Anodic stripping voltammetry of lead and cadmium using a mercury ®lm electrode and thiocyanate

It is well known that lead and cadmium can be determined with good sensitivity using anodic stripping voltammetry (ASV) and a mercury ®lm electrode. However, the mercury ®lm formation is poorly reproducible and cannot fully be removed electrochemically. We have evaluated procedures to improve the reproducibility and sensitivity of the ASV method and found that in the presence of thiocyanate the ®lm can be plated reproducibly and removed fully after each scan. Furthermore we optimised the analytical parameters pH, electrolyte composition, and deposition time and potential. Optimal analytical conditions were found to be a thiocyanate concentration of 5 mM, and a pH of 5.6. Optimisation of the deposition potential unexpectedly showed that the sensitivity was greatly improved by using a very negative potential of ÿ1.5 V, causing a 10-fold increase in the sensitivity of cadmium, and a 3-fold increase for lead. In these conditions the limit of detection (3 ) was 5 pM cadmium and 8 pM lead (standard deviations of 1.7% and 2%) using a deposition time of 5 min. The method was successfully applied to the determination of cadmium and lead in lake waters and certi®ed seawater after UV digestion. # 1999 Elsevier Science B.V. All rights reserved.