Chemistry Reference
In-Depth Information
the electron transfer, a reaction step that cannot be neglected in Equation
2.29. For reversible systems, these parameters do not appear in the equa-
tion, because for reversible systems the kinetics of electron transfer are so
high (even at very small overpotentials) that they do not control the exper-
imentally obtained current signal.
Values for a and
n
a
can be obtained in two alternative ways. First, by
studying the difference between peak potential and half-wave potential:
EE
-
-
1 857
.
RF
Tn
a
[2.30]
p
12
Second, by performing two identical experiments, except for the scan rate
v
.From the shift in peak potential obtained in the experiments with scan
rates
v
1
and
v
2
, the kinetic parameters can be determined:
12
(
)
EE
-=
RF
Tn vv
a
ln
[2.31]
p
,
2
p
,
1
1
2
This shows that for an irreversible process, the peak potential is shifted
towards more negative (reduction reaction) or more positive (oxidation
reaction) potentials by about 0.03 V per decade of increase in the scan rate.
For a totally irreversible reaction, no return peak is observed due to the
fact that the kinetics are so slow that the opposite reaction cannot occur.
The activation energy, overcome by application of a potential, is so high
that it is not possible to apply such a potential under experimental condi-
tions. However, the absence of a return peak does not necessarily imply
slow electron transfer, but can also be due to a fast following chemical
reaction.
Besides electrochemical reactions, cyclic voltammetry is also very useful
in studying electrochemical reactions that are preceded or followed by a
chemical reaction and if adsorption of species at the surface of the elec-
trode is involved
14-24
.A short discussion is given below
25-27
.
For a process in which a reversible electron-transfer reaction is preceded
by a chemical reaction (so-called CE mechanism), the shape of the voltam-
metric wave and the peak is dependent on the rate of that chemical
reaction:
¨
¨
An
OeR
+
-
[2.32]
•The chemical reaction is very slow: the experimental current is con-
trolled by kinetics because the formation of O is very small, thus not
giving the system the chance to build up a diffusion layer. In this case,
a broad wave rather than a peak is observed.
•The chemical reaction rate is very fast: this implies that the formation
of O is very fast, much faster than its consumption in the following elec-
trochemical step. Therefore, the wave is that for a diffusion-controlled
electron transfer. In this case, the depletion of O in the vicinity of
the electrode surface is counter-balanced by formation of O in the
