Chemistry Reference
In-Depth Information
35
30
3
2
25
y
= 9513.1x + 0.0099
R
2
= 0.9995
1
20
y
= 8916.8x + 0.0069
R
2
= 0.9992
15
y
= 7718.5x - 0.0383
R
2
= 0.9994
10
5
0
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
-5
Concentration (mol l
-1
)
7.9
Calibration plot between peak current of the sodium dithionite
oxidation reaction and its concentration at a bare gold electrode
(curve 1) and at a CoTSPc-modified (curve 2) and CoTSPor-
modified (curve 3) gold electrode.
T
=
298.0 K.
However, repetition of the concentration studies demonstrated that this
current is highly reproducible for the modified electrodes, which is not the
case for the unmodified ones. This explains why the error margins (Fig. 7.9)
for unmodified electrodes were relatively high,
particularly for low
concentrations.
The large difference in error margin observed at the bare gold and mod-
ified gold electrode, also has consequences on the detection limit. Taking
into account the criterion that the detection limit corresponds to twice the
standard deviation results in a detection limit of about 2 ¥ 10
-4
mol l
-1
for the
unmodified gold electrode, which was also found in the literature
10
,and
8 ¥ 10
-6
mol l
-1
for the modified electrode. As mentioned above, this is
mainly due to a much more stable and reproducible background current.
From Fig. 7.9, it can also be seen that the sensitivity of the detection of
sodium dithionite has improved on the modified gold electrode because a
higher slope is obtained for the calibration curve. Finally, it can be observed
from the curves in Fig. 7.7 that the second oxidation peak of dithionite (due
to oxidation of sulphite to sulphate
7,8,10,12,13
) does not shift to less-positive
potentials at modified gold electrodes. In other words, this reaction is not
electrocatalysed. This means that an improved peak separation will be
obtained at the modified electrode, or a higher selectivity for dithionite will
be obtained.
