10-17
Q
dl
is the charge due to charging of the working electrode, and Q
ads
is the charge due
to electrolysis of the adsorbed species, and is directly proportional to the surface
concentration of the adsorbate. Of the three components, only Q
diff
is time dependent.
Therefore, the intercept of the Anson plot is Q
dl
+ Q
ads
. One way of calculating Q
ads
is to measure Q
dl
by performing the CC experiment on the background solution.
However, this assumes that Q
dl
is the same in the presence and absence of the
electroactive species; this is not necessarily true. A more accurate method is to use
double step CC, since Q
dl
can be eliminated by calculating the difference between the
intercepts of the forward and reverse Anson plots (Figure 10-9).
Figure 10-9. Forward and reverse Anson plots.
References
1)
BAS Capsule, No. 133 Determination of Electrode Area with
Chronocoulometry
2)
BAS Capsule, No. 143 Determination of Diffusion Coefficients Using
Chrono coulometry
3)
W.R. Heineman, Curr. Seps 7 (1986) 58-67.
4)
BAS Capsule, No. 268 Measurement of Reaction Kinetics Using
Chronocoulometry.
5)
A. J. Bard and L. R. Faulkner "Electrochemical Methods" Wiley, New
York, 1980; p. 204-206
STEP allows a cyclic potential step wave form to be applied to the working
electrode. There is no data acquisition in this mode although the response can be
monitored at the I
OUT
rear panel jack of the BAS 100B. The STEP mode is primarily
designed for electrode cleaning and conditioning. The surface of several types of
electrodes (e.g., Pt, Au, etc.) have been found to be prepared more reproducibly by
applying such a potential wave form to them (the surface is cycled between oxide
formation and reduction) In addition, it is often possible to remove adsorbed species
with this method.
