Homogeneous redox kinetics vs. interfacial electrochemistry: interpretation of the potentiometric characteristics of some oscillatory chemical reactions in terms of mixed electrode potential

Oscillatory chemical reactions in which the concentrations of intermediates vary in a periodic (or more complicated non-monotonic) manner as a function of time, exhibit complex kinetic mechanisms that are often studied using potentiometry. In this concise review a rarely discussed problem of interpretation of the electrode potential in potentiometric monitoring of the course of homogeneous oscillatory reactions, is described. Three processes of that type were selected for detailed discussion: the Belousov-Zhabotinsky, the Briggs-Rauscher and the Orbán (H2O2-SCN- - OH- - Cu2+) oscillators. While different responses of chemically inert (e.g., platinum) electrode and an ion-selective electrode are generally understandable, more sophisticated explanation is needed when different inert electrodes (e.g., Pt and Au) show distinctly different oscillation courses. In an extreme case of the Orbán oscillator, even the phase of the oscillations recorded with glassy carbon or gold electrode differs for ca. 180o from those monitored with platinum or palladium electrodes (antiphase behavior). In order to explain the response of the given inert electrode in a given reaction medium it is necessary to invoke the concept of mixed electrode potential, i.e. to consider the measured potential as positioned between the equilibrium potentials of all redox couples, with relative contributions dependent on their exchange current densities. Numerical modeling of kinetic oscillatory mechanisms combined with the calculations of the mixed potential allows to semiquantitatively reproduce the experimental potentiometric time series. In conclusion, unknown mechanisms of oscillatory chemical reactions should be deduced based on potentiometric measurements involving always more than one (even if inert) electrode and the interpretation of measurements requires the understanding of the physical source of the net potential drop at every electrode/solution interface.