Chemistry Reference
In-Depth Information
Table 2.1
Tabulated relationship of
n
(
E
-
E
1/2
) and
c
rev
12
for the relation between
the measured current,
I
, and the applied potential,
E
, in cyclic voltammetry for
a reversible system
n
(
E
-
E
1/2
) (mV)
c
rev
n
(
E
-
E
1/2
) (mV)
c
rev
5
0.335
-
5
0.400
10
0.328
-
10
0.418
15
0.298
-
15
0.432
20
0.269
-
20
0.441
25
0.240
-
25
0.445
30
0.211
-
30
0.446
35
0.185
-
35
0.443
40
0.160
-
40
0.438
45
0.138
-
45
0.429
50
0.117
-
50
0.421
60
0.084
-
60
0.399
80
0.042
-
80
0.353
100
0.020
-
100
0.315
120
0.009
-
120
0.280
It will be clear that cyclic voltammetry is a powerful tool for a first analy-
sis of an electrochemical reaction occurring at the surface of an electrode
because it will reveal reversibility. Depending on whether the system is
reversible, information will be obtained about half wave potential, number
of electrons exchanged in the reaction, the concentration and diffusion
coefficient of the electroactive species. However, these data can also be
obtained for an irreversible system
11,13
but, in this case, the equations
describing the current-potential curves differ somewhat from Equations
2.21 to 2.27.
For an irreversible system, the peak current may be expressed in terms
of a heterogeneous rate constant, k
h
, and the peak potential by the follow-
ing relation:
[
(
)
]
I
=
0 227
.
n
Fk
Ac
exp
-
a
n
FR
T E
-
E
¢
[2.28]
p
h
a
p
where k
h
is the heterogeneous rate constant of the concerned process on
the condition that the potential of the electrode is equal to the formal
potential of the electrode process
E
¢.
The equation that is valid for the peak current of irreversible redox
systems is derived from Equation 2.28 and is given in Equation 2.29 (at
298.0 K):
12
(
)
[2.29]
I
=
299
.
¥
10
5
n
a
n
A
D
12 12
v
c
p
a
where
n
a
is the number of electrons in the rate-determining step and a is
the transfer coefficient. These two parameters are related to the kinetics of
