The Pu(IV)-Acetate Chemical Equilibria: Intrication between Hydrolysis and Complexation

The complexation of plutonium(IV) by acetate anions as [Pu(AcO)_n]^4-n species does not exist under the experimental conditions used in the literature and in our experiments. Using capillary electrophoresis (CE) coupled with ICP-MS, we discovered that in the Pu(IV)/acetate/water system, two distinct kinetics coexisted for which it was possible to determine the associated constants, namely, the equilibrium constants for the slow hydrolysis and the fast complexation. Basically, each [Pu(OH)_y]^4-y-type hydroxo complex (y = 1-4) is bound by acetate, forming mixed complexes, independently of each other. We show that the speciation of Pu(IV) under acidic conditions (pH < 5.5) is accessible by CE. It leads to the conclusion that previous data in the literature should be reconsidered from the perspective of hydroxo complexes [Pu(OH)_y]^4-y rather than Pu⁴⁺. In addition, a thorough interpretation of experimental data allowed us to demonstrate that CE could probe the hydration sphere and determine size variations during the complexation reaction, and that simple rules could be derived between complexes and electrophoretic mobilities. The resolution of this truly complicated chemical system opens up broad perspectives that go beyond simple thermodynamics. It appears that structural information about ions in solution can be obtained using this technique.