Electrochemical and spectroelectrochemical behaviour of Np(VI) ions in nitric acid solutions

Electrochemical and spectroelectrochemical has been used to investigate the behaviour of neptunium (VI) in 1-8 M HNO₃ solutions. The electrochemical reactions of Np(VI) ions were found to occur quasi-reversibly. The formal redox potentials (E°) for Np(VI)/Np(V) couples were determined to be +0.906, +0.908, +0.909, +0.902, +0.896, +0.895, +0.888, and +0.884V (vs. Ag/AgCl) for Np(VI) ions in 1, 2, 3, 4, 5, 6, 7, 8 M HNO₃ solutions, respectively. The reduction processes of Np(VI) ions were followed spectroelectrochemically by using an optical transparent thin layer electrode cell. It was found that the absorption spectra measured at the applied potentials from +1.10 to +0.60 V versus Ag/AgCl reference electrode redox couple for Np(VI) in HNO₃ solution have clear isosbestic points. These results indicate that the only reduction product of Np(VI) is Np(V), which is of considerable stability in HNO₃ solutions.