KINETIC-STUDY OF THE REACTIONS OF NP(V) AND U(VI) WITH DIPHOSPHONIC ACIDS IN ACETATE BUFFER SOLUTIONS

The results of kinetic investigations for the reactions of Np(V) and U (VI) with methanediphosphonic acid (MDPA), 1-hydroxyethane-1,1-diphosp honic acid (HEDPA), and of Np(V) with Arsenazo III in acetate buffer m edia (I = 0.1 M) are presented. Rate and activation parameters have be en determined for the complex formation reaction of Np(V) with Arsenat e III, complex formation between U(VI) and the diphosphonate ligands, and for both formation and dissociation reactions of the Np(V)-diphosp honate complexes. The activation parameters are generally consistent w ith an associative mechanism for complex formation and dissociation re actions in all systems. Isokinetic plots suggest a common mechanism fo r the actinyl (V and VI) reactions with MDPA and HEDPA. A distinct iso kinetic relationship exists for the actinyl derivatives of chromotropi c acid derivatives Arsenate III and Chlorophosphonazo III. In acetate buffer media, Np(V) reacts more rapidly with the diphosphonate ligands than U(VI) despite its lower formal charge and the smaller thermodyna mic driving force for complex formation.