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

To complement earlier studies of the complexation kinetics of Np(V) and U(V I) by oxydiacetic acid and several diphosphonic acids, the rate of complexa tion of these metal ions by dipicolinic acid (pyridine-2,6-dicarboxylic aci d, DPA) has been investigated by stopped-flow spectrophotometry. This ligan d 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 UO22+ at p[H] 1 and NpO2+ at p[H ] 1 and 3 adhere to the comparatively simple first-order approach to equili brium kinetic model used to describe the earlier results. At p[H] 3, the UO 22+ reaction is characterized by consecutive pseudo-first order reactions. The first is an apparent approach to equilibrium while the second process d emonstrates a saturation effect, indicating the existence of a stable inter mediate complex. Relative rates and activation parameters are discussed in comparison with the previous results.