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

• AGNES determines free concentrations of Zn2+, Cd2+ and Pb2+ despite adsorption.
• The equilibrium between M and Mo (reduced metal) is not affected by other equilibria.
• Special strategy was needed for a strong surfactant stabilizing ZnO nanoparticles.

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.

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