Electrochemical and spectrophotometric investigations of neptunium in alkaline media

The results of electrochemical and spectrophotometric studies of neptunium in concentrated alkali ([OH-]tot=0.9 to 6.5 M) and mixed hydroxo-carbonate solutions are reported. Using cyclic voltammetry, we have demonstrated reversibility of the Np(VI)/(V) couple and quasi-reversibility of the Np(VII)/(VI) couple in basic solutions ([OH-]totabove 1.0 M), implying nearly constant coordination geometries for the predominant Np(V), Np(VI) and Np(VII) species in such media. For the first time, formation of Np(IV) hydroxide in alkaline solutions has been demonstrated using voltammetric methods. The formal redox potential of the Np(VI)/(V) couple in 1 M NaOH was determined to be 0.106±0.001 V/N.H.E. This value is different in NaOH and LiOH media, which we have interpreted to indicate a specific interaction between the lithium cation and a Np anionic species. At [CO32-]totgreater than 0.1 M in 1.8 M NaOH, the Np(VI)/(V) couple was found to be quasi-reversible while the Np(VII)/(VI) couple was irreversible. Formal redox potentials for Np in several alkaline solutions have been evaluated. These results are discussed in comparison with previously reported information.