Chemical speciation of binary complexes of Ca(II), Zn(II) and Mn(II) with L-proline is investigated pH-metrically in acetonitrile-water mixtures. The stability constants are calculated using the computer program MINIQUAD75. The best-fit chemical models are selected based on statistical parameters and residual analysis. The models for the binary species contained ML + , MLH 2+ and ML 2 H + for Ca(II), Zn(II) and Mn(II). The trend in variation of stability constants with change in the dielectric constant of the medium is explained on the basis of structure forming nature of acetonitrile. Distribution of the species with pH at different variations (0.0-60.0% v/v) in acetonitrile-water mixtures is also presented.

Protonation equilibria ofL-proline and L-valine in varying compositions (0.0-60.0% v/v) of 1, 2-Propanediol-water mixtures were investigated pH-metrically. Titrations were performed at 303.0 K and the ionic strength of the medium was maintained at 0.16 mol L-1using sodium chloride. The best fit chemical models of the protonation equilibria were based on crystallographic R-factor, Χ2, skewness, and kurtosis. All the protonation constants of proline and valine increased with increasing propanediol content. This is attributed to the dielectric constant of the medium.

Chemical speciation of ternary complexes of Ca(II), Zn(II) and Mn(II) with L-proline and L-valine was studied in various concentrations (0-60% v/v) of acetonitrile-water mixtures maintaining an ionic strength of 0.16 mol L -1 at 303.0 K. Alkalimetric titrations were carried out with different relative concentrations (M: L: X = 1:2.5:2.5, 1:2.5:5.0, 1:5.0:2.5) of metal (M) to L-proline (L) to L-valine (X). Stability constants of ternary complexes were calculated and various models were refined with MINIQUAD75. The best-fit chemical models were selected based on statistical parameters and residual analysis. The trend of the variation in the stability constants with changing dielectric constant (D) of the medium was explained based on the electrostatic interactions of the ligands, charge neutralisation, chelate effect, stacking interactions and hydrogen bonding. Distribution diagrams with pH at different compositions of aqua-organic mixtures and structures of plausible ternary complexes were also presented.

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