Peruranic acid, H₄UO₆

The existence of uranium tetroxide, or uranium peroxide, UO₄, in the anhydrous condition, has not been established, although Fairley claimed to have obtained it as a heavy crystalline precipitate by adding hydrogen peroxide to a solution of uranyl nitrate in presence of a large excess of sulphuric acid, and allowing the mixture to stand for a few days. The product, when carefully dried, did not lose moisture on heating. It decolorised potassium permanganate solution on warming, and when heated with hydrochloric acid, liberated chlorine.

The hydrate, peruranic acid, UO₄.2H₂O, is obtained as a yellowish-white precipitate by the addition of dilute hydrogen peroxide to a solution of uranyl nitrate or acetate, excess of the uranyl salt being allowed to remain unacted upon. The precipitate may be dried at 100° C. without loss of oxygen. The precipitation has been investigated by Mazzucchelli. It is hindered by the presence of chlorides, sulphates, acetates, oxalates, or tartrates, owing to the tendency to form soluble complexes. In presence of alkali or alkaline earth metals no precipitation occurs owing to the formation of soluble peruranates. The precipitate is very slightly soluble in water and in a solution of ammonium chloride. In the former the solubility, expressed in grams of UO₃ per litre of solution, is 0.0061 at 20° C. and 0.0084 at 90° C. When strongly ignited, the hydrate loses water and oxygen, leaving a residue of urano-uranic oxide. It decolorises permanganate in dilute sulphuric acid solution, the ratio of active oxygen to uranium being 1:1.

Peruranates of the alkali metals are obtained by acting on alkaline solutions of uranyl nitrate with hydrogen peroxide. They are soluble in water, but may be precipitated by the addition of alcohol. Corresponding peruranates of the heavier metals may be obtained by double decomposition with solutions of the sodium salt.

The constitution of peruranic acid and of the peruranates is not yet completely understood. Fairley, from a study of the decomposition of the acid by means of alkalies, suggested the formula (UO₃)₂UO₆. It was shown, however, by Melikoff and Pissarjewsky that, by means of aluminium hydroxide, the alkali peruranates could be quantitatively resolved into the alkali peroxides and uranium tetroxide. This reaction is more in accordance with the constitution, for example, for the sodium salt, (Na₂O₂)₂UO₄. Moreover, carbon dioxide, which has no action on uranic acid, converts insoluble peruranates into metallic hydrogen carbonates, hydrogen peroxide, and free peruranic acid, again supporting the formula (R''O₂)₂UO₄. The action of alkali on the peracid, the latter being partially reduced to uranium trioxide, whilst the alkali peroxide is formed, is also in agreement with such formulation. These considerations lead to the suggestion that the tetroxide is a peroxide of the hydrogen peroxide type, probably



and that the peruranates, instead of being salts of a peruranic acid, are probably additive compounds of the tetroxide with metallic peroxides.

The following value for the heat of formation of peruranic acid has been obtained:

UO₃.H₂O + H₂O₂ = UO₄.2H₂O - 6151 calories.

By drying the precipitate of the dihydrate at ordinary temperatures Fairley obtained a product which he considered to be a tetrahydrate, UO₄.4H₂O, but it has not been established that this is a definite chemical compound. Huttig and von Schroeder also describe a bright yellow hygroscopic trihydrate, UO₄.3H₂O, which, when heated, loses oxygen before all the water is expelled. If heated in a vacuum, it yields the trioxide, not the tetroxide. By means of the Hlittig tensi-eudiometer these investigators also obtained indications of the following hydrates: UO₃.H₂O₂.3.5H₂O; 2UO₃.H₂O₂.5H₂O; 2UO₃.H₂O₂.3H₂O.