Uranium Sulphate, U(SO4)2
|The anhydrous Uranium Sulphate, U(SO4)2.nH2O, has not been prepared, but an extraordinarily large number of hydrates is known. Salts containing 1, 2, 3, 4, 5, 6, 7, 8, and 9 molecules of water have been described, and of these the di-, tetra-, octa-, and nona-hydrates are stable. The actual relation of these hydrates to one another is difficult to determine, as they undergo hydrolysis to a considerable extent when in solution, and the tetra- and octa-hydrates, at least, show a marked tendency to remain in a metastable condition at temperatures far removed from the transition- point between the two phases. Solubility determinations indicate that this transition-point is in the neighbourhood of 20° C., but it has been shown that the octahydrate when heated in absence of air changes into the tetrahydrate5 at 68° to 87° C. when the former hydrate is in a state of metastable equilibrium. |
The solubility data have been obtained, but not without difficulty owing to the gradual formation of insoluble basic sulphate and consequent clouding of the solution.
Fig. shows the relation between the two phases.
| Solubility of the tetra- and octa-hydrates of uranous sulphate. |
The tetra-, octa-, and nona-hydrates are isomorphous with the corresponding hydrates of thorium sulphate, and the relation between the uranium salts is probably similar to that between the sulphates of thorium, which are not so subject to hydrolysis and do not remain in the metastable condition to any extent.
|Temperature, °C.||Grams U(SO4)2 per 100 Grams Saturated Solution.||Phase.|
|Temperature, ° C.||Grams U(SO4)2 per 100 Grams Saturated Solution.||Phase.|
Uranous sulphate, even in acid solutions, is a strong reducing agent and can precipitate silver and gold from solutions of their salts. The sulphate is readily oxidised in solution by atmospheric oxygen. Both these reactions are accelerated by the presence of catalysts, especially copper salts, and in less degree platinum black or traces of iron salts.
The hydrates are usually prepared by reduction of uranyl sulphate by means of alcohol and exposure to light, or directly from uranous compounds.
The dihydrate, U(SO4)2.2H2O, crystallises in grey needles from a solution of uranous oxalate containing concentrated sulphuric acid.
The tetrahydrate, U(SO4)2.4H2O, may be prepared by dissolving urano-uranic oxide in concentrated sulphuric acid, adding water, and evaporating the solution in vacuo; or by exposing a solution of uranyl sulphate containing alcohol to direct sunlight. Giolitti and Bucci, using the latter method, found that by varying the concentrations of uranyl sulphate and alcohol it was possible to get sometimes the tetrahydrate and sometimes the octahydrate. If sodium hyposulphite is added to a solution of uranyl sulphate until a precipitate begins to form, the addition of alcohol to the solution precipitates a mixture of uranous and uranyl sulphates, from which the latter may be removed by washing with aqueous alcohol. On dissolving the residue in dilute sulphuric acid and evaporating at low temperature, crystals of the tetrahydrate are obtained. A further method consists in triturating uranium oxalate with concentrated sulphuric acid and pouring the mixture into alcohol; the precipitate, after washing with alcohol and ether, may be recrystal- lised from dilute sulphuric acid solution.
The crystals obtained in these preparations are green rhombic bipyramids, stable in the air. They dissolve in water, forming a clear solution, which, however, soon goes turbid owing to the separation of basic sulphate.
The octahydrate, U(SO4)2.8H2O, is obtained by exposing to light a solution of uranyl sulphate containing alcohol and sulphuric acid. It may conveniently be prepared by mixing urano-uranic oxide with alcohol and concentrated sulphuric acid, warming gently, and stirring for two or three hours. The product is then dissolved in very dilute sulphuric acid, and the uranous sulphate precipitated by means of alcohol. This is dehydrated at 200° C., the residue dissolved in water, and the solution allowed to crystallise at ordinary temperature over sulphuric acid.
The octahydrate yields small dark green monoclinic prisms, stable in the air. When treated with cold water in large quantity most of it dissolves, about 25 per cent, remaining as a green powdery basic salt, UOSO4.2H2O; with less water the basic salt redissolves and a solution of the normal salt is obtained. The extent to which the sulphate is hydrolysed depends upon the temperature, the solutions being relatively stable between 18° and 105° C. A ten per cent, solution of sulphuric acid dissolves it without hydrolysing it. When heated in absence of air, the octahydrate changes to the tetrahydrate between 68° and 87° C. and becomes anhydrous at 300° C. At the same time it begins to decompose, and at higher temperatures yields uranyl sulphate and finally the green oxide. From an investigation of the rate of dehydration of the octahydrate when kept over sulphuric acid, Giolitti and Bucci formulate the existence of the hydrates: U(SO4)2.7H2O, U(SO4)2.3H2O, and U(SO4)2.2H2O.
The nona- or ennea-hydrate, U(SO4)2.9H2O, is obtained by decomposing a cold solution of uranyl sulphate by means of sodium thiosulphate, and dissolving the precipitate in dilute sulphuric acid. After reprecipitation with alcohol, it is redissolved in the least possible quantity of dilute sulphuric acid, and the solution allowed to crystallise over sulphuric acid. It crystallises in green monoclinic prisms isomorphous with the corresponding thorium salt,
a:b:c = 0.5970:1:0.6555. β = 97° 49'.
The salt effloresces, losing one molecule of water. It gradually loses the remainder of its water on heating, and at red heat decomposes, leaving a residue of urano-uranic oxide.
Uranous sulphate forms double salts with the sulphates of the alkali metals. The salts, K2U(SO4)3.2H2O and (NH4)8U(SO4)6.3H2O, have been prepared, the former crystallising in glistening green plates, the latter in deep green, glistening pyramids. Both decompose in cold water with separation of basic sulphate.
Acid uranous sulphates have been described, e.g. U2(SO4)4.H2SO4, U(SO4)2.H2SO4.10H2O, and UH(SO4)2. The latter, which separates in dark brown leaflets when a solution of uranium trichloride is added to concentrated sulphuric acid at 0° C., may contain trivalent uranium.