Chemistry Reference
In-Depth Information
modified carbon fibre. Two waves can be detected: an oxidation wave
centered around 0.05 V vs. Ag|AgCl and a reduction wave with a peak
potential of -0.65 V vs. Ag|AgCl. Based on the results obtained above in
1.0 mol l
-1
H
2
SO
4
, the oxidation wave corresponds to electrocatalytic oxida-
tion of HSO
3
-
catalysed by Fe in [Fe(II)TSPc]
4-
, which is oxidised at the
same potentials. No wave for SO
2
·
x
H
2
O is observed, because SO
2
is trans-
formed almost totally into HSO
3
-
(Table 12.1) at this pH. A similar voltam-
mogram, as shown in Fig. 12.11, was obtained with sodium sulphite as
starting species. The oxidation wave corresponds to Equation 12.14 with
exchange of two electrons, and is proportional to the analytical concentra-
tion of sulphur dioxide and sulphite (Fig. 12.10, curve 4).
The reduction wave cannot be attributed to reduction of SO
2
·
x
H
2
O or
HSO
3
-
with exchange of two electrons. Its slope is twice as high as the one
for the oxidation (Fig. 12.10, curve 5). Therefore, this wave is assumed to be
the reduction of HSO
3
-
(because this is the only compound in solution) to
S
2
O
3
2-
(Equation 12.16) with exchange of four electrons, because S
2
O
3
2-
does not show electroactive properties over the entire potential region and
is a relatively stable species:
Æ
2HSO
-
+
2H
+
+
4e
-
S O
2
-
+
3H O
[12.16]
3
2
2
Owing to the higher slope obtained for the reduction, a lower detection
limit can be obtained. Detection limits of 3.8 ± 0.1 ¥ 10
-5
and 7.4 ± 0.1 ¥
10
-5
mol l
-1
were obtained for the reduction and oxidation of SO
2
·
x
H
2
O,
respectively, with HSO
3
-
as electroactive species.
12.3.6 Sulphur dioxide and sulphite at pH
=
8 and pH
=
10
buffer at carbon fibre
In a buffer of pH = 8, completely different results were obtained for sulphur
dioxide and sodium sulphite as starting species. Starting from sulphite, only
one oxidation wave is obtained around 0.7 V vs. Ag|AgCl. From Table 12.1
it can be seen that sulphite itself is the main compound in solution, there-
fore it is clear that this wave can be attributed to Equation 12.17. Its slope
(Fig. 12.10, curve 6), obtained by plotting the peak current versus concen-
tration, is situated in the range that allows exchange of two electrons:
Æ
SO
3
2
-
+
H O
SO
4
2
-
+
2H
+
+
2e
-
[12.17]
2
Starting from sulphur dioxide, only one wave around 0.7 V vs. Ag|AgCl is
observed at concentrations lower than 1 ¥ 10
-4
mol l
-1
. At concentrations
higher than this value, this wave disappears and two new oxidation waves
with
E
p
=-0.2 and +1.1 V vs. Ag|AgCl and a reduction wave are observed.
It is well known that SO
3
2-
reacts to form HS
2
O
5
-
in an excess of SO
2
24
.This
