The formation constants of ternary complexes of title systems have been determined pH-metrically in biologically relevant conditions at an ionic strength of 0.16 mol dm -3 and 303 K. The overall stability constants have been evaluated using MINIQUAD75 computer program. The complexation equilibria have been derived on the basis of species distribution diagram. In the present study L-Dopa and 1, 10-phenanthroline are found to be compatible ligands, proving greater stability of ternary complexes as compared to binary ones. The trend in variation of stability constants with change in dielectric constant of medium is explained on the basis of electrostatic and non-electrostatic forces. Distributions of the species with pH at different compositions of propanediol-water mixtures are also presented. The factors responsible for the compatibility of both the ligands have also been discussed.

The protonation constant values of mercaptosuccinic acid, L-dopa and 1, 10 -phenanthroline were determined in dimethylformamide (0-60%v/v) at 303.0 K at ionic strength of 0.16 mol L -1 using pH-metric technique. The protonation constants were calculated with the computer program MINIQUAD75 and selection of the best fit models was based on the statistical parameters. The log K values were found to increase of the organic solvent content. The linear variations of the protonation constants with the reciprocal of the dielectric constant of the medium have been attributed to the dominance of electrostatic forces. Distribution of species, protonation equilibria and effect of influential parameters on the protonation constants have also been presented.