Arsenic Monosulphatotrioxide, As₂O₃.SO₃

Arsenic Monosulphatotrioxide, As₂O₃.SO₃, sometimes called arsenyl sulphate, (AsO)₂SO₄, is obtained by dissolving arsenious oxide in warm concentrated sulphuric acid, the concentration of which should be between H₂SO₄.H₂O and 9H₂SO₄.2H₂O, and evaporating until sulphur trioxide is no longer expelled. On cooling, tabular crystals separate, which in air become moist and form sulphuric acid and arsenious oxide. This compound is somewhat more stable than those with higher sulphur trioxide content and, on gently heating, some volatilisation without decomposition occurs; at 225° C. it begins to lose sulphur trioxide; finally, on heating, fused arsenious oxide remains. From sulphuric acid of weaker concentration than H₂SO₄.H₂O, no sulphato-compound can be obtained.