, and ML. The appropriateness of experimental conditions is verified by introducing errors intentionally in the concentrations of ingredients. The models containing different numbers of species were refined by using the computer program MINIQUAD75. The best-fit chemical models were arrived at based on statistical parameters. The trend in variation of stability constants of the complexes with dielectric constant of the medium is attributed to the electrostatic and nonelectrostatic forces. The species distribution and the plausible equilibria for the formation of the species are also presented.

L-Histidine complexes of Co (II), Ni (II) and Cu (II) metal ions were studied pH-metrically in 0 -60% vyv dioxan -water mixtures at a temperature of 303 K and at an ionic strength of 0.16 mol L À 1 . Exhaustive modeling was performed to identify the species present in the present systems. The models contain MLH, ML 2 H 4 , ML 2 H 2 , ML 2 H and ML 2 for Co (II), ML 2 H 4 , ML 2 H 2 , ML 2 H and ML 2 for Ni (II), and MLH, ML 2 H 2 , ML 2 H and ML 2 for Cu (II). The appropriate experimental conditions were verified by introducing errors intentionally in the concentrations of ingredients. The trend in the variation of stability constants with the dielectric constants of dioxan is explained on the basis of electrostatic and non-electrostatic forces. The bioavailability of metals and their transportation are explained based on the distribution diagrams.

A computer assisted pH-metric investigation has been carried out on the speciation of binary complexes of Ca(II), Mg(II) and Zn(II) with L-histidine. The titrations are carried out with sodium hydroxide in varying concentrations (0-60% v/v) of dioxan-water mixtures at an ionic strength of 0.16 mol L -1 and at a temperature of 303 K. Ca(II), Mg(II) and Zn(II) form the binary complexes of ML 2 H 4 , ML 2 H 3 , ML 2 H 2 , ML 2 H and ML 2 in dioxan-water mixtures. The effect of systematic errors in the concentrations of the substances on the stability constants is in the order acid > alkali > ligand > metal> Log F. The effect of solvent, dielectric constant of the medium and the electrostatic interactions between the complex species on the stability of the complexes are discussed.

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