Solid-state cell for the voltammetric determination of trace electroactive ionic species preconcentrated from high-resistive media at electrodes modified by ion-exchange coatings

Abstract

A solid-state cell is described that allows the extension of ion-exchange voltammetry to the trace determination of small ionic species characterized by low ion-exchange selectivity coefficients, provided that they are dissolved in electrolyte-free media. It consists of a wetted ion-exchange membrane (Nafion), one face of which contacts both the counter and reference electrode, while the other surface is contacted by the electrode modified with the ion-exchange polymeric film used in the preconcentration step. Such a solid-state cell permits voltammetric assays in low-conductive samples of ionic analytes accumulated in the ion-exchange film during the preliminary collection step because when the electrode is held at appropriate potentials they are reduced or oxidized, coupled with an ionic migration through the membrane to maintain electroneutrality. Concurrently, the water that is wetting the Nation membrane reacts at the auxiliary electrode, thus restoring the ionic content of this solid polymer electrolyte interposed between the working and counter electrodes. This device obviates the need to transfer the modified electrode, after preconcentration from low-conductive solutions, into a medium containing an added supporting electrolyte to perform the voltammetric assay of the accumulated analytes, thus avoiding their extensive release owing to the occurrence of significant ion-exchange competition. A suitable reference electrode is adopted which minimizes ion contamination of the solid-state cell. The performance of this device was checked on Pb 2+, Fe 3+, [Fe(CN) 6] 3− and I − ions preconcentrated from electrolyte-free aqueous solutions. Its application to the determination of Pb 2+ in mineral waters by ion-exchange voltammetry is also reported.