Chemical speciation of 2,3-dihydroxybenzoic acid complexes with some biologically essential metal ions in 1, 2-propanediol–water mixtures

Abstract

Chemical speciation of Co(II), Ni(II), Cu(II) and Zn(II) complexes of 2,3-dihydroxybenzoic acid in 0.0-60.0% v/v 1, 2-propanediol-water mixtures maintaining an ionic strength of 0.16 mol dm-3 at 303±0.1 K has been studied pH metrically. The predominant complexes formed are ML, ML2 and ML2H2 for Co(II), Ni(II) and Zn(II) and ML, ML2, ML2H and ML2H2 for Cu(II). Models containing different numbers of species were refined by using the computer program MINIQUAD75. Selection of the best fit chemical models was based on statistical parameters and residual analysis. The trend in variation of complex stability constants with dielectric constant of the medium is explained on the basis of electrostatic and non-electrostatic forces. Distributions of species, formation equilibria and effect of influential parameters on the stability constants have been presented. The possible structures of the various species are elucidated on the basis of the analysis of the pH-metric data.