Page 147 - Analytical Electrochemistry 2d Ed - Jospeh Wang

132 PRACTICAL CONSIDERATIONS

FIGURE 4-25 Normalized calculated concentration pro®les for disk electrodes with
different radii (r ), one second after start of a chronoamperometric experiment. (Reproduced
0
with permission from reference 84.)

conductor with an insulator [e.g. Kel-F=graphite (89)], by impregnation of a porous
conductor with an insulator [e.g., microcellular carbon=epoxy (90)], embedding
carbon ®bers in an insulating epoxy (91), or via deposition of a metal conductor into
the pores of a microporous host membrane (92).
As long as there is a negligible overlap of the diffusion layers from adjacent sites
(i.e., so long as each member maintains its own radial diffusional ®eld), the current
of a composite electrode is the sum of currents of the individual sites. At suf®ciently
long times, the diffusion layers overlap, and the electrode behaves as though its
entire geometric surface were active. For example, a slow scanning cyclic voltam-
metric experiment displays a current peak proportional to the total geometric area.
The exact time scale for the change from isolated to merged diffusion layers depends
on the spacing between the individual electrodes and their size. Larger distances and