Osmium Trichloride, OsCl₃

Osmium Trichloride, OsCl₃, is obtained1 by heating osmium, obtained by reduction of the dioxide in hydrogen, in chlorine at about 1050° C., and cooling the vapour rapidly. It then contains some admixed osmium tetrachloride.

A purer product is prepared by igniting ammonium chlorosmate, (NH₄)₂OsCl₆, in chlorine at 350° C. Osmium trichloride, obtained by the foregoing method, is a brownish black, crystalline powder, which is hygroscopic and readily soluble in water, yielding a dark brown solution. This solution is stable even when boiled, although it possesses a slightly acid reaction.

The trihydrate, OsCl₃.3H₂O, is obtained as follows: osmic acid on prolonged boiling with hydrochloric acid and a little alcohol yields, on evaporation, red crystals of composition OsCl₃.OsCl₄.7H₂O, which are attacked by moist air yielding an olive-green mass. On solution in alcohol, and addition of potassium chloride, crystals of potassium chlorosmate, K₂OsCl₆, are obtained, the filtrate yielding, on evaporation, dark green crystals of the hydrated trichloride, OsCl₃.3H₂O. These, on heating in a current of hydrogen, are reduced to osmium, hydrogen chloride and water vapour being evolved. At the same time a minute trace of yellow sublimate is obtained, which Moraht and Wischin suggest may be a higher chloride, possibly the octachloride, OsCl₈.