Chemical elements
    Physical Properties
    Chemical Properties
      Uranium Difluoride
      Uranium Tetrafluoride
      Uranous Oxyfluoride
      Uranium Hexafluoride
      Uranyl Fluoride
      Uranium Trichloride
      Uranium Tetrachloride
      Uranium Pentachloride
      Uranyl Chloride
      Uranyl Chlorate
      Uranyl Perchlorate
      Uranium Tetrabromide
      Uranyl Bromide
      Uranium Tetra-iodide
      Uranyl Iodide
      Uranyl Iodate
      Uranous Oxide
      Uranous Hydroxide
      Uranium Pentoxide
      Urano-uranic Oxide
      Uranium Trioxide
      Ammonium Diuranate
      Ammonium Hexa-uranate
      Hydroxylamine Uranate
      Hydroxylamine Potassio-uranate
      Barium Uranate
      Barium Diuranate
      Bismuth Uranate
      Iron Uranate
      Lithium Uranate
      Potassium Uranate
      Potassium Diuranate
      Potassium Tetra-uranate
      Potassium Hexa-uranate
      Rubidium Uranate
      Silver Diuranate
      Sodium Uranate
      Sodium Diuranate
      Sodium Triuranate
      Sodium Penta-uranate
      Strontium Uranate
      Zinc Uranate
      Peruranic acid
      Ammonium Peruranate
      Barium Peruranates
      Lithium Peruranate
      Nickel Peruranate
      Potassium Peruranate
      Sodium Peruranates
      Uranium Monosulphide
      Uranium Sesquisulphide
      Uranium Disulphide
      Uranium Oxysulphide
      Uranyl Sulphide
      Uranium Sulphite
      Uranyl Sulphite
      Complex Uranyl Sulphites
      Uranium Sulphate
      Uranium Dithionates
      Uranyl Sulphate
      Uranyl Pyrosulphate
      Uranyl Thiosulphate
      Uranyl Dithionate
      Uranium Sesquiselenide
      Uranium Diselenide
      Uranyl Selenide
      Uranyl Selenite
      Uranyl Selenate
      Uranium Telluride
      Uranium Nitrides
      Uranous Nitrate
      Uranyl Nitrate
      Uranium Monophosphide
      Uranous Phosphide
      Uranyl Hypophosphite
      Uranyl Phosphite
      Uranous Phosphates
      Uranyl Phosphates
      Complex Uranyl Phosphates
      Uranyl Aminophosphates
      Uranous Arsenide
      Uranyl Metarsenite
      Uranous Arsenate
      Uranyl Arsenates
      Complex Uranyl Arsenates
      Uranous Antimonide
      Uranous Antimonate
      Uranium Carbide
      Complex Uranyl Carbonates
      Ammonium Uranyl Carbonate
      Calcium Uranyl Carbonate
      Potassium Uranyl Carbonate
      Sodium Uranyl Carbonate
      Thallium Uranyl Carbonate
      Potassium Uranyl Ferrocyanide
      Uranyl Platinocyanide
      Uranyl Cyanate
      Uranyl Thiocyanate
      Uranium Silicide
      Uranium Boride
      Uranyl Perborate
    PDB 1anv-3pu4

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.

Hydrates of Uranium Trioxide

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.

Colloidal Uranic Acid

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.

Salts of Uranic Acid

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.
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