Chemistry Reference
In-Depth Information
of SO
2
was detected. However, if a bare-carbon fibre is used, ill-defined
and poorly visible waves were obtained. This is a first indication that
[Fe(II)TSPc]
4-
electrocatalyses the observed reactions.
Two main waves (IV and V) can be observed in Fig. 12.9 and are attrib-
uted to oxidation (Equation 12.13) and reduction of sulphur dioxide, respec-
tively. (The latter reaction is discussed later.)
Æ
SO
+
2H O
SO
4
2
-
+
4H
+
+
2e
-
[12.13]
2
2
Besides these main waves, three other waves are observed in Fig. 12.9.
Waves I and II are attributed to oxidation of reaction products formed in
wave V. This was proved by cycling the potential between -0.3 and 0.6 V vs.
Ag|AgCl where waves I and II were absent. By addition of sodium dithion-
ite to the solution, waves I and II again appeared, indicating that the reac-
tion product formed in wave V is dithionite. Waves I and II were also
observed at a bare-carbon-fibre electrode after addition of sodium dithion-
ite. Therefore, it is not clear whether these reactions are electrocatalysed by
[Fe(II)TSPc]
4-
. However, when cycling between -0.3 and 0.6 V vs. Ag|AgCl,
wave III still occurred under these conditions, indicating that this wave can
be attributed to a species in solution.
Both waves IV and V in Fig. 12.9 correspond to diffusion-controlled reac-
tions, because a linear relationship is obtained between the peak current
and the square root of the scan rate. However, the slope of the relationship
between the peak current and SO
2
·
x
H
2
O concentration is slightly higher
for the reduction than for the oxidation (Fig. 12.10, curves 1 and 2, respec-
tively). A different number of electrons exchanged in both reactions cannot
explain the small peak-current differences between these waves. As pointed
out earlier, wave III (corresponding to curve 3 in Fig. 12.10) was attributed
to a species present in solution. Moreover, it can be seen that the sum of
the peak currents of waves III (curve 3) and IV (curve 2) is equal to the
peak current of wave V (curve 1 in Fig. 12.10). Therefore, it is presumed that
wave III corresponds to the oxidation of bisulphite (reaction 4), which is
present at about 1.5% of the analytical sulphur dioxide concentration
(Table 12.1). This wave is observed at similar potentials as those for the oxi-
dation of the Fe-metal ion in [Fe(II)TSPc]
4-
.Therefore, it is assumed that
the bisulphite oxidation is electrocatalysed by the central metal ion of the
[Fe(II)TSPc]
4-
:
Æ
HSO
-
+
H O
SO
4
2
-
+
3H
+
+
2e
-
[12.14]
3
2
However, an additional condition, that k
1
in Equation 12.11 is relatively
small, needs to be fulfilled. This is indeed the case and is confirmed by two
additional experiments. In a first experiment, the pH was varied from 0 to
2 in five steps. The peak height of wave III in Fig. 12.9 increased while the
one of wave IV decreased with increasing pH value, due to formation of
