Kinetics of the reaction of U(VI) with benzene-1,2-diphosphonic acid

The kinetics of the reaction between dioxouranium(VI) and benzene-1,2-diphosphonic acid (BzDPA) has been investigated by stopped-flow spectrophotometry. The rate of reaction of uranyl ions with Arsenazo III (2,7-bis(2,2'-arsonophenylazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid) in 50:50 methanol-water solutions was also determined. Both formation and dissociation rate constants for the 1:1 complex between uranyl-BzDPA in acidic solutions were resolved. To gain insight into the effect of solvation on the progress of the reaction, the system was studied in triply distilled water, in 50:50 methanol-water, 80:20 methanol-water and in 50:50 tert-butanol-water as a function of temperature at pH 1.0. The rates of complex formation and dissociation reactions decrease as methanol substitutes for water in the medium and further decrease as tert-butanol replaces methanol as co-solvent. Activation parameters are most consistent with an associative process governing the progress of both complex formation and dissociation reactions. Introduction of alcoholic co-solvents results in notably more negative activation entropies for both complex formation and dissociation reactions, while the activation enthalpies are only slightly reduced in the mixed methanol-water medium. These results are compared with the kinetic features of other U(VI) systems.