Acid Calcium Sulphates

By cooling, or by precipitation with hot water of saturated solutions of calcium sulphate in hot concentrated sulphuric acid, several different acid sulphates may be obtained, CaSO₄.H₂SO₄, CaSO₄.3H₂SO₄, and 2CaSO₄.H₂SO₄. Kendall and Davidson, on studying the solubility curve of calcium sulphate in anhydrous sulphuric acid, found two modifications of the compound CaSO₄.3H₂SO₄. Only one of them was stable over the range of temperature of the experiment, namely 0°-63° C. These compounds are all decomposed by water or by moist air. The compound CaSO₄.H₂SO₄ has been obtained in the gelatinous state. D'Ans separated an acid double sulphate of calcium and sodium, but it was probably an isomorphous mixture.