Formal oxidation potentials of the Np(VI)-Np(V) couple and schemes of formal oxidation potentials of Np and am ions in HNO3 solutions containing potassium phosphotungstate K10P2W17O61

Abstract

The formal oxidation potentials E f 0 of the Np(VI)-Np(V) couple in 1–3 M HNO3 solutions containing (1–3) × 10−3 M potassium phosphotungstate K10P2W17O61 (KPW) were measured by the potentiometric method with spectrophotometric identification of the valence states. The potentials slightly decrease with an increase in the HNO3 and KPW concentrations, and the shift of the Np(VI)-Np(V) potential toward negative values relative to its value in 1 M HClO4 reaches 0.08 V. The formal oxidation potentials of the Np(V)-Np(IV) couple were calculated from the experimentally determined equilibrium constants (K eq) of the redox reaction of the Np(V) disproportionation. Under the examined conditions, these potentials also vary insignificantly, and the shift of E f 0 toward positive values relative to its value in 1 M HClO4 is 0.53 V. The schemes of potentials of the Np and Am couples in 1 M HNO3 in the presence of KPW are suggested.