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 profiles without any assumption even when diffusion coefficients differ significantly. The validity of the method is established experimentally through the study of the one-electron reversible oxidation of the Fe(CN)63-/Fe(CN)64- couple in aqueous KCl, under steady-state or under transient diffusion conditions. Under steady-state conditions, the results compare excellently with those we obtained by means of the potentiometric method reported in Part II of this series. Under transient diffusion conditions, the measured concentration profiles match perfectly those predicted for planar diffusion, which demonstrates the interest of the method for the analysis of dynamic diffusion-kinetic problems. The validity of the method in a complex diffusional situation is established as previously (Part II) by the investigation of the effect of a conproportionation reaction taking place during the second reduction of tetracyanoquinodimethane (TCNQ) in DMF.