Evaluation of stability constants of consecutive ternary complex equilibria in aqueous and aqua-dioxan media

摘要

The stability constants of the mixed ligand complexes of L-3,4-dihydroxyphenylalanine (L-dopa) (L) and 1,10-phenanthroline (phen) (X) with Ca~(II), Mg~(II) and Zn~(II) ions were determined pH-metrically at 303.0 K and an ionic strength of 0.16 mol L~(-1) (NaCl) in varying concentration of dioxan-water (0-60% v/v) mixtures. These studies establish the binding mode of the ambidentate L-dopa and phen in the ternary complexes. The trend in the variation of stability and concentrations of the ternary species with the dielectric constant of the medium was explained on the basis of electrostatic and non-electrostatic forces. The formation and distribution of different species with relative concentrations of metal and ligands, with varying pH were represented in the form of distribution diagrams. Influence of the solvent on the speciation was discussed based on the dielectric constant of the medium.