Interactions of Np(V) and U(VI) with dipicolinic acid

Abstract

To complement earlier studies of the complexation kinetics of Np(V) and U(VI) by oxydiacetic acid and several diphosphonic acids, the rate of complexation of these metal ions by dipicolinic acid (pyridine-2,6-dicarboxylic acid, DPA) has been investigated by stopped-flow spectrophotometry. This ligand is distinguished from the previously studied species by the rigid planar arrangement of the ligand functional groups and the presence of a pyridine nitrogen donor atom. Reactions of DPA with UO 2 2+ at p[H] 1 and NpO 2 + at p[H] 1 and 3 adhere to the comparatively simple first-order approach to equilibrium kinetic model used to describe the earlier results. At p[H] 3, the UO 2 2+ reaction is characterized by consecutive pseudo-first order reactions. The first is an apparent approach to equilibrium while the second process demonstrates a saturation effect, indicating the existence of a stable intermediate complex. Relative rates and activation parameters are discussed in comparison with the previous results.