Lanthanum sulphate, La₂(SO₄)₃

Of all the rare earth sulphates, Lanthanum sulphate, La₂(SO₄)₃, is the most difficult to decompose completely by heat. Its specific heat (0°-100°) is 0.1182.

One part of the anhydrous sulphate dissolves in 6 parts of ice-cold water; from the solution at 0° C. a hydrate, La₂(SO₄)₃.16H₂O, may be crystallised.

The hexagonal enneahydrate, La₂(SO₄)₃.9H₂O, is the only hydrate stable in contact with the solution between 0° and 100°. It is readily prepared by warming a 15 per cent, ice-cold solution of lanthanum sulphate to 30°. The solubility diminishes with the temperature as follows: -

Temp. °C.	0°	14°	18°	30°	50°	75°	100°
Grams of La2(S04)3	3.0	2.6	2.1	1.9	1.6	1.0	0.7

The figures represent grams of anhydrous salt per 100 grams of water.

A hexahydrate, La₂(SO₄)₃.6H₂O, is said to crystallise from a solution in aqueous sulphuric acid (Frerichs and Smith).

The acid sulphate, La(HSO₄)₃, and the basic sulphate, La₂O₃.SO₃, are known. The following double salts have been described: -

La₂(SO₄)₃.Cs₂SO₄.2H₂O; 3La₂(SO₄)₃.2Cs₂SO₄;
La₂(SO₄)₃.Rb₂SO₄.2H₂O; La₂(SO₄)₃.Rb₂SO₄; 3La₂(SO₄)₃.2Rb₂SO₄;
La₂(SO₄)₃.K₂SO₄.2H₂O; La₂(SO₄)₃.3K₂SO₄; La₂(SO₄)₃.5K₂SO₄;
La₂(SO₄)₃.Na₂SO₄.2H₂O;
La₂(SO₄)₃.(NH,)₂SO₄.8H₂O; 2La₂(SO₄)₃.5(NH₄)₂SO₄; La₂(SO₄)₃.5(NH₄)₂SO₄.

Lanthanum sulphate also combines with hydrazine sulphate. The diminution in solubility of lanthanum sulphate brought about by the addition of potassium, sodium, or ammonium sulphate will be seen from the following data, given by Barre, and denoting grams of anhydrous salts present in solution per 100 grams of water (solid phases not stated by Barre): -

Temperature 16.5°.		Temperature 18°.		Temperature 18°.
K2SO4.	La2(SO4)3.	Na2SO4.	La2(SO4)3.	(NH4)2SO4.	La2(SO4)3.
0.00	2.21	0.00	2.13	0.00	2.13
0.25	0.73	0.40	1.00	4.01	0.39
0.50	0.27	0.69	0.35	8.73	0.28
0.85	0.19	0.77	0.30	18.24	0.25
1.03	0.05	1.14	0.13	27.89	0.48
1.16	0.02	2.48	0.04	36.11	0.28
2.50	0.00	3.80	0.02	47.49	0.14
...	...	5.55	0.02	65.29	0.011