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

Abstract

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.