Cobalt Polysulphides

A study of the freezing-point diagram of the cobalt sulphides suggests the possibility of the formation of the complexes Co₄S₃ and Co₆S₅. Hiortdahl claimed to have obtained the former of these by heating tricobalt tetroxide to redness in a current of hydrogen sulphide. It is doubtful, however, if it should be regarded as a separate chemical entity.

According to Chesneau, a polysulphide, Co₂S₇, is precipitated as a black mass when a solution of sulphur in aqueous sodium monosulphide is added to one of cobalt chloride. If this precipitate is dissolved in sodium polysulphide solution and the liquid evaporated in vacuo, black crystalline plates separate out.