystallises out in stable, reddish yellow crystals, readily soluble in water and aqueous alcohol. On heating to 50-60° C. the whole of the water of crystallisation is expelled, the anhydrous salt remaining as a yellow powder.