Uranyl ions stability

Among the elements known before transuranium elements started to be synthesized in 1940, uranium has a unique characteristic, the extreme stability of the triatomic uranyl ion OUO+z. Not only are the numbers of uranium(VI) compounds larger than of U(IV), and far larger than of the two other oxidation states U(V) and U(III) known from non-metallic compounds, but until the preparation of UOFj discussed below, the only two U(VI) compounds known to contain less than two oxygen atoms per uranium atom were the octahedral molecules UFe and UQ6. 

Uranium is usually found in compounds which can be metabolized and recomplexed to form other compounds. In body fluids, tetravalent uranium is likely to oxidize to the hexavalent form followed by formation of uranyl ion. Uranium generally complexes with citrate, bicarbonates, or protein in plasma (Cooper et al. 1982 Bounce and Flagg 1949 Stevens et al. 1980). The stability of the carbonate complex depends on the pH of the solution, which will differ in different parts of the body (BEIRIV 1988). The low-molecular-weight bicarbonate complex can be filtered at the renal glomerulus, and be excreted in urine at levels dependent on the pH of the urine. The uranium bound to the protein (primarily transferrin) is less easily filtered and is more likely to remain in blood. In the blood, the uranyl ion binds to circulating transferrin, and to proteins and phospholipids in the proximal tubule (Wedeen 1992). 

Not only is the uranyl ion thermodynamically robust, it is also kinetically inert. Experiments designed to measure the rate of isotopic oxygen exchange between the oxo atoms and water at room temperature, establish that the exchange half-life is greater than 40,000 hours [19]. This overall chemical stability accounts for an extensive coordination chemistry which is exploited, for example, in the solvent extraction separation processes used in the nuclear fuel cycle [20]. 

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 lanthanides are all unreactive except for Eu, Yb and Sm which have stable divalent ions. Stability problems have restricted studies of the actinides. However, the uranyl ion reacts with ejq with a rate coefficient of 7.4 X 10 1 mole" sec [46]. 

The chemical properties span a range similar to the representative elements in the first few rows of the periodic table. Francium and radium are certainly characteristic of alkah and alkaline earth elements. Both Fr and Ra have only one oxidation state in chemical comhina-tions and have little tendency to form complexes. Thallium in the 1+ oxidation state has alkah-like properties, but it does form complexes and has extensive chemistry in its 3+ state. Similarly, lead can have alkaline earth characteristics, hut differs from Ra in forming complexes and having a second, 4+, oxidation state. Bismuth and actinium form 3+ ions in solution and are similar to the lanthanides and heavy (Z > 94) actinides. Thorium also has a relatively simple chemistry, with similarities to zirconium and hafiuum. Protactinium is famous for difficult solution chemistry it tends to hydrolyze and deposit on surfaces unless stabilized (e.g., by > 6 M sulfuric acid). The chemistry of uranium as the uranyl ion is fairly simple, hut... 

Stability aspects of SLMs were investigated in DC18C6-facilitated, diffusion-limited transport of uranyl ion across a flat-sheet SLM. Solvent effects on the cation flux and diffusion coefficients were evaluated (44). The results of this study indicated that the stability and transmembrane fluxes depend on the physico-chemical characteristics of the carrier-diluent combination, not on the characteristics of the diluent alone. The physico-chemical properties of some diluents tested in the study are given in Table 5. Greater membrane stability was obtained with a membrane solvent of low volatility and aqueous solubility. Among the various diluents tested, a mixture of o-dichlorobenzene and toluene (3 7 by volume) gave the best combination of stability, regeneration capability, and transport rates. 

In the second example we present a study where theoretical helped unravel an unexpected result in the laboratory a reaction expected to lead to the thermodynamically most stable species (uranium oxo), instead generated a uranium imido species. As mentioned earlier, the uranyl ion possesses U-O bonds with high thermodynamic stability and extreme kinetic inertness. As a result, the majority of uranyl ion reaction chemistry involves substitution of equatorially coordinated ligands while leaving the U-O bond unaffected. Its isoelectronic bis(imido) [U(NR)2] + analog, while possessing many of the bonding features found in UO2+ [46] exhibits reactivity quite distinct from UO. ... 

The order of solvents with respect to solvate stability is opposite to that with respect to the stability of the trlnltratouranyl complex. This suggests a conpetltlon between solvent molecule and nitrate Ion for coordination with the uranyl ion. ... 

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