Np(v) complexation with carbonate in aqueous solutions studied by spectrophotometric titration at various temperatures

Abstract

The complexation of neptunium(V) with carbonate has been studied at temperatures from 10 to 70 °C in 0.1 M LiClO₄ by spectrophotometry. Three NpO₂⁺–CO₃²⁻ complex species, NpO₂(CO₃)_n^(2n−1)− (n = 1, 2, 3), are identified and the stability constants are calculated by using the absorption spectra in the near-IR region collected from titrations at varying temperatures. The enthalpies and entropies are calculated with van't Hoff equations in the temperature range of 10 to 70 °C, indicating that the formation of all NpO₂⁺–CO₃²⁻ complexes is mainly entropy driven. The structures of the NpO₂⁺–CO₃²⁻ complex species in aqueous solutions are also reviewed. Based on the molar absorptivity of Np(V) in the near-IR region the structure of NpO₂(CO₃)₂³⁻ is re-constructed as NpO₂(CO₃)₂(H₂O)³⁻of low symmetry but not as NpO₂(CO₃)₂(H₂O)₂³⁻of high symmetry as suggested in a previous study.