Mapping concentration profiles within the diffusion layer of an electrode Part II. Potentiometric measurements with an ultramicroelectrode

A platinum-disk ultramicroelectrode is used to monitor electrochemical pote ntial variations inside the steady-state diffusion layer created by a large r electrode. The experimental potential variations result from combination of a pure electrochemical component (viz., as given by the Nernst law) with a smaller ohmic drop contribution, which varies linearly with the distance from the working electrode surface so that it can be readily eliminated. F rom the ensuing corrected potential variations, the concentration profiles of the electroactive species present within the diffusion layer can be reco nstructed by application of the Nernst law. The validity and great interest of the method are demonstrated experimentally by the study of the one-elec tron reversible oxidation of the Fe(CN)(6)(3-)/Fe(CN)(6)(4-) couple in aque ous KCI solutions. As a correlation, this demonstrates also for the first t ime the validity of the Nernst layer approximation. The method is then appl ied to examine the specific structure of the diffusion layers, which result from the involvement of a conproportionation reaction during the second re duction of tetracyanoquinodimethane (TCNQ) in DMF. (C) 2000 Elsevier Scienc e S.A. All rights reserved.