The impact of electrodic adsorption on Zn, Cd and Pb speciation measurements with AGNES

Abstract

The systems assayed in this work (Cd with iodide, Cd with polyacrylic acid, Pb or Zn with xylenol orange, etc.) as well as others previously studied in our lab (Pb/Cd/Zn with humic acids, Pb with pyridinedicarboxylic acid, Pb with organic extracts from river waters, etc.), exhibit induced adsorption of the metallic electroactive species on the mercury electrode. This fact, however, does not hinder the possibility of a correct determination of the free metal concentration in the solution with AGNES (Absence of Gradients and Nernstian Equilibrium Stripping). The equilibrium state (the target situation aimed at the end of the deposition stage of AGNES) is not disturbed by induced adsorption, while different strategies (e.g. the use of charge as stripping analytical signal) can avoid a possible kinetic interference on the quantification stage. The impact of adsorption on the blanks proves to be very small and, if needed, it could be further minimized by choosing a sufficiently high gain or variants of AGNES (such as AGNELSV or AGNES-SCP) that do not require the explicit subtraction of the blank. Some surfactants in solution, even without complexing the metal, might block the electrode surface, so that refined methodologies have to be used in such systems. In the case of the dispersant accompanying Nanotek ZnO nanoparticles, the complete blocking of the electrode can be reached in around 100 s, and, thus, electrodes with renovating drops are key for dealing with such kind of difficulties. The free Zn concentration in equilibrium with these nanoparticles determined by AGNES (with minimization of the deposition times and suppression of any stirring) is consistent with the one expected according to its primary particle size.

Experiments with PAA were conducted with the help of Antoni Ortín and Anna Sedó, whose contribution is gratefully acknowledged. The authors gratefully acknowledge support of this research by the Spanish Ministry Spanish Ministry of Education and Science (Projects CTQ2009-07831 and CTM2012-39183), by the “Comissionat d’Universitats i Recerca de la Generalitat de Catalunya” and by the European Union Seventh Framework Programme (FP7-NMP.2012.1.3-3) under Grant Agreement No. 310584 (NANoREG). R. Wallace, N. Hondow, A.P. Brown, S.J. Milne and R. Brydson (Institute for Materials Research, University of Leeds, UK) are gratefully acknowledged for providing a characterized sample of the Nanotek ZnO nanoparticles (EN-Z-1) with funding from the European Union Seventh Framework Programme FP7-NMP-2008-1.3-2, grant agreement 229244 (ENNSATOX).