Chemical elements
  Uranium
    Isotopes
    Energy
    Production
    Application
    Physical Properties
    Chemical Properties
    Compounds
      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

Urano-uranic Oxide, U3O8






Urano-uranic Oxide, the Green Oxide of Uranium, U3O8 or UO2.2UO3, occurs in nature in a more or less pure condition as pitchblende. It may be obtained by heating to redness any of the other oxides of uranium; by strongly igniting, at a temperature above 900° C., ammonium diuranate; or by heating to redness the uranium salt of any volatile acid. The uranyl salts of commerce are invariably impure with alkali, so that it is impossible to obtain the pure oxide from them, unless they are first purified by a series of recrystallisations. To obtain pure urano-uranic oxide, free from alkali, the following method may be used: Sodium uranyl acetate (100 g.) is dissolved in a mixture of water (4 l.) and hydrochloric acid (50 c.c.), and precipitated by adding concentrated ammonia solution (300 c.c.). The precipitate is washed ten times with a 2 per cent, ammonium chloride solution, and then redissolved in hydrochloric acid. The precipitation with ammonia and washing are repeated twice, and the final precipitate, on calcination, yields pure urano-uranic oxide.

Urano-uranic oxide is an amorphous powder, varying in colour, according to its mode of preparation, from green to black, but in all cases it leaves a green streak on unglazed porcelain. The density, according to Karsten, is 7.193; according to Ebelmen, 7.31. The specific heat at ordinary temperatures is 0.07979, but the value falls considerably at lower temperatures, as shown by the following determinations:

Temperature Range, ° C.Specific Heat.
0 to -770.0616
-77 to -1900.0428


The oxide is paramagnetic, the susceptibility being +0.95×10-6, a value very near to that for the metal itself. The single electrode potential of the oxide, measured by pasting it in a finely powdered condition with gelatin on a platinum electrode and immersing it in a solution of uranyl nitrate containing 14.3 grams per litre, is:

U3O8/UO2(NO3)2 = 0.776 volt.

This value is identical with that obtained for uranous oxide.

Urano-uranic oxide is insoluble in water and in dilute acids; in concentrated mineral acids it dissolves slowly, forming a mixture of uranous and uranyl salts. It is very stable at high temperatures, and for this reason uranium is usually converted into this oxide for gravimetric estimation. It may be heated in a vacuum for three hours at 1000° C. without any appreciable loss of oxygen, and in order to obtain the dioxide from it by simple dissociation a temperature of 2000° C. is necessary. It is reduced when heated in hydrogen, the sole product being the dioxide. The action commences at about 625° C. and can be completed at 650° C. It also yields the dioxide when heated with sulphur. It is reduced to the metal by heating with carbon in the electric furnace; also by heating with magnesium. When heated in a stream of carbon dioxide at about 1120° C. it yields the pentoxide.

The heat of formation of the oxide has been determined by Mixter as follows:

3U + 4O2 = U3O8 + 845,200 calories.

The chemical behaviour of urano-uranic oxide indicates the presence of tetra- and hexa-valent uranium and supports the view that it is an association of uranous and uranic oxides, thus: UO2.2UO3. The properties of the substance do not support the view of Groth that it is uranous uranate, U(UO4)2.

The oxide is sometimes used for producing a black glaze on porcelain. In uniform films it may be used as a standard of α-ray activity.


Hydrates of Urano-uranic Oxide

When an aqueous solution of uranyl oxalate, or an alcoholic solution of uranyl acetate, is exposed to sunlight, a violet precipitate of hydrated urano-uranic oxide is formed. Moreover, any uranyl salt in solution in presence of a readily oxidisable organic substance, such as alcohol, ether, or glucose, when exposed to light, or better to ultra-violet rays, yields a violet precipitate which invariably contains the acid present in the original salt. The acid, however, can be removed by boiling with water, and a green hydrated oxide, U3O8.2H2O, results. Similar violet compounds are also obtained, without the action of light, whenever uranous and uranyl salts are present together in neutral or feebly acid solution at the requisite temperature.
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