Arsenic Pentachloride, AsCl₅

By the action of chlorine on arsenic trichloride at a low temperature Baskerville and Bennett obtained a greenish-yellow liquid which they believed to be arsenic pentachloride, AsCl₅. Pure solid arsenic trichloride was placed in a vessel surrounded by carbon dioxide snow and saturated with chlorine, the temperature being -33° C. The liquid approximated in composition to AsCl₅. It was soluble in carbon disulphide and in ether, and the ether solution, on evaporation at -30° C., left yellow crystals which readily decomposed with slight rise in temperature. The formation of this compound has not been confirmed, however, and its existence is doubtful.

At -23° C. almost sufficient chlorine is absorbed by arsenic trichloride to produce the pentachloride, but there is no evidence of compound formation and the chlorine may be expelled by warming or by passing a current of air through the liquid. If arsenic trichloride is saturated with chlorine at 0° C. the mixture, on cooling, solidifies at about -30° C. with considerable diminution in volume. If more chlorine is passed into the liquid before solidification occurs, there is further absorption and the resulting yellow liquid may be cooled below -60° C. without freezing. If this liquid is brought into contact with water, the excess of chlorine is rapidly evolved and the aqueous solution contains tervalent arsenic only, so that again there is no evidence of the formation of the pentachloride. Solid arsenic trichloride at -35° C. is not acted upon by liquid chlorine, and if the mixture is gently heated until the former melts, the two liquids mix without reaction.

The freezing point curve of mixtures of chlorine and arsenic trichloride is smooth and shows no indication of combination.