Characteristics of subtractive anodic stripping voltammetry of Pb and Cd at silver and gold electrodes

Silver and gold electrodes are useful for the quantitative determination of lead and cadmium with subtractive anodic stripping voltammetry (SASV). The use of SASV is essential for achieving good separation between the two peaks, to eliminate the interference of nitrates when cadmium is present and to allow analysis at very low concentrations without the removal of oxygen. The deposition and dissolution of Pb²⁺ and Cd²⁺ proceed at underpotential (UPD) on both electrodes. The UPD properties of the deposits are the main factor determining the analytical characteristics of the ASV method and are strongly affected by the type and concentration of the electrolyte. The effects of anions (Cl⁻, Br⁻, SO₄²⁻, NO₃⁻) and acids (HNO₃, HClO₄, H₂SO₄, HCl) are shown. The two electrodes complement each other and, in addition, enable the qualitative identification of Pb²⁺ and Cd²⁺, since the peaks appear in opposite order on the two electrodes. Analysis of mixtures of the two analytes is restricted on gold but not on silver. At gold the two peaks overlap: (i) at concentrations of cadmium higher than 250 nM at deposition times greater than 30 s, (ii) in the presence of copper at concentrations higher than 1 μM, and (iii) in the presence of Triton X-100 at concentrations above 10 mg/l. The repeatability at 10 nM analyte is better than 2.5%. The detection limits for Pb²⁺ and Cd²⁺ at 120 s deposition time and 3500 rpm rotation rate are: dl_Pb/Ag=0.04 nM; dl_Cd/Ag=0.7 nM; dl_Pb/Au=0.1 nM; dl_Cd/Au=0.3 nM. The analysis of lead and cadmium in natural waters has been performed.