Arsenic Trisulphatotrioxide, As₂O₃.3SO₃

Arsenic Trisulphatotrioxide, As₂O₃.3SO₃, sometimes called arsenic sulphate, As₂(SO₄)₃, is obtained by heating arsenious oxide with sulphur trioxide at 100° C. and maintaining the mixture at this temperature until excess of the latter oxide is expelled. It may also be separated from the mother liquor from the preparation of the tetra- sulphato-compound. The trisulphato-compound is less stable than the mono- and di-compounds described above.