Chemistry Reference
In-Depth Information
200
6 7
180
4
5
1
160
140
2
120
100
80
60
40
3
20
0
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
Concentration (mol l
-1
)
12.10
Calibration plots of the oxidation and reduction of SO
2
and its
related compounds at a carbon-fibre electrode modified with
[Fe(II)TSPc]
4
. (1) SO
2
reduction in 1.0 mol l
-
1
H
2
SO
4
, (2) SO
2
oxidation in 1.0 mol l
-
1
H
2
SO
4
, (3) HSO
3
-
oxidation in 1.0 mol l
-
1
H
2
SO
4
, (4) HSO
3
-
oxidation in pH
=
4 buffer, (5) HSO
3
-
reduction
in pH
=
4 buffer, (6) SO
3
2
-
oxidation in pH
=
8 buffer and (7) SO
3
2
-
oxidation in pH
=
10 buffer.
HSO
3
-
with increasing pH. In a second experiment, a preconditioning of the
electrode at a potential of +0.6 V vs. Ag|AgCl was applied prior to cycling
from +0.6 to +1.3 V vs. Ag|AgCl. With increasing preconditioning time, the
peak height of wave IV decreased, indicating that during the precondi-
tioning SO
2
·
x
H
2
O is transformed (however slowly) to HSO
3
-
, which is in
turn oxidised at the preconditioning potential. Despite the fact that k
1
in
Equation 12.11 is relatively small, reaction of SO
2
·
x
H
2
O to HSO
3
-
still
occurs. Indeed, a much higher ratio of
I
p,III
/
I
p,IV
is obtained than was found
for the actual concentration ratio of [HSO
3
-
]/[SO
2
·
x
H
2
O] in solution (Table
12.1). This shows that, during the oxidation of HSO
3
-
, fresh HSO
3
-
is pro-
duced by Equation 12.11. This result also suggests that the peak current of
waves III and IV in Fig. 12.9 cannot be attributed to the actual concentra-
tions of bisulphite and sulphur dioxide in solution. However, the sum of
these peak currents is proportional to the analytical concentration of
sulphur dioxide and can therefore be used for analytical purposes.
Another possibility is to make use of the reduction wave V. No reduction
prewave of HSO
3
-
was observed, and since k
-1
in Equation 12.11 is high, it
can be concluded that HSO
3
-
is transformed rapidly into SO
2
·
x
H
2
O, during
