Indium sulphate, In₂(SO₄)₃

Indium sulphate, In₂(SO₄)₃, may be prepared by dissolving indium, indium sesqui-oxide, or indium hydroxide in sulphuric acid, and carefully heating to expel water and excess of sulphuric acid. It is a white solid, very hygroscopic and extremely soluble in water, from which it is exceedingly difficult to crystallise out a hydrated sulphate. An ennea-hydrate, In₂(SO₄)₃.9H₂O, is said to exist, and an acid salt of the composition In₂(SO₄)₃.H₂SO₄.8H₂O is described as separating from an acid solution of the sulphate when evaporated over concentrated sulphuric acid.

The anhydrous sulphate has a density of 3.44 and a specific heat (between 0° and 100°) of 0.129.

Various double sulphates have been prepared by mixing indium sulphate solutions with solutions of other sulphates. The ammonium-, rubidium-, and caesium-indium alums, M₂SO₄.In₂(SO₄)₃.24H₂O (M = NH₄, Rb, or Cs), can be thus prepared, the first having been discovered by Roessler and the others by Soret. Lithium, sodium, and potassium alums are not known, but sodium indium sulphate, Na₂SO₄.In₂(SO₄)₃.8H₂O, and the corresponding potassium, ammonium, and thallium salts have been prepared.

The three alums separate out in regular octahedra or combinations of the octahedron and cube. Their aqueous solutions have an acid reaction, and when heated become turbid owing to the separation of basic sulphates or, in the case of the caesium salt, indium hydroxide. The melting-points and solubilities of the alums in water (in grams of crystalline salt per 100 grams of water) are as follows : -

		Solubilities
Temperature, °C.		15°	16.5°	25°	30°
Ammonium indium alum	m.p. 36°	...	c. 200	...	c. 400
Rubidium indium alum	m.p. 42°	44.28	...	...	...
Caesium indium alum	m.p. ?	...	3.04	c. 11.7	...

In the case of the ammonium alum, the "melting-point" is known to denote the transition-point, above which the alum breaks up into the octahydrate and water (Roessler): -

(NH₄)₂SO₄.In₂(SO₄)₃.24H₂O ⇔ (NH₄)₂SO₄.In₂(SO₄)₃.8H₂O + 16H₂O.

The densities and refractive indices of the alums for the D-line at the ordinary temperature are given by Soret as follows: -

	NH4	Rb	Cs
density	2.011	2.065	2.241
μ	1.4664	1.4638	1.4652