TERNARY COMPLEXES OF Pb(II),Cd(II) AND Hg(II) WITH L-GLUTAMIC ACID AND L-METHIONINE IN 1,2-PROPANEDIOL-WATER MIXTURES

摘要

Speciation of ternary complexes of Pb(II),Cd(II) and Hg(II) with L-glutamic acid and L-methionine has been studied in varying concentrations (0.0-60.0%) of 1,2-pro-panediol-water mixtures maintaining an ionic strength of 0.16 mol dm~(-3)(NaNO3) at 303 K.Titrations were carried out in the presence of different relative concentrations of metal (M) to L-glutamic acid (L) and L-methionine (X) ( M :L :X = 1:2:2,1:2:4,1:4:2) 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 species detected are MLXH for Pb(II);MLX,MLXH,MLXH2 and ML2X for Cd(II) and MLX and MLXH2 for Hg(II).Extra stability of ternary complexes compared to their binary complexes is believed to be due to electrostatic interactions of the side chains of ligands,charge neutralisation,chelate effect,stacking interactions and hydrogen bonding.The species distribution with pH at different compositions of 1,2-propanediol and plausible equilibria for the formation of species are also presented.