The influence of pH and complex formation on the ASV peaks of Pb, Cu and Cd

The effects of pH, electrolyte composition and complex formation on the size and position of the ASV peaks of Pb, Cu and Cd have been systematically evaluated, with an instrument equipped with a mercury thin-film electrode and by applying a linear ramp voltage scan. The peak heights change with pH and the magnitude of the pH effect varies with base electrolyte composition. Anions such as chloride and acetate reduce the signal, as does the presence of excess of ligands such as 2,2'-bipyridyl, NTA and EDTA. Formation of stable chelates (e.g., with EDTA) can lead to total loss of signal, but dissociation of labile complexes can be enhanced by reducing the pH and/or increasing the magnitude of the applied deposition potential, thus producing measurable peaks. The peak potentials vary with pH, and in copper systems there are additional shifts in the presence of citrate, 2,2'-bipyridyl and chloride. With the last two, double peaks are formed and these are attributed to the formation of both Cu(I) and Cu(II) oxidation products. The varied response, particularly in the case of copper, which can follow changes in the base electrolyte composition, supports the need for careful control of the chemical environment in quantitative determination, and raises some queries about the feasibility of using direct ASV for speciation purposes.