Solution Equilibrium Study of Complexation of Pb(II), Cd(II) and Hg(II) With L-histidine and L-glutamic acid in organic media-water mixture

Abstract

Speciation of mixed ligand complexes of Pb(II), Cd(II) and Hg(II) with L-Histidine and L-glutamic acid were studied in varying amounts (0.0–50v/v) of Dimethylformamide-Water mixture by maintaining an ionic strength of 0.16mol dm^-3 (NaNO₃) at 310K. 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 in the case of Pb (II) and Cd(II) and 1.0:5.0:5.0, 1.0:5.0:10.0, 1.0:10.0:5.0 in the case of Hg(II)) were carried out with 0.4 mol dm^-3 NaOH as titrant in Dimethylformamide- water mixture. Stability constants of ternary complexes were re?ned with MINIQUAD75. The best-?t chemical models were selected based on statistical parameters and residual analysis. The species detected were MLXH₃, MLXH₂, MLXH and ML₂X for Pb(II), MLX₂H₂ and ML₂X for Cd(II) and MLXH₃ and MLXH₂ for Hg(II). Extra stability of ternary complexes compared to their binary complexes was believed to be due to interactions outside the coordination sphere such as the formation of hydrogen bonds between the coordinated ligands, charge neutralization, chelate effect and stacking interactions and hydrogen bonding. The species distribution with pH at different compositions of Dimethylformamide and plausible equilibria for the formation of species were also presented. The bioavailability of the toxic metal ions is explained based on the speciation.