Bismuth Sulphate

In addition to the normal sulphate, a number of other compounds, most of which are basic, has been obtained. The products obtained by crystallisation from solutions of bismuth trioxide in sulphuric acid vary in composition according to the temperature and concentration of the solution employed. From an investigation of the equilibrium of the system Bi₂O₃.4SO₃-H₂SO₄-H₂O, it is found that the solid phase in equilibrium with liquid containing 60 to 90 per cent, of sulphuric acid is Bi₂O₃.4SO₃; when the concentration of the liquid phase falls below 47.5 per cent, of sulphuric acid the solid phase in equilibrium is Bi₂O₃.2SO₃, when the concentration falls below 1.37 per cent, acid the solid phase is a mixture of Bi₂O₃.SO₃ and Bi₂O₃.2SO₃, and when the concentration falls below 1.09 per cent, acid Bi₂O₃.SO₃ alone remains as the solid phase.

Normal bismuth sulphate may be obtained by dissolving bismuth trioxide, bismuth trisulphide or bismuth nitrate in excess of concentrated sulphuric acid, evaporating the solution to dryness and heating the residue very carefully. It is a white, powdery or finely crystalline substance, very hygroscopic, forming a hydrate Bi₂(SO₄)₃.7H₂O, which loses water at 100° C., becoming a dihydrate, Bi₂(SO₄)₃.2H₂O. It can be heated to 400° C. without decomposition, but above that temperature decomposition sets in, basic salts being produced. On strongly heating, Bi₂O₃.SO₃ is obtained, while at red heat all the sulphuric anhydride is driven off. It can be reduced to metal by heating in a current of hydrogen or ammonia. It is hydrolysed slowly by cold water, more rapidly by hot water, forming in each case Bi₂O₃.SO₃. With hydrogen chloride it forms addition compounds, the following having been obtained: Bi₂(SO₄)₃.4HCl, Bi₂(SO₄)₃.2HCl, Bi₂(SO₄)₃.HCl. It forms double salts with the sulphates of the alkali metals and ammonium. With lithium sulphate two compounds have been described, Li₃Bi(SO₄)₃.2H₂O and Li(BiO)SO₄.H₂O. With sodium sulphate the compound Bi₂(SO₄)₃.Na₂SO₄ has been described; with potassium sulphate the double salt Bi₂(SO₄)₃.3K₂SO₄ is obtained in the form of hexagonal crystals; the existence of the compound Bi₂(SO₄)₃.K₂SO₄, which had previously been described, has not been confirmed. The ammonium compound Bi₂(SO₄)₃.(NH₄)₂SO₄ has also been described. Two compounds with cerium sulphate, Bi(OH)SO₄.Ce(SO₄)₂.5H₂O and 2Bi₂(SO₄)₃.Ce(SO₄)₂.15H₂O, have been described, and solid solutions are also found in this system.

Bismuth sulphate is isomorphous with the sulphates of yttrium, lanthanum and "didymium." It is said to be able to confer upon certain other substances the property of phosphorescence.

Acid Bismuth Sulphate, Bi₂O₃.4SO₃.7H₂O, is obtained by the action of moderately concentrated sulphuric acid upon bismuth trioxide, the basic sulphate first formed being redissolved in a large excess of sulphuric acid.

In addition to the foregoing, several other sulphates, both acid and basic, have been described; it is possible that many of them are mixtures of the normal sulphate, bismuth oxide and water. The following may be mentioned: Bi₂O₃.SO₃, Bi₂O₃.SO₃.2H₂O, Bi₂O₃.SO₃.3H₂O, 3Bi₂O₃.SO₃.3H₂O, 5Bi₂O₃.11SO₃.17H₂O, Bi₂O₃.4SO₃.3H₂O, Bi₂O₃.4SO₃.H₂O, Bi₂O₃.4SO₃.10H₂O.