Study of Ternary Complex Stability Constants of Some Toxic Metal Ions with l-Histidine and l-Glutamic Acid in Dioxan–Water Mixtures

Abstract

The stability constants of ternary complexes of Pb(II), Cd(II) and Hg(II) ions with l-histidine and l-glutamic acid were determined pH metrically in the concentration range of 0–60 % v/v dioxan–water mixtures maintaining an ionic strength of 0.16 mol l−1 at 303.0 K. Alkalimetric titrations were carried out in different relative concentrations [M:L:X = 1.0:2.5:2.5, 1.0:2.5:5.0, 1.0:5.0:2.5 for Pb(II) and Cd(II) and 1.0:5.0:5.0, 1.0:5.0:10.0, 1.0:10.0:5.0 for Hg(II)] of metal (M) to histidine (L) to glutamic acid (X). Stability constants of ternary complexes were calculated and various models were refined with MINIQUAD75. The best fit chemical models were selected based on statistical parameters and residual analysis. The species detected are MLXH3, MLXH2, MLXH and ML2X for Pb(II), MLX2H2 and ML2X for Cd(II) and MLXH3 and MLXH2 for Hg(II). The relative stabilities of the mixed ligand complexes were compared with those of the binary complexes. The enhanced stability associated with the ternary complex is attributed to chelate effect, stacking interactions and hydrogen bonding.