The importance of concentration effects at the electrode surface in anodic stripping voltammetric measurements of complexation of metal ions at natural water concentrations

The influence of the ligand/metal ion concentration ratio on the shape, peak current and peak potential of curves obtained by anodic stripping voltammetry (a.s.v.) at the hanging mercury drop electrode is described, particularly with respect to the use of a.s.v. for speciation of metal ions at very low concentrations as is often found in natural waters. The lead(II)/triethylenetetramine system is used as a model of a fully labile reversible system. It is shown that the total metal ion concentration at the electrode surface (C^o_M) during the stripping step may be much larger (30–300 times in typical conditions) than that in the bulk solution (C_M), the exact value depending on the deposition time t_d. Consequently, changes in the peak characteristics are observed when the ligand/metal concentration ratio in the bulk of the solution, C_L/C_M, is less than 1000. Semi-empirical equations, experimentally tested, are given, which enable C^o_M/C_M to be estimated for a specified solution and a.s.v. conditions, which correct for the “surface concentration effect” when a.s.v. is used to measure complexation, and which describe the influence of the parameters such as stirring efficiency, radius of the mercury drop and C_L/C_M. The implications of the results are discussed for determinations of total metal ion in complex media, of speciation based on peak-potential shifts or stripping voltammetric curves, and of complexation capacity.