Potassium Thiodichromite, K₂Cr₄S₇

Potassium Thiodichromite, K₂Cr₄S₇, is obtained when 1 part of potassium chromate is fused with 24 parts of potassium carbonate and 24 parts of sulphur to a bright red heat, the mass cooled slowly, and then being thoroughly but rapidly washed with cold water. The crystals, which are reddish or bluish-grey hexagonal plates, of density 2.77 at 15° C., are stable in air.

By the action of hydrochloric acid on these substances in absence of air, thiochromous acid, H₂Cr₂S₄, and thiodichromous acid, H₂Cr₄S₇, are respectively obtained.

Thiochromites of the heavy metals may be obtained by boiling solutions of their salts with sodium thiochromite. Thus compounds of the type M^••Cr₂S₄, where M^• = Cu, Ag₂, Zn, Cd, Sn, Pb, Mn, Fe, Co, Ni, have been prepared. They are greyish-black, insoluble substances, unacted upon by hydrochloric acid, but readily decomposed by nitric acid or aqua regia.