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between -0.5 and -0.8 V vs. Ag|AgCl of the curves shown in Fig. 6.6, which
indicates a different condition of the platinum surface, possibly due to the
presence of an oxide or hydroxide layer. Arvia
et al
.
39
also suggested that
some oxide and/or hydroxide coverage remains at the surface of platinum
and gold during the cycling procedure, with vertex potentials located in the
region of the hydrogen- and oxygen-evolution reaction.
It can be concluded from the curves in Fig. 6.6 that a reproducible plat-
inum surface is obtained after three consecutive scans, which allows use of
the electrode for the study of the oxidation reactions of sodium dithionite
and sulphite. However, before going into quantitative measurements, first
a qualitative analysis was performed for the oxidation of sulphite and
dithionite.
6.3.2
Oxidation of sulphite
In section 6.3.1, it was shown that for the second oxidation wave of sodium
dithionite (sulphite oxidation to sulphate), an influence of electron-
transfer rate (kinetics) is observed that cannot be neglected. Therefore, first
the oxidation of sulphite is studied and described, followed by the oxida-
tion reaction of sodium dithionite.
In Fig. 6.2, five successively recorded current-potential curves are shown,
obtained at a platinum rotating-disc electrode (
N
= 1000 rpm) in a 7.9 ¥
10
-3
mol l
-1
Na
2
SO
3
solution.The platinum surface was previously pre-treated
at -0.5 V vs.Ag|AgCl for 5 min in the electrolyte solution (0.6 mol l
-1
NaClO
4
,
pH = 12.5), and the following observations were made. First, it is clear that
the oxidation reaction of sulphite occurs in the potential region where the
oxidation of chemisorbed OH
-
to platinum oxide proceeds (see Fig. 6.5 and
Fig. 6.6). The fact that sulphite is not oxidised at platinum or at platinum
covered with chemisorbed OH
-
indicates that the electron transfer between
sulphite and electrode surface is determined by a PtO-SO
3
interaction and
not by PtOH-SO
3
, PtOH-OSO
2
, Pt-SO
3
, Pt-OSO
2
or HOPt-SO
3
and
HOPt-OSO
2
. Second, the oxidation reaction of sulphite appears at initial
potentials somewhat less positive in the first scan compared with the current
signals of the other scans, but the slope of the transient is smaller than the
one for the subsequent scans. This is an indication of slower oxidation kinet-
ics in the first scan. It can also be seen that the limiting-current plateau of the
first scan is not as flat as it should be for a transport-controlled reaction. This
also indicates that even in the limiting-current plateau, some kinetic influ-
ence is still present, despite a relatively high overpotential.
These slow kinetics can be explained by the following hypothesis. In the
first scan, it is presumed that the relative amount of PtO sites present (nec-
essary for sulphite oxidation) is small compared with the Pt and PtOH sites
(not favourable for sulphite oxidation) owing to the pre-treatment of the
