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

Electrochemical behavior of NpO ₂ ²⁺ on platinum (Pt) and glassy carbon (GC) electrodes have been studied in an aqueous 3M HNO ₃ solution using cyclic voltammetry and spectroelectrochemical method. The NpO ₂ ²⁺ 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 NpO ₂ ²⁺ in an aqueous 3M HNO ₃ 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 NpO ₂ ²⁺ is NpO ₂ ⁺. Further details for reaction mechanism of Np(VI) are discussed on the basis of digital simulation of the experimental cyclic voltammograms.