Potassium Thioarsenates

Potassium Orthothioarsenate, K₃AsS₄.H₂O, may be obtained by treating an aqueous solution of the pyro-salt with alcohol and evaporating the mixture, or by adding excess of potassium sulphide or hydrosulphide to a solution of arsenious sulphide in the aqueous reagent, and evaporating in a vacuum. It crystallises in pale yellow, four-sided prisms, which are hygroscopic. Potassium pyrothioarsenate, K₄As₂S₇, crystallises in rhombic plates2 when an aqueous solution of potassium monohydrogen orthoarsenate is saturated with hydrogen sulphide and allowed to evaporate. If carbon dioxide is passed into the solution, arsenic pentasulphide separates. The addition of alcohol to an aqueous solution of the ortho-salt precipitates potassium metathioarsenate, KAsS₃. The ortho-salt is not decomposed on heating, but both the pyro- and meta-salts are converted to thio- arsenites, with loss of sulphur.