POTENTIAL SHIFTS AT ELECTRODES COATED WITH ION-EXCHANGE POLYMERIC FILMS

Voltammetry at electrodes modified with ion-exchange polymers, named ' 'ion exchange voltammetry'', has been recently developed for character izing and determining quantitatively ionic electroactive analytes prec oncentrated at the electrode surface. Like lor other voltammetric tech niques, characterization is based on the position of the response on t he potential scale, but an appreciable difference is frequently observ ed between the formal half-wave potential for redox couples incorporat ed within ion-exchange polymeric films and those for the same redox co uples in solution as measured at bare electrodes. Such a difference ha s been rationalized here by a generalized equation, inferred from a su itable elaboration of the Nernst equation, whose validity has been tes ted by a thorough investigation performed at glassy carbon electrodes modified with either cationic (Nafion) or anionic (Tosflex) polymeric films. With this purpose, the effect of both charge and concentration of the analyte and of the loading counterion, this last introduced as the cation or anion of the supporting electrolyte, of the ion-exchange selectivity coefficients of the redox partners and of their stoichiom etric coefficients, as well as of the number of electrons involved in the charge transfer has been evaluated. The results obtained agree qui te well with theoretical expectations and indicate that the potential shifts found are mainly conditioned by both charge and concentration o f the counterion from the supporting electrolyte and by the ratio of t he ion-exchange equilibrium constants for the two redox partners invol ved. Other parameters considered have no influence on the potential sh ift or lead to negligible effects, provided that the quantities of the redox partners incorporated within the ion-exchange coating represent s less than 5% of the film capacity. Again in agreement with theoretic al expectations, positive shifts are found for increasing supporting e lectrolyte concentrations when cationic redox species incorporated wit hin cationic films are involved, while the opposite effect is found fo r anionic redox species incorporated within anionic films.