Speciation of binary complexes of Ca(II), Mg(II) and Zn(II) with L-3,4-dihydroxyphenylalanine (dopa) in (0 -60% vyv) dioxane (DOX) -water mixtures have been studied pH-metrically at a temperature of 303 K and at an ionic strength of 0.16 mol L À 1 . The models for binary species contain MLH, ML 2 H 2 , ML 2 H and ML 2 for Ca(II) and Mg(II) and MLH, ML, ML 2 H 2 , ML 2 H and ML 2 for Zn(II). The trend in the variation of stability constants with the mole fraction of dioxane was explained on the basis of electrostatic and nonelectrostatic forces. Distribution of the species with pH at different compositions of DOX -water media was also presented.

Protonation equilibria of L-Dopa and 1,10 phenonthroline have been studied in varying concentrations (0-60% v/v) of propylene glycol-water mixtures maintaining an ionic strength of 0.16 mol dm -3 at 303 K using pH metric method. The protonation constants have been calculated with the computer program MINIQUAD75 and the best fit models are arrived at based on statistical grounds employing crystallographic R factor, χ 2 , skewness and kurtosis. Dopa has three dissociable protons and one amino group which associate with proton. It exists as LH 4 + at low pH and gets deprotonated with the formation of LH 3 , LH -2and LH 2-successively with increase in pH. Phen forms LH 2 2+ at low pH and gets deprotonated with the formation of LH + and L with increase in pH. Secondary formation functions confirm the existence of 3 and 2 protonation equilibria for dopa and phen, respectively. The linear increase of log values of protonation constants of Dopa with decreasing dielectric constant of PG-water mixtures indicates the dominance of electrostatic forces in the protonation-deprotonation equilibria. Phen exhibits non-linear trend indicating the dominance of non-electrostatic forces.