Redox behavior plutonium ions

A second reason for the wealth of chemical investigations of the early actinide elements is the relative diversity of their chemistry. While the chemistry of the later actinides is most often restricted to that of the tri- and tetravalent oxidation states, compounds of the early actinides can be isolated in all oxidation states from +3 to +7. The accessibility of a range of oxidation states is the impetus for signficant chemical interest in the early actinides, but also vastly complicates investigation of these elements under some circumstances, such as aqueous redox behavior. In the case of plutonium, ions in four different oxidation states (+3, +4, +5, and - -6) can exist simultaneously in comparable concentrations in the same solution. 

Several cerium(IV) complexes of various bidentate and tetradentate hydroxypyrodinonate (HOPO) complexes have been studied as model compounds for plutonium(IV) complexes (Xu et al., 2000). Bidentate HOPO monoanions are isolelectronic with catecholate dianions and they display a similar complex formation behavior towards cerium(IV) ions. However, HOPO ligands are more acidic and form stable complexes with cerium(IV) at lower pH values than catechol. The tetradentate ligands form more stable complexes than the corresponding bidentate ligands. New types of chelators for cerium(IV) and pluto-nium(IV) are the 2,3-dihydroxyterephthalamides (Gramer and Raymond, 2004 Xu et al., 2004). Some authors have made comparisons between the coordination chemistry and the redox behavior of cerium and berkelium (Lebedev et al., 1975 Milyukova et al., 1980 Yakovlev et al., 1982). 

Studies of ligands which might provide specificity in binding to various oxidation states of plutonium seems a particularly promising area for futher research. If specific ion electrodes could be developed for the other oxidation states, study of redox reactions would be much facilitated. Fast separation schemes which do not change the redox equilibria and function at neutral pH values would be helpful in studies of behavior of tracer levels of plutonium in environmental conditions. A particularly important question in this area is the role of PuOj which has been reported to be the dominant soluble form of plutonium in some studies of natural waters (3,14). 

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... 

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