Potentiometric Investigation on Complexation Equiibria and Stabilities of Divalent Metal Ion Complexes of L-Cysteine and Glycine in Aqueous Solutions

A.M. Radalla
Beni-Suef University,

Keywords: potentiometric investigation, complexation equiibria and stabilities, divalent metal ion complexes, l-cysteine, aqueous solutions


Abstract Binary and Ternary complex formation of the divalent metal ions, Ca2+ and Zn2+ with the bio-relevant α-mino acids, L-cysteine and glycine were investigated by means of the potentiometric technique. The dissociation constants of the bio-Ligands were determined and used for determining the stability constants of their binary and ternary complexes formed in aqueous solutions at 25 oC and I = 0.20 mol•dm-3 NaNO3. The acid-base properties of glycine were investigated at different ionic strengths, ranging from 0.01 to 0.30 mol.dm-3 NaNO3 at 25.0 ± 0.1oC. The formation of the different 1:1 and 1:2 binary complexes and 1:1:1 ternary complexes were inferred from the potentiometric pH-metric titration curves. The ternary complex formation occurred in a stepwise manner with L-cysteine acting as the primary ligand. The stability constants of these binary and ternary complex systems were evaluated. The values of ∆ log10 K, percentage of relative stabilization (%R.S.) and log10 X for the ternary complexes were discussed. The concentration distribution of the different glycine species formed in solution due to variation of pH was evaluated and discussed.