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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.

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