Determination of lead and cadmium in the presence of quercetin-5'-sulfonic acid by adsorptive stripping voltammetry with a hanging mercury drop electrode and a nafion-coated mercury film electrode

The use of a hanging mercury drop electrode (HMDE) and a glassy carbon electrode modified with Nafion-Hg (NHgFE) to determine Pb(II) and Cd(II) by adsorptive stripping voltammetry in the presence of quercetin-5'-sulfonic acid (QSA) is reported. With a HMDE it is possible to determine Pb(II) and Cd(II) individually because the peak current of Pb-QSA is maximum at pH 6.1 (E _peak -0.50 V), while the peak current of the Cd-QSA complex is maximum at pH 8.9 (E _peak -0.66 V). The linear calibration curves ranged from 0.5-40.0 μg L ^-1 for Pb(II) and 0.5-45.0 μg L ^-1 for Cd(II). The detection limits (3σ) were estimated to be around 0.3 and 0.1 μg L ^-1 and the relative standard deviations were 2.0 and 1.7 % at the 9.4 μg L ^-1 level of Pb(II) and Cd(II) with 30 s of accumulation (n=7) (CQSA 2.7 μmol L-1; Eads -0.10 V). On the other hand, it is found that with the prepared Nafion-mercury film electrode Pb(II) and Cd(II) can be analyzed simultaneously at pH 6.1 (E _peak -0.55 V, -0.73 V). The detection limits were 0.2 μg L ^-1 for both metal ions, with a linear range until 18.0 μg L ^-1. The methods were validated using synthetic sea water (ASTM D665) spiked with several metal ions, and reference material for measuring of elements in water (TMDA-61) and waste waters (SPS-WW1). Finally, the method was successfully applied to the determination of Pb(II) and Cd(II) in tap water after UV digestion.