Determination of nanomolar concentrations of lead and cadmium by anodic-stripping voltammetry at the silver electrode

Abstract

Lead and cadmium have been determined by subtractive anodic-stripping voltammetry (SASV) in the square-wave mode at a silver electrode without removal of oxygen. The sensitivities and detection limits for the two metals differ considerably. Detection limits of 0.05 nM for lead and 1 nM for cadmium have been achieved following 90 s electrodeposition. The repeatability of consecutive SASV runs is good (for lead 0.5% at 20 nM for 30 s electrolysis, 5% at 0.3 nM for 60 s electrolysis; for cadmium 2.5% at 20 nM for 30 s electrolysis, 5% at 5 nM for 60 s). Hundreds of runs can be carried out without any pretreatment of the electrode. The high stability is attributed to renewal of the electrode surface that takes place during the electrodeposition step in a two-electrode cell: the silver counter/quasi-reference electrode generates silver ions that codeposit with lead and cadmium at the Ag-RDE, thus ensuring a continuity of the latter. Underpotential deposition (UPD) plays a central role in anodic-stripping voltammetry (ASV). During the deposition step, the adatom coverage of trace elements is in the range of 0.01–1% and no bulk deposition is invoked for metals that exhibit UPD. The UPD properties and, as a result, the ASV signals are strongly affected by the type and concentration of the supporting electrolyte. The effects of Cl −, Br −, SO 4 2− and NO 3 − are shown. The analysis of lead and cadmium in natural waters has been performed. Surfactants distort the SASV signal. In order to ensure surfactant-free solutions, the samples were pretreated by wet ashing.