Separating boundary potential changes at thin solid contact ion transfer voltammetric membrane electrodes

Thin ion-selective membrane films deposited on solid electrode substrate are useful tools to study ion transfer processes. This is because the experimental conditions may be chosen such that diffusion processes within the membrane and contacting aqueous solution are not rate limiting. In an ideal case, therefore, equilibrium considerations may be used to describe the resulting ion transfer voltammograms. For example, the electrochemical oxidation of an electrically neutral redox molecule in the membrane results in a cationic oxidized form. To preserve electroneutrality, a cation is transferred out of the membrane into solution, freeing the cation-exchanger of the membrane to become the counterion of the oxidized redox molecule. This work describes a model system that agrees well with thermodynamic theory, using the lipophilic (1-dodecyl-1H-1,2,3-triazol-4-yl)ferrocene as redox molecule and a monovalent reference cation for ion transfer. The full peak width at half maximum was found as 0.110 V, in agreement with theory, and with peak current proportional to scan rate supporting thin layer behavior. The charge passed during the voltammetric scan was related to ion-exchanger concentration available for ion extraction as a function of potential. Subtraction of the ion transfer potential using the reference ion from the experimental one for each charge increment gave the potential change for the electrochemical ion-to-electron transducer. In one application, the potential change of the polymeric transducing layer poly(3-octylthiophene) (POT) film upon electrochemical oxidation within the membrane was characterized. A non-linear potential–charge curve was observed, in contrast to earlier assumptions.