Uranous Oxide, UO₂

Uranous Oxide, Uranium Dioxide, UO₂, owing to its stability and metallic appearance, was until 1842 thought to be the element. It may be obtained as a brown amorphous powder, usually pyrophoric, by reducing the green oxide, U₃O₈, or uranyl oxalate, by heating in a current of hydrogen; by igniting the higher oxides in an indifferent atmosphere, such as carbon dioxide or nitrogen; or by heating urano-uranic oxide with ammonium chloride, ammonium chloride and sulphur, or anhydrous oxalic acid. It is also produced by reducing uranyl sulphate or nitrate in hydrogen; or by heating together uranyl phosphate, potassium carbonate, and potassium cyanide; or by electrolysis of a solution of uranyl nitrate. It is obtained as a black powder by reducing an aqueous solution of uranyl chloride by means of powdered aluminium or magnesium.

Uranous oxide is also obtained in the form of black microscopic crystals when urano-uranic oxide is reduced with carbon or heated with a little hydrofluoric acid; when crystallised uranic hydroxide is heated in hydrogen; when uranyl chloride is heated with potassium in a current of hydrogen, or with a mixture of ammonium and sodium chlorides; or when the double chloride, Na₂UCl₆, is heated to redness in a moisture-laden stream of carbon dioxide. Especially fine black cubic crystals are obtained by fusing together 1 part of sodium diuranate and 4 parts of magnesium chloride. The amorphous form may be readily converted into the crystalline variety by fusion with borax, the product being washed with dilute hydrochloric acid, when jet-black octahedra are obtained.

A brick-red variety of uranous oxide has been obtained by calcining uranyl bromide in the air. It is very stable at high temperatures, and is converted to the black modification by heating in hydrogen without any appreciable loss of oxygen.

On a commercial scale uranous oxide is prepared by fusing at red heat a mixture of 35 parts of common salt and 20 parts of sodium uranate with 1 part of powdered charcoal, the heating being continued until the escape of gas ceases. After cooling, the mass is lixiviated with water, and the residue of uranous oxide is washed by decantation. By washing with 5 per cent, hydrochloric acid, any iron, aluminium, or vanadium compounds may be removed, and a commercial product of purity equivalent to 97 per cent. U₃O₈ is obtained. If the uranous oxide is required for the production of ferro-uranium, the complete removal of iron is not necessary.

Uranous oxide exists in various forms, depending, as indicated above, on the mode of formation. The crystals are usually regular in form, of density 10.95 to 11.0 at 4° C., and are isomorphous with those of cerium and thorium dioxides. The specific heat, according to Regnault, is 0.0619. The magnetisability of uranous oxide is greater than that of the metal or of the higher oxides, the magnetic susceptibility being +7.51×10^-6. When heated in an atmosphere of nitrogen, the oxide melts at 2176° C. It is the most stable of the oxides of uranium at high temperatures, and in many respects behaves like an element. It cannot be reduced by heating in a current of hydrogen, but it is reduced by carbon at about 1500° C. It combines directly with chlorine and bromine at red heat, uranyl compounds being produced. When heated in oxygen, it is readily oxidised, the action commencing at about 185° C., and the only product is urano-uranic oxide. The heat of the reaction is as follows:

3UO₂ + 2O = U₃O₈ + 75,300 calories.

Uranous oxide is only difficultly soluble in hydrochloric and sulphuric acids, even when concentrated. With the latter acid, insoluble uranium sulphate is formed. It readily dissolves, however, in dilute nitric acid forming uranyl nitrate; it is also soluble in aqua regia. The amounts of the oxide dissolving in these acids in a given time vary widely with the mode of preparation of the oxide.

It resembles the dioxide of molybdenum in being able to reduce salts of silver in ammoniacal solution, with precipitation of silver, thus:

UO₂ + 2Ag^• + 2OH' → UO₃ + 2Ag + H₂O.

During the electrolysis of a solution of uranyl chloride a black reduction product is precipitated at the cathode. This precipitate dissolves in water, forming a dark-coloured colloidal solution, which by titration with potassium permanganate, before and after reduction with zinc, has been shown to contain uranous oxide. A similar solution may be obtained by the reduction of uranyl chloride by zinc or copper in dilute acid solution. When very dilute, the solution has a yellow tinge. In an electric field it becomes decolorised at the anode and precipitation takes place at the cathode. It behaves as a typical positive colloid with coagulating agents.