On the interpretation of the potentiometric response of the inert solid electrodes in the monitoring of the oscillatory processes involving hydrogen peroxide

Abstract

Solid inert electrodes are frequently used in potentiometry. However, potentiometric responses may significantly depend on the inert electrode material, a fact which may manifest itself particularly distinctly for the dynamical chemical systems—oscillating processes. We found that for the homogeneous oscillators involving hydrogen peroxide and either thiocyanates or thiosulfates, the periodic variations of the platinum and palladium indicator electrode potential are both not in phase with the variations of the potential of the gold and glassy carbon electrodes, the latter two exhibiting in turn concordant, in-phase responses. Potentiometric responses were compared with the impedance characteristics of the electrodes during the oscillations. In spite of high mechanistic complexity of the studied homogeneous oscillatory systems, we explain different responses of inert electrodes in terms of the concept of the mixed electrode potential, i.e., determined by more than one redox couple of different kinetic characteristics (exchange current densities). In our model explanation, two coupled Ox1/Red1 and Ox2/Red2 redox systems are considered. It is suggested that for Au or glassy carbon electrodes, the mixed potential is largely determined by the Ox1/Red1 couple. For Pt or Pd electrodes, due to the catalytic effect of their surfaces on the Ox2/Red2 couple, its exchange current largely controls the measured mixed potential. Our concept is supported by numerical calculations involving the classical Brusselator as the model generator of chemical oscillations. The proper interpretation of potentiometric kinetic data is crucial for the diagnosis of the correct reaction mechanism.