Molybdenum Thioarsenates

Berzelius stated that molybdic orthothioarsenate was formed in solution when sodium orthothioarsenate was added to a solution of a molybdic salt. A series of complex thio-salts of arsenic and molybdenum has been prepared, most of which appear to be derived from a hypothetical acid of composition H₄As₂S₇.(MoS₃)₂, or may be regarded as double salts of metathioarsenic and pyrothiomolybdic acids: 2HAsS₃.H₂Mo₂S₇. The alkali salts are best prepared by adding salicylic acid to a solution of molybdenum trisulphide in aqueous alkali orthothioarsenate, the three compounds being in molecular proportions, and precipitating with alcohol. The sodium salt, Na₄As₂S₇.(MoS₃)₂.14H₂O, crystallises in lustrous, dark red, hexagonal prisms, and the potassium salt, K₄As₂S₇.(MoS₃)₂.8H₂O, in bright red needles. Both are readily soluble in water, are not hygroscopic, but decompose on keeping or when acted upon by mineral acids. The ammonium salt, (NH₄)₄As₂S₇.(MoS₃)₂.5H₂O, is obtained by adding salicylic acid to a concentrated solution of ammonium thiomolybdate and pyrothioarsenate and precipitating with alcohol. It yields scarlet, needle-shaped crystals, which are very unstable and are difficult to obtain pure. The aqueous solution decomposes with evolution of hydrogen sulphide and ammonia. The barium salt, Ba₂As₂S₇.(MoS₃)₂.14H₂O, crystallises in red needles from a solution of arsenic penta-sulphide in aqueous barium thiomolybdate cooled at -15° C. The corresponding salts of cobalt, copper, bivalent mercury, nickel, silver and zinc are obtained as red or brown precipitates by adding the potassium salt to an aqueous solution of a salt of the metal. When the potassium salt is triturated in the presence of water with salicylic acid, in molecular proportions 2: 3, and the mixture heated at about 50° C. until clear and then precipitated with alcohol, a brick-red amorphous powder of composition KAsS₃.MoS₃.4H₂O is obtained. It is insoluble in water, but soluble in aqueous alkali or ammonia. The sodium salt, NaAsS₃.MoS₃.6H₂O, is obtained in a similar manner and resembles the potassium salt in properties.