Kinetics of the complexation reactions between Th(IV), Zr(IV) and chlorophosphonazo III

The rate of complexation of Th(IV) and Zr(IV) by the chelating phosphonic acid ligand Chlorophosphonazo III has been investigated at 2 M ionic strength and acidities between 0.1-2.0 M by stopped-flow spectrophotometry. In the zirconium system, the kinetic traces are resolved as two parallel first order reactions, probably correlated with the dominant monomeric zirconium species Zr4+ and ZrOH3+. For thorium, a single first order reaction was adequate for data resolution at 2 M acid. More complex reactions were indicated at lower acidity and higher concentrations. The Th complex is formed via a dissociative reaction [ΔH* = 60(±6) kJ mol-1, ΔS* = 88(±9) J mol-1 K-1] in contrast to the associative process reported elsewhere for the corresponding uranyl complex. The zirconium complex also is formed by a dissociative process for both first order pathways [ΔH* = 90(±15) kJ mol-1, ΔS*1 = 158(±27) J mol-1, K-1, ΔH*2 = 94(±16) kJ mol-1, ΔS*2 = 156(±26) J mol-1 K-1]. The fastest reaction for zirconium is 50 times slower than that for thorium, suggesting that the strength of the cation-water interaction is important in the rate controlling process. (orig.)