Magnesium Thioarsenic Salts

A suspension of arsenic tri- sulphide in water is decolorised by the addition of magnesia - soluble magnesium thioarsenite and insoluble magnesium hydrogen arsenite being formed -

2As₂S₃+5MgO+H₂O = Mg₃(AsS₃)₂+2MgHAsO₃.

On boiling the solution of magnesium thioarsenite, decomposition occurs according to the equation -

Mg₃(AsS₃)₂+7H₂O = 2MgHAsO₃+6H₂S+MgO.

Nilson obtained a brown mass of MgS.As₂S₃.5H₂O by evaporating a solution of arsenic trisulphide in calcium hydrosulphide. He also obtained 2MgS.As₂S₃.8H₂O as a yellow crystalline difficultly soluble salt, 3MgS.As₂S₃.9H₂O as a pure yellow crystalline salt, and 5MgS.2As₂S₃.3OH₂O as a yellow crystalline salt.