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

Abstract

The speciation of lead and cadmium in model solutions of a well characterized fulvic acid (FA), Laurentian FA, was investigated by anodic stripping voltammetry (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 metal ion, M, and the metal complexes, ML, and the heterogeneity of naturally-occurring organic complexants, L, such as fulvic acid. The pseudopolarograms were used for each metal system to determine the differential equilibrium 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 Nafion-coated thin mercury film-rotating disk electrode (NCTMF-RDE). The NCTMF-RDE was investigated to determine its usefulness in preventing adsorption of surface-active organic complexants such as fulvic acid on the mercury electrode surface. LSASV was found to be the least susceptible to adsorption. DPASV was affected by adsorption in the lead-FA model solutions, and SWASV 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 prevent adsorption when DPASV or SWASV was used. However, the results suggest that the Nafion-coated TMF-RDE may be useful in preventing adsorption when LSASV is used.