Kinetic studies on the complexation of uranium(VI) with chromotropic acid and chromotrope-2B

Summary

Kinetic studies on the complexation of U^VI by chromotropic acid, LH₄ (1) and chromotrope-2B, L'H₄, (2) in aqueous solution (pH=3.8–4.5) have been carried out, using stopped-flow spectrophotometry, in the presence of excess U^VI to maintain pseudo-first order conditions and formation of 1∶1 complexes only. Under the experimental conditions complex formation appears to be quantitative for (1), but with (2), which is a much weaker acid, an equilibrium mixture of reactants and the product complex is formed. The kinetic data suggest concurrent reactions of UO ²⁺₂ and UO₂(OH)⁺ with a partly deprotonated ligand, LH³⁻ and L'H ²⁻₂ . The reactions appear to involve fast unidentate binding of the ligand followed by rate-determining ring closure. Activation parameters corresponding to each of the rate constants have been evaluated, and plausible mechanisms are discussed.