Strategies for the characterization and optimization of adsorptive stripping voltammetry with catalytic enhancement for ultratrace element determination: The case of iron 2,3-dihydroxynaphthalene complex with catalytic enhancement by atmospheric oxygen

A comprehensive approach to the characterization and setup of metal determination by adsorptive stripping voltammetry with catalytic enhancement (CAdSV) is presented. The focus is on the understanding of the chemical features of these procedures to demonstrate which parameters can influence the analytical performances: the CAdSV method for the determination of iron at trace level using 2,3-dihydroxynaphthalene (DHN) is taken as case study. First, the ligand degradation was investigated by 1H NMR spectroscopy highlighting a significant degradation at alkaline pH of around 33% in 12 h. The use of degraded DHN had a detrimental effect on the analytical sensitivity, highlighting the need to frequently prepare the ligand. The thermodynamics of ligand and complex adsorption onto the working electrode (hanging mercury drop electrode, HMDE) was subsequently studied: both showed a strong adsorption onto the mercury surface (βDHN = 2.5·103 ± 5·102 L mol−1; βFe-DHN = 5.5·105 ± 8·104 L mol−1), but no competition for the mercury surface between the ligand and the complex was evident as determined by the multicomponent Langmuir isotherm. The mechanism of the electrode reaction was also investigated with and without the catalytic enhancement of the signal caused by air oxygen. The reduction of the complex Fe-DHN in purged solution showed α = 0.57 and k0 = 79 cm s−1, highlighting a quasireversible mechanism. The apparent catalytic constant (k'cat) was 168 s−1 for 1 mL sample volume: the simultaneous study of the kinetic and catalytic constant showed that the current signal was mostly influenced by the kinetic of the reaction.