Ternary Complexes of some Divalent Metal Ions with Potentially Tridentate Ligands in Dioxane-Water Mixtures

Abstract

Chemical speciation of mixed ligand complexes of Ca(II), Mg(II) and Zn(II) with L-histidine and L-glutamic acid have been studied in varying concentrations (0.0–60.0 % v/v) of 1, 4-dioxane-water mixtures maintaining an ionic strength of 0.16 mol L^–1 sodium chloride at 303.0 K. Titrations were carried out in the presence of different relative concentrations (M:L:X = 1.0:2.5:2.5, 1.0:2.5:5.0, 1.0:5.0:2.5) of metal (M) to L-histidine (L) to L-glutamic acid (X) with sodium hydroxide. Stability constants of ternary complexes were refined with MINIQUAD75. The best-fit chemical models were selected based on statistical parameters and residual analysis. The predominant species detected were ML₂XH₂, MLXH₂ and MLX₂ for Ca(II), Mg(II) and Zn(II). The formation and distribution of different species with relative concentrations of metal and ligands with varying pH are represented in the form of distribution diagrams. The influence of the solvent on the speciation is discussed in terms of the dielectric constant of the medium.

Keywords: Speciation, mixed ligand complexes, L-histidine, L-glutamic acid, essential metals, dioxane