Thallous sulphate, Tl₂SO₄

Thallous sulphate, Tl₂SO₄, is prepared by dissolving thallium in hot sulphuric acid, or by adding thallous hydroxide or carbonate to aqueous sulphuric acid, evaporating, and crystallising.

Thallous sulphate crystallises in rhombic prisms (a:b:c = 0.5555:1:0.7328) of density 6.765 at 20°. It is therefore isomorphous with potassium sulphate (for which a:b:c = 0.5727:1:0.7418), and these two salts form a continuous series of mixed crystals. Thallous sulphate melts at 632°, and at temperatures above dull redness it volatilises. The mean refractive index, 1.8708 for the D-line, is extremely high, the value for potassium sulphate being 1.4952 (Tutton).

One hundred grams of water dissolve the following amounts of thallous sulphate: -

Temperature, °C.	0°	10°	20°	30°	40°	50°	60°	70°	80°	90°	99.7°
Grams of Tl2SO4	2.70	3.70	4.87	6.16	7.60	9.21	10.92	12.74	14.61	16.53	18.45

At a red heat hydrogen reduces thallous sulphate to a mixture of sulphide and metal, and ammonia reduces it to thallous sulphide.

Numerous double sulphates containing thallous sulphate are known. With the sulphates of the tervalent metals aluminium, chromium, iron, vanadium, gallium, and rhodium, thallous sulphate forms alums, of the general formula Tl₂SO₄.R₂^II(SO₄)₃.24H₂O. With the sulphates of magnesium, zinc iron, nickel, cobalt, copper, manganese, and cadmium, thallous sulphate forms a series of isomorphous, monoclinic, double sulphates of the general formula Tl₂SO₄.MSO₄.6H₂O, isomorphous with the corresponding series of salts containing ammonium, potassium, rubidium, and caesium in place of thallium. The crystallographic constants, determined by Werther and by Tutton (zinc salt), are as follows: -

Salt	a:b:c	β
Tl2SO4.MgSO4.6H2O	0.7422:1:0.5002	106°24'
Tl2SO4.ZnSO4.6H2O	0.7413:1:0.5010	106°16'
Tl2SO4.FeSO4.6H2O	0.7366:1:0.4964	105°52'
Tl2SO4.NiSO4.6H2O	0.7431:1:0.4988	106°39'
K2SO4.ZnSO4.6H2O	0.7413:1:0.5044	104°48'

The density of the thallium-zinc salt is 3.720 at 20°; the mean refractive index for the D-line is 1.6064, while that of the potassium zinc salt is 1.4859 (Tutton).