Square-wave anodic stripping voltammetry of lead and cadmium at cylindrical graphite fiber microelectrodes with in situ plated mercury films

Abstract

Square-wave anodic stripping voltammetry (SWASV) at a cylindrical, 8-μm graphite fiber microelectrode with an in situ plated thin mercury film was investigated in oxygen-free and air-saturated solutions containing high and low concentrations of a supporting electrolyte. Owing to the non-planar (cylindrical) diffusion-enhanced mass transport at the microelectrode surface, very high steady-state current densities were observed at the microelectrode, e.g. 1.2 mA 1 mmol −1 cm −2 for a −1.0 V reduction of 0.59 mM Hg(II) in 0.1 M acetate buffer. To obtain such high current densities at a conventional rotating disk electrode (RDE), the RDE must be rotated ar 1600 rpm. Because of the high speed and background rejection features of square-wave voltammetry, SWASV measurements (without background subtraction) of lead and cadmium at the in situ mercury-plated microelectrodes are not affected by the presence of dissolved oxygen, despite a high yield of oxygen diffusion to the microelectrode. Enhanced SWASV peaks were recorded for lead and cadmium in millimolar solutions of supporting electrolytes. In a non-deaerated and quiescent solution of 0.84 mM acetate buffer, for example, the method yielded a detection limit of 5 × 10 −10 M for each metal. This represents a very impressive sensitivity of the method, especially when one considers the very small dimensions of the microelectrode, the absence of solution stirring and purging and the relatively short deposition time used (2 min). In addition to improved sensitivity, SWASV at in situ mercury-plated microelectrodes offers other advantages over the commonly used anodic stripping methods such as differential-pulse anodic stripping voltammetry, particularly in terms of speed, convenience and applicability. Because of the elimination of both the oxygen removal step and solution stirring, and because of the use of the very fast stripping technique, the method is significantly faster and technically simpler, and thus more convenient for analysis. Also, because of the incorporation of cylindrical microelectrodes, the method does not require high concentrations of supporting electrolytes or sophisticated instrumentation, and can be applied for the analysis of microvolume samples.