Electrochemical and spectroelectrochemical properties of neptunium(VI) ions in nitric acid solution

Electrochemical behavior of NpO22+ on platinum (Pt) and glassy carbon (GC) electrodes have been studied in an aqueous 3M HNO3 solution using cyclic voltammetry and spectroelectrochemical method. The NpO22+ species was found to be reduced to Np(V) quasi-reversibly with the formal redox potential of +0.907 V (Pt) and +0.909 V (GC). The reduction process of NpO22+ in an aqueous 3M HNO3 solution was also followed spectroelectrochemically by using an optical transparent thin layer electrode cell. It was found that the absorption spectra measured at the applied potentials in the range of +1.05 to +0.65 V have isosbestic points at 1051 and 1124 nm and the evaluated electron stoichiometry is 0.93. These results indicate that the reduction product of NpO22+ is NpO2+. Further details for reaction mechanism of Np(VI) are discussed on the basis of digital simulation of the experimental cyclic voltammograms.