Dissociation Kinetics and Complexation Equilibrium Studies of a ZnII Complex with 1,5-Bis(2-hydroxy-5-sulfophenyl)-3-cyanoformazan during Capillary Electrophoretic Separation Process

Abstract

Abstract The dissociation kinetics and the complexation equilibrium of a ZnII complex with 1,5-bis(2-hydroxy-5-sulfophenyl)-3-cyanoformazan (hscf: H5L) during capillary electrophoretic (CE) separation process were studied. The solvolytic dissociation reaction rate constants of the ZnII–hscf complex ([ZnL]3−) (kd) in the pH range of 6.4 to 8.0 were determined with a CE-based dissociation kinetic analysis method. Small kd values on the order of 10−5 s−1 showed the remarkable kinetic inertness of [ZnL]3−, which was detectable in the CE system. The stability constant of [ZnL]3− (KZnL) at 293 K and I = 0.10 was estimated to be [ZnL3−]/{[Zn2+][L5−]} = 1024.2±0.3 through the spectrophotometric titration. The thermodynamic stability of [ZnL]3− in the pre-capillary complexation stage was thus demonstrated. In contrast, it was shown that the CE separation environment is thermodynamically unsuitable for the metal complexes based on the conditional stability constant of [ZnL]3−, which was determined under CE separation conditions, and the concentration of free HSCF in the complex band in the capillary. Thus, the “stability” of metal complex in the CE system strongly depends on kinetic inertness rather than thermodynamic stability, though their thermodynamic stability is necessary for the pre-capillary derivatization step.