title: 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(+)-CO3(2-) 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(+)-CO3(2-) complexes is mainly entropy driven. The structures of the NpO2(+)-CO3(2-) 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)2(3-) is re-constructed as NpO2(CO3)2(H2O)(3-)of low symmetry but not as NpO2(CO3)2(H2O)2(3-)of high symmetry as suggested in a previous study.
authors.affiliation: Radiochemistry Department, China Institution of Atomic Chemistry, Beijing P. O. Box 275-126, Beijing 102413, China. gtiab@ciae.ac.cn.
authors.forename: Suliang
authors.initials: S
authors.lastname: Yang
authors.forename: Yaping
authors.initials: Y
authors.lastname: Zhao
authors.forename: Guoxin
authors.initials: G
authors.lastname: Tian