Chemistry Reference
In-Depth Information
opposite sweep direction back to the initial potential. Curve 1, recorded in
the absence of Ce(IV), shows that over a potential range from about 0.2
to 0.4 V vs. Ag|AgCl, no electrochemical reaction of the gold surface is
observed. From a potential of about 0.4 V vs. Ag|AgCl, a voltammetric wave
with a relatively small limiting-current is observed (Fig. 12.13). According
to Burke and O'Sullivan
.
86
, this wave could be attributed to the oxidation
of Au adatoms (Au*) to Au(I). The slow kinetics of the equilibrium between
Au and Au* explain why the limiting-current is low. From a potential of 1.1
V vs. Ag|AgCl, a relatively broad peak is observed resulting from the for-
mation of Au(III). This peak is composed of several peaks, but is not con-
sidered further here because it is beyond the scope of this topic. At a
potential of 1.55 V vs. Ag|AgCl, the current starts to increase exponentially,
corresponding to the oxygen-evolution reaction.
In the cathodic sweep direction, a sharp reduction peak (
E
p
= 0.95 V vs.
Ag|AgCl) is observed, which can be attributed to the reduction of the
Au(III) formed in the anodic-inclined branch. Up to a potential of 0.2 V vs.
Ag|AgCl, again an electro-inactive region is observed. Finally, the onset of
the reduction reaction of H
+
is observed from a potential of 0.2 V vs.
Ag|AgCl. Curves 2-6 of Fig. 12.13, recorded with increasing amounts of
Ce(IV) present in solution, show a Ce(IV) reduction wave. The formation
of a limiting-current plateau indicates that in the 0.2-0.7 V vs. Ag|AgCl
potential region, transport of Ce(IV) determines the overall reaction rate.
This limiting-current is proportional to the Ce(IV) concentration. Curve
(6), recorded at the highest Ce(IV) concentration, clearly shows the pres-
ence of an hysteresis effect. Transport as rate-determining step in the 0.2-
0.7 V potential region was confirmed by carrying out the same experiment
at a gold rotating-disc electrode. The limiting Ce(IV) reduction current is
proportional to the square root of the rotation rate of the disc electrode as
expected from the Levich equation
87
. Cyclic voltammetry at a stationary-
disc electrode showed that the peak current is proportional to the square
root of the polarisation rate (Sevcik-Randles equation
87
).
The results obtained and presented in Fig. 12.14 show that chrono-
amperometry, with an applied potential situated in the limiting-current
plateau of the Ce(IV) reduction reaction, can possibly be used for on-line
measurement of the concentration of Ce(IV). In Fig. 12.14, the current signal
of Ce(IV) reduction is given as a function of concentration in a 1.0 mol l
-1
H
2
SO
4
solution at an applied potential of 0.25 V vs. Ag|AgCl and a flow rate
of 2.36 ml min
-1
. It can be seen that the current signal is relatively stable, and
not dependent on time. It shows a linear relationship with the Ce(IV) con-
centration. This experiment was repeated eight times. Taking into account
that the faradaic current
I
is superimposed on a time-dependent background
current
I
bg
, this resulted in the following empirical equation for the rela-
tionship between current and Ce(IV) concentration. In the electrode
