Chemistry Reference
In-Depth Information
sulphur dioxide reduction. As was pointed out earlier, dithionite is supposed
to be the reaction product, therefore wave V is attributed to:
Æ
2SO
-
+
2e
-
S O
4
2
-
[12.15]
2
2
Based on the reduction of SO
2
·
x
H
2
O, a detection limit of 8.5 ± 0.1 ¥ 10
-5
mol l
-1
is obtained, which is lower than the one obtained by using waves III
and IV (1.2 ± 0.1 ¥ 10
-4
mol l
-1
). The determination of the detection limit was
based on the criterion that the detection limit corresponds to an electrode
signal of twice the background current. However, an advantage of using the
oxidation is that sulphate is formed as a reaction product instead of dithion-
ite. The latter species itself is electroactive and relatively unstable; decom-
position occurs easily into sulphite and related products. Dithionite can
therefore become an influencing species in continuous measurement or for
measurements in small-volume cells.
12.3.5 Sulphite and sulphur dioxide in pH
=
4 buffer at
a carbon fibre
In Fig. 12.11, current-potential curves are shown for different sulphur
dioxide concentrations, recorded in pH = 4 buffer at a [Fe(II)TSPc]
4-
-
I
(nA)
30
7
4
20
6
1-3
5
10
0
-1.3
-0.8
-0.3
0.2
0.7
1
-10
4
E
(V) vs. Ag
|
ACl
5
2-3
-20
-30
6
-40
-50
7
-60
12.11
Current-potential curves recorded in pH
=
4 buffer solution for
increasing SO
2
concentrations at a carbon-fibre electrode
modified with [Fe(II)TSPc]
4
-
. SO
2
concentrations are (1) 0, (2)
3.89
¥
10
-
5
, (3) 6.02
¥
10
-
5
, (4) 8.79
¥
10
-
5
, (5) 1.51
¥
10
-
4
, (6) 2.71
¥
10
-
4
and (7) 4.71
¥
10
-
4
mol l
-
1
;
v
=
100 mV s
-
1
.
