Electrochemical oxidation of Am(III) with lacunary heteropolyanions and silver nitrate

Anodic oxidation of Am(III) with tungstophosphate and silver nitrate is an efficient way to prepare Am(VI). This article considers the phenomenon in g reater detail. Thermodynamic data on the reactions are first assessed: redo x potentials of Am pairs are computed for a 1 M nitric acid medium containi ng Various amounts of lacunary heteropolyanion (LHPA) ligands such as tungs tosilicates or tungstophosphates. These results are based on data on the co mplexation of Am(III) and Am(IV) with the relevant LHPA. Am(IV)/Am(III) and Am(V)/Am(IV) pairs depend strongly on the [LHPA((total))]/[Am] ratio and c onsequently on the stoichiometry of the Am-LHPA complexes. With tungstosili cate, for example, the Am(V)/Am(IV)LHPA pair potential is about 1.7 V/SHE, compared with about 2.2 V/SHE for Am(V)/Am-IV(LHPA)(2). These potentials ac count for the observed oxidizability of Am(III)-LHPA and Am(IV)-LHPA comple xes. The speciation of the Am during the process can be estimated from UV-v isible spectra. An oxidation mechanism is proposed on the basis of the results obtained. It assumes the oxidation of Am(III) to Am(IV)LHPA (1:1 or 1:2) complexes; 1:1 Am(IV) complexes can be easily oxidized to Am(V) with an anode potential o f 1.9 V/SHE, whereas higher anode potentials are needed to oxidize 1:2 comp lexes. The Am(V) formed is assumed to be instantly oxidized to Am(VI) so th at no Am(V) is detected in the oxidation process. A kinetic model based on this mechanism has also been developed and accounts for the existence of an optimum [LHPA(total)]/[Am] ratio.