Molybdenum Tetrasulphide, MoS₄

Molybdenum Tetrasulphide, MoS₄, is formed by decomposition of a perthiomolybdate with a dilute acid.

A solution is made of potassium dimolybdate (30 grams MoO₃ fused with 14.4 grams K₂CO₃) in 120 c.c. of hot water; this solution, after filtration, is diluted with 350 c.c. of water, and saturated at 15° C. with hydrogen sulphide. The liquid is then evaporated on the water-bath up to crystallising point. There is deposited on cooling not only potassium perthiomolybdate, KHMoS₅, but also the di- and tri-sulphides of molybdenum, and gelatinous thiomolybdates. The mass is thoroughly extracted with ice-cold water, then dissolved in water at 30° to 40° C., and precipitated with hydrochloric acid. This precipitate, well washed with water, alcohol, and carbon disulphide, and dried in an atmosphere of hydrogen sulphide at 140° C., is the anhydrous tetrasulphide.

This brownish powder, which is somewhat unstable in the air, and which is soluble in potassium sulphide solution, is also obtained in small quantity by the interaction of molybdenum trisulphide and potassium sulphide.

Of the thio-acids corresponding to the sulphides of molybdenum, two have been prepared, namely, hexathiomolybdic acid, HMoS₆, and per-thiomolybdic acid, H₂MoS₅. The former has been obtained by decomposition of the ammonium salt with hydrochloric acid; the latter is precipitated as a reddish-brown powder on the addition of dilute acetic acid to a solution of its potassium salt, KHMoS₅.