Uranyl ions disproportionation

Partial regeneration of the product of the electrode reaction by a disproportionation reaction is characteristic for the reduction of the uranyl ion in acid medium, which, according to D. H. M. Kern and E. F. Orleman and J. Koutecky and J. Koryta, occurs according to the scheme... 

In the reduction of hexavalent uranyl ions, one electron is involved the product, pentavalent uranyl, disproportionates spontaneously into tetra- and hexavalent uranium... 

Trivalent uranium ion reduces water to hydrogen. Hence, stable aqueous solutions of trivalent uranium compounds cannot be prepared. Compounds of tetravalent uranium are generally similar to those of zirconium or thorium, except that some uranium compounds can be oxidized to the hexavalent form. Compounds of pentavalent uranium are of little importance because they disproportionate readily into tetravalent and hexavalent forms. The properties of hexavalent uranium are generally similar to those of hexavalent molybdenum or tungsten. In aqueous solution hexavalent uranium forms the uranyl ion UO2 ... 

The most prevalent form of uranium in aqueous solution is the tight yellow, fluorescent uranyl ion U02. The U cation (green in solution) can be obtained by strong reduction of U(V1), but readily oxidizes back to U02 in air. The pentavalent ion U02 can be reversibly formed by reduction of U02, but it readily disproportionates into U(IV) and U(VI). The trivalent U can be formed by reduction of U(IV) but is unstable to oxidation in aqueous solution. 

The uranium(v) ion, U02, is extraordinarily unstable towards disproportionation and has a transitory existence under most conditions, although evidence for its occurrence can be obtained polarographically. It is also an intermediate in photochemical reductions of uranyl ions in presence of sucrose and similar substances. The ion is most stable in the pH range 2.0-4.0 where the disproportionation reaction to give U4+ and U02+ is negligibly slow. By contrast, reduction of U02+ in dimethyl sulfoxide gives U02 in concentrations sufficiently high to allow the spectrum to be obtained and disproportionation occurs with a half-life of about an hour.42 As noted above, Uv can be stabilized in HF solutions as UF, as well as in concentrated Cl- and C03 solutions.42... 

The photodecomposition is the result of a disproportionation of the carboxylic acid catalyzed by the excited uranyl ion. The reduction product of the uranyl ion is oxidized back to UO2 by the organic acid. The secondary photochemical reactions vary, however, from acid to acid. ... 

These reaction products can be explained on the basis of the pathway shown in Scheme 8.4. This pathway involves an initial photoinduced one-electron transfer in a uranyl formate complex to give UO2 and a formate radical, followed by disproportionation to give and carbon dioxide. The photochemistry of the uranyl ion with either acetic acid or its higher homologues follows a similar type redox pathway. Such a pathway with acetic acid leads to the formation of carbon dioxide, ethane, and carbon dioxide ... 

General. For the most part, studies of aqueous solutions of fissile materials for use in homogeneous reactors have dealt with hexavalent uranium salts of the strong mineral acids. Hexavalent uranium in aqueous solutions appears as the divalent uranyl ion, IJOj Tetravalcnt uranium salts in aqueous solutions arc relatively unstalile, being oxidized to the hexavalent condition in the presence of air. Other valence states of uranium either disproportionate or form very insoluble compounds and have not been seriously proposed as fuel solutes. 

The oxidation-reduction behavior of plutonium is described by the redox potentials shown in Table I. (For the purposes of this paper, the unstable and environmentally unimportant heptavalent oxidation state will be ignored.) These values are of a high degree of accuracy, but are valid only for the media in which they are measured. In more strongly complexing media, the potentials will change. In weakly complexing media such as 1 M HClOq, all of the couples have potentials very nearly the same as a result, ionic plutonium in such solutions tends to disproportionate. Plutonium is unique in its ability to exist in all four oxidation states simultaneously in the same solution. Its behavior is in contrast to that of uranium, which is commonly present in aqueous media as the uranyl(VI) ion, and the transplutonium actinide elements, which normally occur in solution as trlvalent... 

In uranyl(v) solutions, an analogous tricarbonato complex, U02(C03)3, is formed, differing only in one unit of charge from the uranyl(vi) complex [248]. As might be expected from the lower charge of the central ion, the stability is much lower, however, log P3 = 13.3. Any formation of a trinuclear complex (1702)3(003)5 is precluded by the disproportionation of uranyl(v) in the carbonate concentration range where such a complex might exist. 

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