Faradaic depolarization in the electrokinetics of the metal-electrolyte solution interface

Streaming potentials (E-str) have been measured in a flat thin-layer cell with gold and aluminum surfaces. The conventional relation between E-str and the zeta-potential is shown to be applicable only as long as charge transfer reactions at the metal-electrolyte solution interface are insignificant in terms of the ensuing contribution to the overall cell conductivity. Owing to the irreversibility of the reduction/oxidation of water at most metal surfaces, streaming potentials can be obtained over a very broad range of pressure gradients for metallic substrates in electrolytes such as KNO3. The situation changes drastically in the presence of a reversible redox couple like Fe(CN)(6)(3-)/Fe(CN)(6)(4-). Even small streaming potentials are then greatly diminished due to the extensive conduction that results from the bipolar electrolysis at the metal surface. For gold and aluminum in the presence of various electroinactive and electroactive electrolytes, the measured values for E-str are shown to be consistent with their conventional voltammetric characteristics. (C) 2003 Elsevier Science (USA). All rights reserved.