The electrochemistry of neptunium in tri-n-butyl phosphate

Neptunium's electrochemical behavior was studied in solutions of undiluted, acid-equilibrated tri-n-butyl phosphate (TBP) as well as in TBP solutions diluted by dodecane. Careful experimental choices and electromagnetic noise suppression allowed voltammetry in the low-conductivity 30 vol% TBP/n-dodecane solvent, bypassing the approximation of using undiluted TBP for this widely used solvent composition. In undiluted TBP, the Np(VI/V) couple is quasireversible and has a half-wave potential of 0.722 ± 0.012 V vs. Ag/AgCl. Diffusion coefficients for the Np(VI) and Np(V) species were found to be (4.3 ± 0.8) × 10^-7 cm² s^-1 and (3.5 ± 0.6) × 10^-7 cm² s^-1, respectively. The Np(IV/III) couple is irreversible in TBP indicating that Np(III) is unstable; however, like the Np(VI/V) couple, it shows stabilization of the more-oxidized state by TBP complexation. Spectrophotometry showed that Np(V) is unstable in undiluted TBP-HNO₃, oxidizing to Np(VI) over time. Switching macroelectrodes for ultramicroelectrodes enables measurement of neptunium redox couples in TBP diluted with dodecane, and increased dilution shifts the Np(VI/V) couple to more negative potentials. Voltammetry in even 30 vol% TBP was possible with a minor amount of supporting electrolyte. The presence of di-n-butyl phosphoric acid (HDBP), a common TBP radiolysis product, stabilizes Np(VI) relative to Np(V), with exchange of one HDBP upon reduction of Np(VI).