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Thorium Sulphate, Th(SO4)2

Thorium Sulphate, Th(SO4)2, is obtained in an anhydrous state by dissolving the dioxide in concentrated sulphuric acid and heating the solution till the excess of sulphuric acid has been evaporated, or by heating the octahydrate Th(SO4)2.8H2O to about 400° C. The anhydrous sulphate has a density of 9.225. It dissolves sparingly in concentrated sulphuric acid, forming an acid sulphate, which is also formed when a saturated aqueous solution of the sulphate is precipitated with concentrated sulphuric acid. The thorium hydrogen sulphates ThH2(SO4)3 and 2Th(SO4)2.H2SO4.2H2O have been obtained.

Thorium sulphate can be dissolved in four times its weight of ice-cold water, forming a solution that is metastable with respect to various hydrates. The salt is noteworthy on account of its different hydrates; the following are known:

Enneahydrate, Th(SO4)2.9H2O, monoclinic prisms isomorphous with U(SO4)2.9H2O.
Octahydrate, Th(SO4)2.8H2O, monoclinic crystals.
Hexahydrate, Th(SO4)2.6H2O.
Tetrahydrate, Th(SO4)2.4H2O, fine needles, isomorphous with U(SO4)2.4H2O.
Dihydrate, Th(SO4)2.2H2O.

Solubility of thorium sulphates
Solubility curves of the hydrates of thorium sulphate.
These different hydrates have different solubility curves, which have been studied by Roozeboom and others. The hydrates with 9 and 4 molecules of water present a contrast in properties, since the solubility of the former increases with rise of temperature whilst that of the latter decreases. Consequently, whilst Th(SO4)2.9H2O crystallises from a solution at low temperature, Th(SO4)2.4H2O will separate from the same solution at high temperature. The solubility curves of the two salts are represented by AC and BE on the accompanying diagram (Fig.). These two curves intersect at B (43°C.), at which point both salts are in equilibrium with one and the same saturated solution. If the temperature is lowered from this point the 9-hydrate will separate; if raised, the 4-hydrate. Nevertheless each hydrate can exist in a metastable state, so that the solubility curve of Th(SO4)2.9H2O has been traced up to 55° C. (C), and that of Th(SO4)2.4H2O down to 18° C. (E). Thus, while the true equilibrium between these two salts and their saturated solution occurs at 43° C., there is not necessarily a transformation of one salt into the other through the medium of the solution when this temperature is departed from, though the further the temperature is removed from 43° C. towards 18° C. or 55° C. the more likely is it that respective transformation into the 9- or 4-hydrate will take place.

The solubility curve FG on the diagram is that of the salt Th(SO4)2.8H2O, and the curve HK of the salt Th(SO4)2.6H2O. The salt Th(SO4)2.2H2O separates from the solution at about 100° C.

Under conditions favourable to the production of the enneahydrate it very frequently happens that the octahydrate is obtained. The latter hydrate, on standing in contact with the solution, becomes converted into the former, but the rate of change is extremely slow.

Solubility of thorium sulphate in sulphuric acid
Solubility of thorium sulphate in sulphuric acid solutions.
Technically, the octahydrate is a very important salt. Its solubility in aqueous sulphuric acid at 20°, 30°, and 100° C. is shown graphically in Fig. At 30° C. the octahydrate passes into the tetra-hydrate when the concentration of sulphuric acid exceeds 33 per cent. At the same temperature, and with hydrochloric and nitric acids as solvents, the concentrations at which the change of hydration occurs are 19.5 and 31 per cent, respectively, and the solubilities of the hydrates are as follow:

Per cent, of Acid.Grams of Th(SO4)2 per 100 grams of Solution at 30° C.Per cent, of Acid.Grams of Th(SO4)2 per 100 grams of Solution at 30° C.

By the prolonged heating of the tetrahydrate with water an insoluble basic sulphate, ThOSO4.2H2O is produced; the salt ThOSO4.H2O has also been obtained by heating a concentrated solution of thorium sulphate in a sealed tube at 160°-180° C. ThOSO4.5H2O is also known.
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