Uranyl Sulphate, UO₂SO₄

Uranyl Sulphate, UO₂SO₄, is obtained as amber-yellow non- fluorescent crystals when uranyl hydroxide is dissolved in concentrated sulphuric acid and the solution is evaporated; or when either of its hydrates is heated to 300° C. The trihydrate, UO₂SO₄.3H₂O, crystallises from a solution of uranyl hydroxide in dilute sulphuric acid; or it may be obtained by dissolving urano-uranic oxide in concentrated sulphuric acid, diluting the solution, and boiling with nitric acid in order to oxidise the uranous salt present; or by triturating potassium hydrogen sulphate with uranyl hydroxide, taking up with water, and concentrating the solution. It forms yellowish-green prismatic crystals, which under the microscope show a beautiful fluorescence. They have density at 16.5° C. = 3.280. In the air the crystals slowly effloresce; on heating they lose more water, and at 115° C. yield the monohydrate. This hydrate, in moist air, takes up water again and reforms the trihydrate.

Uranyl sulphate is readily soluble in water, but accurate solubility determinations have not been made. According to O. de Coninck, the trihydrate dissolves in about twenty times its weight of water at ordinary temperature, and the solubility decreases with rise in temperature. The heat of solution at 18° to 20° C. is +5100 calories.

The equivalent conductivities of solutions containing ½UO₂SO₄ in v litres at 25° C. are as follows:

v =	4	8	16	32	64	128	256	512	1024
Λ =	17.1	20.5	25.5	32.0	41.5	53.2	67.7	85.2	103.4

The difference between Λ₃₂ and Λ₁₀₂₄, 71.4, is, as in the case of uranyl chloride and nitrate, unusually high, probably because dissociation takes place in stages. If the solutions are kept for some time, the conductivity increases owing to the increase in hydrogen ions due to hydrolysis. The migration velocity of the uranyl ion at 25° C. is, according to Dittrich, 56. The conductivities at higher temperatures have been determined as follows (for ½UO₂SO₄ in v litres):

v =	8	16	64	256
Λ at 35° C. =	48.2	61.9	82.9	117.2
Λ at 50°C. =	59.1	69.8	98.9	141.3
Λ at 65° C. =	70.2	87.0	114.3	162.2

Freezing-point determinations show that uranyl sulphate dissociates in dilute solutions to a much less extent than the chloride or nitrate.

Solutions of uranyl sulphate are fairly stable in diffused daylight, even in presence of alcohol, but readily undergo reduction when placed in direct sunlight or in ultra-violet light.

Uranyl acid sulphate, UO₂SO₄.H₂SO₄, is obtained in fine yellow fluorescent crystals when a solution of uranyl sulphate in moderately concentrated sulphuric acid is evaporated at 200° C. It is very deliquescent and readily soluble in water. By evaporating at 60° C. a solution containing uranyl sulphate (1 molecule) and sulphuric acid (5 molecules), similar crystals of composition 2UO₂SO₄.H₂SO₄.5H₂O are obtained.

Basic uranyl sulphates of composition 3U₂O₃.SO₃.2H₂O and 4U₂O₃. SO₃.7H₂O have been obtained by evaporating at 250° C. solutions of uranyl sulphate containing 3 and 15 per cent, respectively. The former yields citron-yellow, and the latter greyish-yellow, microscopic crystals.

Uranyl sulphate forms yellow addition compounds with ammonia, of composition UO₂SO₄.xNH₃, where x = 2, 3, or 4, the colour of which deepens with increasing ammonia content.

Complex uranyl sulphates, of composition

R^•₂SO₄.UO₂SO₄.2H₂O, (R^• = NH₄, K, Rb),
R^•₂SO₄.UO₂SO₄.3H₂O, (R^• = NH₄, Cs, Na, K, Tl),
Li₂SO₄.UO₂SO₄.4H₂O,
MgSO₄.UO₂SO₄.5H₂O,

have been described. They are usually obtained from a solution of the diuranate in sulphuric acid, or by crystallisation in vacuo of solutions of the mixed salts.

The potassium salt, K₂SO₄.UO₂SO₄.2H₂O, may also be prepared by triturating uranyl hydroxide with potassium hydrogen sulphate, taking up the mixture with water, and evaporating the solution. It was in this salt that Becquerel first discovered radioactivity. It yields yellow fluorescent crystals of density at 191° C.=3.363. The crystals, like those of the corresponding ammonium and rubidium salts, belong to the monoclinic system. The salt is stable towards water. When heated, it becomes anhydrous at 120° C.

The double salt, 2K₂SO₄.UO₂SO₄, is obtained by dissolving uranyl sulphate and excess of potassium sulphate in hot water, and crystallising from the hot solution. At temperatures below 80° C. the salt is decomposed by water thus:

2K₂SO₄.UO₂SO₄ → K₂SO₄.UO₂SO₄ + K₂SO₄.

A double salt with hydroxylamine sulphate, (NH₂OH)₂H₂SO₄.2UO₂SO₄.5H₂O, and an ethylenediammonium uranyl sulphate, C₂H₄(NH₂)₂H₂SO₄.UO₂SO₄.4H₂O, have been prepared.