Equilibrium study on complex formation of malonic acid with Co(II), Cu(II), Ni(II) and Zn(II) has been investigated pH-metrically in dioxan-water mixtures (0.0-60.0% v/v) at 303±0.1 K and 0.16 mol L-1 ionic strength. The predominant species detected for Co(II) and Cu(II) are ML, ML2, ML2H2 and ML2H and those for Ni(II) and Zn(II) are ML2, ML2H2, ML2H. 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 possible structure, species distribution and the plausible equilibria for the formation of the species are also presented.
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