Voltammetric metal speciation in mixtures of inert and labile macromolecular complexes at any ligand-to-metal ratio: differential pulse polarographicstudy of the Zn(II)-nitrilotriacetate-polymethacrylate system

Voltammetric metal speciation in mixtures of inert and labile complexes is discussed, especially for the case of the titrations of ligand mixtures wit h metal ion solutions, frequently used in determining complexing capacities . A new voltammetric model is applied which, for the case of non-excess of ligands forming inert complexes, produces a set of equations that can be us ed to obtain complexation parameters of both labile and inert complexes fro m titration data. The applicability of these equations is tested by simulat ion and by fitting them to the experimental results obtained by differentia l pulse polarography for the system Zn(II) + nitrilotriacetate + polymethac rylate in the presence of succinate buffer. The study showed that the metho d provides an accurate measure of the total concentration of the 'inert' li gand, and a rough estimate of the stability constants of both inert and lab ile complexes. It was also noticed that, when macromolecular labile complex es are present, the complexation constant thus determined can be a meaningl ess parameter unless such complexes are considered. (C) 1999 Elsevier Scien ce S.A. All rights reserved.