Uranium Trioxide, UO₃

Uranium Trioxide, Uranic Oxide, Uranic Anhydride, or Uranyl Oxide, UO₃, is obtained when uranic acid, ammonium diuranate, or ammonium uranyl carbonate is heated to a temperature not exceeding 300° C. When uranyl nitrate is similarly heated the product always contains basic nitrate, but if this product is heated in a current of oxygen at 500° C. it yields pure uranium trioxide.

The oxide appears to exist in two modifications, orange-yellow and red, the latter probably being a polymeride of the former. The yellow variety produces the red on prolonged heating. Both forms are amorphous. The oxide is paramagnetic, the susceptibility being +1.08×10^-6. When strongly heated in air or oxygen it yields the green oxide, U₃O₈, and in the latter case traces of ozone are also formed. It is reduced when heated with hydrogen to uranous oxide; with carbon in the electric furnace at 3000° C. it yields the metal.

The following values for the heat of formation of uranium trioxide have been determined as follows:

U + 3O = UO₈ + 303,900 calories
UO₂ + O = UO₃ + 34,200 calories
U₃O₈ + O = 3UO₃ + 16,200 calories.

Uranium trioxide is slightly basic, but with the exception of uranium hexafluoride, the salts formed by interaction with acids still contain two-thirds of its oxygen in the form of the uranyl radical, compounds of the type UO₂R'₂ being produced. On the other hand, however, the oxide acts towards strong bases as an acid anhydride, similar to chromic anhydride, and produces stable uranates. In contact with water it readily forms uranic acid, UO₂(OH)₂.

Two well-defined hydrates are known: (1) the monohydrate, UO₃.H₂O; and (2) the dihydrate, UO₃.2H₂O. The former is also known as uranic acid, H₂UO₄, or uranyl hydroxide, UO₂(OH)₂, and acts both as an acid and a base. It may be obtained in the amorphous condition by heating an alcoholic solution of uranyl nitrate for some time at a temperature just below its boiling- point, and then washing the yellow precipitate produced with boiling water; by exposing a solution of uranium oxalate to light until the precipitated violet hydroxide turns yellow, and then washing and drying the product; or by heating the dihydrate, UO₃.2H₂O, in dry air at 80° C. It is also obtained in the crystalline form by exposing the violet hydrate to air, and then boiling it with water for several hours; by heating a 2 per cent, solution of uranyl acetate in a sealed tube at 175° C. for 100 hours; by digesting precipitated copper hydrate with a solution of uranyl nitrate; by dissolving the amorphous form in a concentrated solution of uranyl nitrate, evaporating to dryness, and extracting the residue with dry ether; or by the electrolysis of a solution of uranyl nitrate with current intensity less than 1 ampere. It is also formed, mixed with the dihydrate, when a solution of uranyl nitrate is evaporated almost to dryness on a water-bath.

Uranic acid, when amorphous, is a yellow powder, of density 5.92. In the crystalline form it yields, according to its mode of formation, rectangular orthorhombic tables or hexagonal crystals. When heated to 300° C. it loses water, but it does not appear to be completely dehydrated until it begins to lose oxygen.

The single potential of the acid electrode, determined in the manner described for the green oxide, is as follows:

UO₃.H₂O/UO₂(NO₃)₂ = -0.860 volt.

It dissolves in acids to form yellow solutions which on concentration yield well-crystallised uranyl salts, while with inorganic and organic bases it yields uranates.

The dihydrate, UO₃.2H₂O, appears to be analogous to the corresponding tungsten compound, and behaves as the monohydrate of uranic acid, H₂UO₄.H₂O. It is formed by the slow oxidation in air of the violet hydrate; by boiling a solution of ammonium uranyl carbonate, when it falls as a yellow precipitate; by evaporating a solution of uranyl nitrate at ordinary temperature over sulphuric acid, and extracting the residue with ether; or by heating uranous oxide with a solution of hydrogen peroxide. The hydrate is a lemon-yellow powder, lighter in colour than uranic acid. It may be obtained in hexagonal prisms. On heating at 80° C. in dry air, it yields the monohydrate. It is very soluble in acids, and reacts in the same way as uranic acid.

The subhydrate, 2UO₃.H₂O, or pyroranic acid, H₂U₂O₇, corresponding to the stable diuranates, has not as yet been shown to exist.

By the addition of potash to a solution of uranyl nitrate or chloride in the presence of sugar, Graham obtained, in the cold, a deep orange-yellow coloured solution, from which all acid and alkali could be removed by dialysis. The solution contained hydrated uranic oxide, and showed considerable stability. It coagulated, however, on the addition of an electrolyte. Szilard prepared a similar solution by gradually adding uranic acid to a hot dilute solution of uranyl nitrate until the former is no longer dissolved. The resulting solution contained a little uranyl nitrate, but was very stable. Colloidal uranic acid may also be obtained by precipitating all the chlorine from an aqueous solution of uranyl chloride trihydrate by means of silver oxide, and dialysing after filtration.

It has been observed that colloidal uranic hydroxide, in very dilute solution, acts as a powerful catalyst in the synthesis of formaldehyde when an aqueous solution of carbon dioxide is exposed to direct sunlight.

Uranic acid, UO₃.H₂O, is a dibasic acid, and yields salts of the type R'₂UO₄. As in the case of the corresponding acids of the other elements of the group, in addition to the normal salts, other types, richer in acid content, also exist. For example, salts of the alkali metals of types R'₂O.xUO₃, where x may be 1, 2, 3, 4, 5, or 6, are known. The acids corresponding to such derivatives have not been isolated. The most important salts are the diuranates, from the hypothetical diuranic or pyro-uranic acid, H₂U₂O₇, which are usually precipitated when metallic oxides, hydroxides, or carbonates are added to solutions of uranyl salts. The normal uranates are generally obtained by fusion. They are yellow in colour, insoluble in water, but soluble in acids. Salts containing the more electro-positive metals only have been prepared.