DETERMINATION OF METAL-ORGANIC COMPLEXATION IN NATURAL-WATERS BY SWASV WITH PSEUDOPOLAROGRAMS

A novel electrochemical method is described for the examination of metal-organic complexation in natural waters. The technique is based upon the comparison of square wave anodic stripping voltammetry (SWASV) 'pseudopolarograms' (current vs. deposition potential plots) for natural samples with those obtained for metal complexation with selected 'model' binding ligands at constant ionic strength. For electrochemically active zinc species in seawater, the pseudopolarogram approach allowed the estimation of thermodynamic stability constants (K-ML) within a stability constant window of approximately 10(4)-10(6) M(-1) up to greater than or equal to 10(15) M(-1). For a natural rainwater sample, the pseudopolarogram indicated only one zinc class, probably a mixture of 'free' and/or inorganically complexed species. In an application to a coastal seawater sample, three species were identified: (1) weakly complexed, log K-ZnL = 4.14 +/- 0.92 M(-1); (2) moderately complexed, log K-ZnL = 7.77 +/- 0.62 M(-1) (3) strongly complexed, log K-ZnL = 11.45+/-0.95 M(-1). Results are compared to those obtained using complexometric titration with SWASV at a thin mercury film-glassy carbon rotating disk (TMF-GCRD) electrode. The latter detected a single strong complexing ligand (or ligand class), with [L'] = 15.25 +/- 0.04 nM and log K-ZnL(') greater than or equal to 10.9 M(-1).