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

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

Potentiometric Studies on the Formation Equilibria of Ternary Complexes of Lead (II), Cad-mium (II), and Mercury (II) with Maleic Acid and L-Phenylalanine in Dimethylformamide-Water Mixture

The stability constants of ternary complexes of Pb(II), Cd(II) and Hg(II) ions with Maleic Acid (MA) and L Phenylalanine (F) were determined pH metrically in the concentration range of 0–50 % v/v in Dimethylformamide-Water Mixtures maintaining an ionic strength of 0.16 mol l?1 using NaNO3 as an electrolyte at 303.0 K. The Value of ? log K that is responsible for the extra stability of ternary complexes was discussedbased on statistical parameters and the nature of the species formed. The formation of various ternary species was established by modeling studies using the computer program MINIQUAD75. The best-fit chemical models were selected based on statistical parameters and residual analysis. The species detected are MLXH2, ML2X and HML2XH2for Pb (II), ML2XH, ML2XH2 and MLX2H3for Cd (II) and ML2XH, ML2XH2and MLX2 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.Influence of the solvent on the speciation was discussed based on the dielectric constant of the medium. The stability of the mixed ligand complexesis discussed in terms of the molecular structure of Maleic acid and L- Phenylalanine as well as the nature of the metal ion