Estimation of on-capillary dissociation characteristics of Zn(II) complex with 1,5-bis(2-hydroxy-5-sulfophenyl)-3-cyanoformazan by a capillary electrophoretic reactor

The on-capillary dissociation reaction properties of the Zn(II) complex with 1,5-bis(2-hydroxy-5-sulfophenyl)-3-cyanoformazan ([Zn-HSCF]) have been studied using the Capillary Electrophoretic Reactor (CER). The solvolytic dissociation reaction rate constant (k _d) of [Zn-HSCF] under the optimal conditions for CE separation at pH 8.80 was determined to be 4.3 × 10 ^-5 s ^-1 (half-life period was 4.5 hours) with ligand substitution mode CER. Thus, it is clear that [Zn-HSCF] is extremely inert at pH 8.80 in a kinetic sense. On the other hand, [Zn-HSCF] was not detected at pH 5.10 in the CE system. The k _d value at pH 5.10 was determined to be 3.7 × 10 ^-3 s ^-1 by a batch-wise method with a ligand substitution technique using EDTA as a substitution ligand. The half-life period was calculated to be only 3 minutes, suggesting that almost all [Zn-HSCF] dissociates on the time scale of CE separation at pH 5.10. From an estimation of the free ligand concentration profiles in the capillary during CE separation processes, it was shown that CE separation gives an almost thermodynamically unsuitable environment for [Zn-HSCF] at both pH 8.80 and 5.10. Thus, it is concluded that the detection selectivity of metal complexes in the pre-capillary derivatization CE depends on the kinetic stability, i.e. the dissociation reaction inertness of the metal complexes, though their thermodynamic stability is necessary only for the pre-capillary derivatization step.