Aluminium sulphate, Al₂(SO₄)₃

Aluminium sulphate, Al₂(SO₄)₃, is prepared in the anhydrous state by heating the crystalline, hydrated salt. The latter melts in its water of crystallisation, swells up, and eventually leaves a porous, white residue of anhydrous sulphate.

The anhydrous sulphate has a density of 2.713 at 17°, and its specific heat (0° to 100°) is 0.1855. At a red heat it decomposes, leaving a residue of alumina; decomposition becomes appreciable at 770°. It dissolves slowly in water.

A solution of aluminium sulphate is readily prepared by dissolving aluminium hydroxide in dilute sulphuric acid. The solution crystallises with difficulty, the hydrate Al₂(SO₄)₃.18H₂O being deposited in thin, six-sided, nacreous plates. This hydrate has also been obtained in the form of tetrahedra. At low temperatures the hydrate Al₂(SO₄)₃.27H₂O separates in trigonal crystals (a:c = 1:0.5408). Other hydrates, with 16H₂O, 12H₂O, 10H₂O, 9H₂O, 6H₂O, 3H₂O, and 2H₂O have been described. The hydrates with 9H₂O and 10H₂O are said to be precipitated by alcohol, and to absorb water from a damp atmosphere, forming the hydrate with 18H₂O. The hexahydrate results from the action of concentrated sulphuric acid on the hydrate with 18H₂O; the trihydrate forms regular tetrahedra. The system aluminium sulphate - water has not yet been systematically investigated.

The hydrate Al₂(SO₄)₃.18H₂O is practically insoluble in alcohol. Its density is 1.69, and its specific heat (15° to 52°) is 0.353. As a white, fibrous efflorescence on shale and other rocks, this hydrate occurs as the mineral alunogen.

Aluminium sulphate is used in paper-making, as a mordant, and for the precipitation of sewage. The crude, very impure commercial preparations are manufactured by heating powdered, calcined china-clay or bauxite with sulphuric acid, the product being caused to solidify into blocks. When aluminium sulphate is required free from iron salts it is prepared from aluminium hydroxide, and sulphuric acid. It is impossible to separate ferric sulphate from aluminium sulphate by crystallisation, although the two sulphates form neither a compound nor mixed crystals; but commercial aluminium sulphate may be freed from iron by crystallisation if the iron sulphate present is reduced to the ferrous state by hydrogen sulphide, sulphuric acid, etc.

The acid sulphate Al₂(SO₄)₃.H₂SO₄.3H₂O has been prepared by Baud, and also by Wirth; while Silberberger has described an acid sulphate of the composition Al₂(SO₄)₃.3H₂SO₄.7H₂O. When aluminium sulphate is boiled with concentrated hydrochloric acid and the liquid cooled, crystals of aluminium chlorosulphate, AlClSO₄.6H₂O are deposited, which are decomposed by water.

About twenty different basic sulphates of aluminium have been described, but beyond the statement that the substance Al₂O₃.2SO₃ and one or two hydrates of it are probably definite compounds, practically nothing can be said about them. A hydrated basic salt of the composition Al₂O₃.SO₃.9H₂O occurs naturally as the mineral websterite.