The speciation of L-histidine complexes of Pb(II), Cd(II) and Hg(II) has been investigated pH-metrically in dioxan-water mixtures (0-60% v/v) at 303K and 0.16 mol L-1 ionic strength. The predominant species detected for Pb(II) and Cd(II) are ML2H4, ML2H3, ML2H2, ML2H and ML2 and those for Hg(II) are ML2H4, ML2H3, ML2 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 non-electrostatic forces. The species distribution and the plausible equilibria for the formation of the species are also presented.
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