Perthiocarbonic Acid, H₂CS₄

Perthiocarbonic Acid, H₂CS₄, has not been obtained in a perfectly pure condition, but a dark brown liquid consisting largely of perthiocarbonic acid is obtained when the ammonium salt is treated with 98 per cent, formic acid. The liquid is rapidly decomposed by acetone, the action being accompanied by crackling, the colour disappearing and sulphur being deposited:

H₂CS₄ → CS₂ + H₂S₂,

H₂S₂ → H₂S + S.

When the liquid is distilled under reduced pressure, using a small fractionating column, carbon disulphide is first obtained, followed at 70° C. by hydrogen disulphide, leaving a large residue of sulphur.

If concentrated hydrochloric acid is used instead of formic acid in the foregoing preparation, the resulting red liquid consists of a solution of sulphur in trithiocarbonic acid which dissolves quietly in acetone to give a yellow solution, the dissolved sulphur then gradually precipitating out.