Investigation of mercury-free Potentiometric Stripping Analysis and the influence of mercury in the analysis of trace elements lead and zinc

The application of Potentiometric Stripping Analysis (PSA), without any mercury, to the determination of trace elements lead and zinc, results in linear responses between stripping-peak areas and concentrations within 0 to 2000 ng/g. The best response, as determined by the size of stripping areas, was obtained with an electrode prepared with mercury but without mercury ions in the electrolyte. Lead is analysed by a freshly polished glassy-carbon electrode in 0.09-0.1 mol/L HCl while the analysis of zinc requires an electrode activation procedure. The electrode activation is performed by stripping zinc in an electrolyte containing 0.1 mol/L HCl and 2 μg/g Zn²⁺ and electrolysis at - 1400 mV (SCE). The concentration range of linear response occurs where the electrode is not fully covered by metal clusters during the electrolysis step. The influence of mercury is investigated and a model is proposed which explains the co-deposition of mercury and test metals in the electrolysis step in terms of a chargedistribution parameter. The model explains that the decrease of stripping peak area, as a function of concentration, is entirely due to mercury ions being simultaneously reduced together with the ions of the test metal in the electrolysis step. The influence of hydrogen evolution and oxygen reduction together with possible improvements of the method are discussed.