Chemistry Reference
In-Depth Information
2500
I
(
m
A)
5-7
4
2000
3
2
1500
1000
1
500
0
-2.0
-1.5
-1.0
-0.5
0
0.5
1.0 1.5
E
(V) vs. SCE
-500
1
3
4-7
-1000
-1500
4.5
Current-potential curves of hydrogen peroxide with a glassy-
carbon electrode at pH
=
11.72 and
T
=
298.0 K, with rotation rates
of (1) 100, (2) 400, (3) 900, (4) 1600, (5) 2500, (6) 3600 and (7) 4900
revolutions per minute;
C
per
(hydrogen peroxide concentration)
=
0.11 mol l
-
1
.
Apparently, the current in the ascending part of the wave and the pseudo-
limiting current are mostly determined by transport-independent, kinetic
factors. The limiting current of the second wave, obtained after correction
for the IR voltage drop, satisfies the Levich relation (Chapter 1, Equation
1.15) and is hence determined by the transport rate of hydrogen peroxide
to the electrode surface. This wave will not be further discussed since it is
of no use for the aim of this investigation.
If one wishes to verify whether the prewave can form the basis for an
amperometric sensor, one would preferably dispose of as much informa-
tion as possible concerning the nature and the properties of this wave. An
obvious technique for diagnosis is cyclic voltammetry. Hydrogen peroxide
can be oxidised as well as reduced at glassy-carbon electrodes; however, the
potential ranges within which the reactions occur are situated relatively
distant from each other, as can be seen in Fig. 4.3 and Fig. 4.5.
When varying the hydrogen peroxide concentration, pH and polarisation
rate (
v
) in a broad range, in no case could a reduction reaction connected
with the oxidation wave be detected in cyclic voltammograms. With a lower
polarisation rate, the same S-shaped curve was described in the opposite
potential variation as in the departing potential variation. With higher
v
-
values, hysteresis effects were observed; however, this did not provide any
relevant information. This behaviour strongly reduces the diagnostic pos-
sibilities. The effectiveness of cyclic voltammetry as a diagnostic instrument
depends considerably on the voltammetric signal obtained in the opposite
potential variation, because this signal contains information about the reac-
tion product that has arisen from the forward potential variation.
