A study of Nafion-coated and uncoated thin mercury film-rotating disk electrodes for cadmium and lead speciation in model solutions of fulvic acid

The speciation of lead and cadmium in model solutions of a well characteriz ed fulvic acid (FA), Laurentian FA, was investigated by anodic stripping vo ltammetry (ASV) using a thin mercury film-rotating disk electrode (TMF-RDE) and a Nafion-coated TMF-RDE. Pseudopolarograms were fitted to a model that accounted for differences in the diffusion coefficients between the free m etal ion, M, and the metal complexes, ML, and the heterogeneity of naturall y-occurring organic complexants, L, such as fulvic acid. The pseudopolarogr ams were used for each metal system to determine the differential equilibri um function which is independent of the experimental conditions and allows the stability constants for other metal loading conditions to be predicted. The experiments were done using linear-scan anodic stripping voltammetry ( LSASV), differential pulse anodic stripping voltammetry (DPASV), and square wave anodic stripping voltammetry (SWASV) with an uncoated TMF-RDE and a N afion-coated thin mercury film-rotating disk electrode (NCTMF-RDE). The NCT MF-RDE was investigated to determine its usefulness in preventing adsorptio n of surface-active organic complexants such as fulvic acid on the mercury electrode surface. LSASV was found to be the least susceptible to adsorptio n. DPASV was affected by adsorption in the lead-FA model solutions, and SWA SV was strongly affected by adsorption in both the lead-FA model solutions and the cadmium-FA model solutions. The Nafion-coated TMF-RDE did not preve nt adsorption when DPASV or SWASV was used. However, the results suggest th at the Nafion-coated TMF-RDE may be useful in preventing adsorption when LS ASV is used. (C) 2000 Elsevier Science B.V. All rights reserved.