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Uranium trioxide

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Uranium trioxide

γ-UO3:

Systematic name Uranium trioxide
Uranium(VI) oxide
Other names Uranyl oxide
Uranic oxide
Molecular formula UO3
O3U (NIST)
Molar mass 286.03 g/mol
CAS number
Density 5.5-8.7 g/cm
Solubility (water) Partially soluble
Lung solubility half-time < 5 days
(dog, inhalation)
Melting point ca. 500 °C decomp. (s)
Disclaimer and references

Uranium trioxide (UO3), also called uranyl oxide, uranium(VI) oxide, and uranic oxide, is the hexavalent oxide of uranium. The toxic, teratogenic, and radioactive solid may be obtained by heating uranyl nitrate to 400 °C. The gas is produced when uranium burns in oxygen or air, and condenses at standard temperature and pressure.

Its most commonly encountered polymorph, γ-UO3, is a yellow-orange powder.

Cameco Corporation, which operates at the world's largest uranium refinery at Blind River, Ontario, produces high-purity uranium trioxide.

Health and safety hazards

Like all hexavalent uranium compounds, UO3 is hazardous by inhalation, ingestion, and through skin contact. It is a poisonous, radioactive substance, which may cause shortness of breath, coughing, acute arterial lesions, and changes in the chromosomes of white blood cells and gonads leading to congenital malformations if inhaled.

UO3-Material Safety Data Sheet.

Chemistry

In addition to the monomeric gas which may be produced by uranium combustion, solid UO3 may be obtained by heating uranyl nitrate (UO2(NO3)2·6H2O) at 400 °C. This conversion is used in the reprocessing of nuclear fuel, which begins with the dissolution of the fuel rods in HNO3.

Uranium trioxide is also an intermediary compound in the conversion of sodium diuranate yellowcake (Na2U2O7·6H2O) to uranium hexafluoride and is shipped between processing facilities in the form of a UO3 gel. In the jargon of the uranium refining industry, the chemical solution containing the concentrated uranium trioxide is called "OK liquor". Upon heating, this material liberates ammonia, giving UO3.

Combustion product of uranium

UO3 gas vapor is produced by combustion of uranium metal in air (Cotton, 1991.) The burning temperature reaches 2800 K (Mouradian and Baker, 1963.) UO3 gas will eventully condense under normal atmospheric conditions.

Uranium-nitrogen salts UN and UN2, form above 700 deg. C. Professor Simon Cotton (1991) Lanthanides and Actinides (New York: Oxford University Press) also writes, on page 127: "Aerial oxidation of any uranium compound eventually results in the formation of a uranyl compound."

Uranium trioxide gas molecules are a known intermediate in the chemical transition reactions forming U3O8 crystals from combustion products including uranium oxides. According to Wilson (1961), Ackermann et al. (1960) show that U3O8 crystals result from the two step process:

/3 U3O8(s) + /6 O2(g) → UO3(g) at T1;
UO3(g) → /3 U3O8 + /6 O2(g) at T2;
where T2 < T1.

The uptake of oxygen by U3O8 is "not infrequently ignored" (Gmelin vol. U-C1, p. 98.)

Individual UO3 molecules will not decompose below the burning temperature of uranium in air, because uranium monoxide requires additional energy to form, as does the release of O2 by a single UO3 molecule. (Hoekstra and Siegel 1958; Wanner and Forest (2004) p. 98.)

Structure

Structure of uranyl oxide. "The formation of two short U=O bonds in the uranyl ion prevents the close approach of a third oxygen" - from Cotton (1991) p. 128.

The geometry of the uranyl ion has been the subject of much debate. The close approach of two oxygen atoms to uranium, with each linear O-U-O bond from 1.7 to 1.9 Å, prevents the close approach of a third or more. Bond angles are 90 and 180 degrees. d-p and f-p bonding have been suggested to explain the short U-O bonds. (Cotton, 1991)

The only well characterized binary trioxide of any actinide is UO3, of which several polymorphs are known. Solid UO3 loses O2 on heating to give green-colored U3O8: reports of the decomposition temperature in air vary from 200–500 °C. Heating at 700 °C under H2 gives dark brown uranium dioxide (UO2), which is used in MOX nuclear fuel rods.

The most frequently encountered polymorph is γ-UO3, whose x-ray structure has been solved from powder diffraction data. The compound crystallizes in the space group I41/amd with two uranium atoms in the asymmetric unit. Both are surrounded by somewhat distorted octahedra of oxygen atoms. One uranium atom has two closer and four more distant oxygen atoms whereas the other has four close and two more distant oxygen atoms as neighbors. Thus it is not incorrect to describe the structure as , that is uranyl uranate. (Engmann, 1963).

Infrared spectroscopy of molecular UO3 isolated in an argon matrix indicates a T-shaped structure (point group C2v) for the molecule. This is in contrast to the commonly encountered D3h symmetry exhibited by most trioxides. One could think of the compound as "uranyl monoxide", O. From the force constants the authors deduct the U-O bond lengths to be between 1.76 and 1.79 angstroms. The authors did not attempt to deduce bond angles. (Gabelnick, Reedy, Chasanov, 1970)

In the gas phase

At elevated temperatures gaseous UO3 and O2 are in equilibrium with solid U3O8.

/3 U3O8(s) + /6 O2(g) ⇄ UO3(g)

With increasing temperature the equilibrium is shifted to the right. This system has been studied at temperatures between 900 and 1500 °C. The vapor pressure of UO3 is low but appreciable, about 10 bar at 980 °C, rising to 10 bar at 1400 °C (in the presence of 1 bar oxygen). At these partial pressures, uranium trioxide is monomeric. (Ackermann, 1960)

Uranium oxide in ceramics

UO3-based ceramics become green or black when fired in a reducing atmosphere and yellow to orange when fired with oxygen. Orange-coloured Fiestaware is a well-known example of a product with a uranium-based glaze. UO3-has also been used in formulations of enamel, uranium glass, and porcelain.

Prior to 1960, UO3 was used as an agent of crystallization in crystalline coloured glazes. It is possible to determine with a Geiger counter if a glaze or glass was made from UO3.

Related anions and cations

Uranium oxide is amphoteric and reacts as acid and as a base, depending on the conditions.

As an acid:

UO3 + H2O → UO4 + H

Dissolving uranium oxide in a strong base like sodium hydroxide forms the doubly negatively charged uranate anion (UO4). Uranates tend to agglomerate, forming diuranate, U2O7

As a base:

UO3 + H2O → UO2 + OH

Dissolving uranium oxide in a strong acid like sulfuric or nitric acid forms the double positive charged uranyl cation. The uranyl nitrate formed (UO2(NO3)2ˑ6H2O) is soluble in ethers, alcohols, ketones and esters; for example, tributylphosphate. This solubilty is used to separate uranium from other elements in nuclear reprocessing, which begins with the dissolution of nuclear fuel rods in nitric acid. The uranyl nitrate is then converted to uranium trioxide by heating.

From nitric acid one obtains uranyl nitrate, trans-UO2(NO3)2·2H2O, consisting of eight-coordinated uranium with two bidentate nitrato ligands and two water ligands as well as the familiar O=U=O core.

See also

Accidental teratogens:

References

Peer-reviewed
United Nations invitation-only
  • H. R. Hoekstra and S. Siegel (1958) "Recent Developments in the Chemistry of the Uranium-Oxygen System," in the Proceedings of the Second International Conference on Peaceful Uses of Atomic Energy, (Geneva: UN) 7, 394-400.
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