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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.

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