Electrochemical and spectroscopic studies of neptunium(VI), -(V) and -(IV) in carbonate-bicarbonate buffers

The behavior of neptunium(VI), -(V) and -(IV) in carbonate-bicarbonate buffers (1 M) has been examined electrochemically and spectroscopically. Np(VI) undergoes a one-electron quasi-reversible reduction to Np(V) at + 0.22 V vs SSCE. Np(V) is reduced irreversibly to Np(IV) at ca. - 1.8 V. Although Np(V) is readily oxidized to Np(VI), no oxidation of Np(IV) was observed. Spectroscopic results indicate that the Np(VI)-(V) reaction involves one reactant and one product. The spectrum of Np(IV) in 1 M carbonate-bicarbonate has been quantitatively analyzed. Comparisons with results for uranium under similar conditions are presented. (author)