Mapping concentration profiles within the diffusion layer of an electrode Part III. Steady-state and time-dependent profiles via amperometric measurements with an ultramicroelectrode probe

A platinum-disk ultramicroelectrode is used to monitor amperometrically the concentrations of the electroactive substrate and of its electrogenerated product(s) inside the diffusion layer created by a larger working electrode . This allows a direct monitoring of the target species concentration profi les without any assumption even when diffusion coefficients differ signific antly. The validity of the method is established experimentally through the study of the one-electron reversible oxidation of the Fe(CN)(6)(3-) /Fe(CN )(6)(4-) couple in aqueous KCl, under steady-state or under transient diffu sion conditions. Under steady-state conditions, the results compare excelle ntly with those we obtained by means of the potentiometric method reported in Part II of this series. Under transient diffusion conditions, the measur ed concentration profiles match perfectly those predicted for planar diffus ion, which demonstrates the interest of the method for the analysis of dyna mic diffusion-kinetic problems. The validity of the method in a complex dif fusional situation is established as previously (Part II) by the investigat ion of the effect of a conproportionation reaction taking place during the second reduction of tetracyanoquinodimethane (TCNQ) in DMF. (C) 2000 Elsevi er Science S.A. All rights reserved.