Adsorptive Stripping Voltammetric Determination of Germanium(IV) at a Mercury-Free Bismuth Film Electrode Using Quercetin: Substituent Effect of the 5′-Sulfonate Group on Complex Adsorption and Analytical Sensitivity

The increasing use of germanium in advanced technologies and its potential release into aquatic environments demand sensitive and accessible analytical methods for its determination at trace levels. This work reports the development of an adsorptive stripping voltammetric method for Ge(IV) determination in freshwater samples using an ex situ bismuth film electrode and quercetin as a complexing agent. The stoichiometry of the Ge(IV)–quercetin complex was established as 1:2 by UV-Vis spectrophotometric titration and non-linear least-squares fitting. The effect of the sulfonic substituent was evaluated by comparing quercetin with quercetin-5′-sulfonic acid under otherwise identical conditions. The method was optimized with respect to pH, buffer concentration, ligand concentration, accumulation potential and time, and electrode cleaning conditions. Under optimized conditions, the method exhibited a linear range of 0.23–24.4 µg L−1 (3.17–335.9 nmol L−1), a limit of detection of 0.08 µg L−1 (1.10 nmol L−1), and a repeatability of 4.41%. The higher sensitivity achieved with quercetin relative to quercetin-5′-sulfonic acid was attributed to the neutral charge of the Ge(IV)–quercetin complex, which favors adsorption at the electrode surface. The method was successfully applied to groundwater, river water, and tap water samples with relative errors below 5.0%, demonstrating its suitability for environmental water analysis.