Chemical speciation of ternary complexes of Co~(II), Ni~(II) and Cu~(II) with L-glutamine and succinic acid in CTAB-water mixtures

摘要

Chemical speciation of ternary complexes of Co~(II), Ni~(II) and Cu~(II) with L-glutamine and succinic acid have been studied pH metrically in the concentration range of 0.0-2.5% w/v of cetyltrimethylammonium bromide (CTAB) in aqueous solutions maintaining an ionic strength of 0.16 mol L~(-1) (NaCl) at 303.0 K. Alkalimetric titrations were carried out in the presence of different relative concentrations (M : L : X = 1 : 2 : 2, 1 : 4 : 2, 1 : 2 : 4) of metal (M) to L-glutamine (L) to succinic acid (X) with sodium hydroxide. 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 predominant species detected were ML2X, MLX, MLXH and MLXH2 for Co~(II), Ni~(II) and Cu~(II). Extra stability of ternary complexes compared to their binary complexes was believed to be due to electrostatic interactions of the side chains of ligands, charge neutralization, chelate effect, stacking interactions and hydrogen bonding. The species distribution with pH at different compositions of CTAB and plausible equilibria for the formation of species were also presented.