The impact of high Zn° concentrations on the application of AGNES to determine free Zn(II) concentration

AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) determination of free Zn(II) in a solution can be affected by the reaching of high Zn° concentrations inside the amalgam. At concentrations about the solubility limit of Zn° in mercury, the formation of dendrites and powders around the mercury surface can be seen with an optical microscope. At concentrations of Zn° quite below the solubility limit, an anomalous stripping current appears which increases with decreasing supporting electrolyte concentration. The current along the stripping time exhibits a convex shape, which is labelled here as “anomalous convex behaviour” (acb). The origin of acb is tentatively ascribed to different kinetic reasons (amongst which the electroneutrality limitation due to low ionic strength outstands), but more than one cause is necessary for a full account of the experimental observations. With various strategies, like monitoring the charge as response function or by application of lower gains, AGNES can successfully probe these high concentrations.