Chemistry Reference
In-Depth Information
In the backward-sweep direction, a limiting-current plateau for dithion-
ite is obtained, with limiting-current values that are almost equal to those
obtained in the forward-sweep direction (second oxidation wave of the first
scan, first wave of the other scans). This again is proof that the electron
transfer of dithionite at PtOH and PtO is fast compared with the rate of
transport. In addition, for the first wave (second and following scans) of
dithionite, a hysteresis effect is observed. At a potential of about 0.25 V vs.
Ag|AgCl, the reduction of PtO to PtOH starts to occur. Owing to the simul-
taneously occurring rearrangement of PtOH to OHPt, a dissimilar surface
condition is present compared with the PtOH condition prevailing in the
forward-scan direction. Slow kinetics for the oxidation of dithionite at
rearranged platinum hydroxide can possibly explain the kinetically con-
trolled decrease of dithionite signal leading to the observed hysteresis
effect.
From what is described in this section, it can be concluded that the kinet-
ics of the oxidation reaction of sulphite and dithionite at a platinum elec-
trode in alkaline solution are strongly affected by the nature of the platinum
surface. This is important when a platinum electrode is used for a quanti-
tative investigation of the kinetics of the oxidation of sodium dithionite
and/or sulphite or as electrode material in the development of a sensor for
the measurement and/or control of dithionite and/or sulphite concentra-
tions. However, for sodium dithionite, it has no serious consequences
because the limiting-current at 0.45 V vs. SCE does not change as a func-
tion of scan number. However, this oxidation is still irreversible (no return
peak observed) which means that, in the onset of the voltammetric wave,
the current is controlled by charge-transfer kinetics. Therefore, it is possi-
ble to investigate and obtain the mechanism of the oxidation of sodium
dithionite, which is explained in the next section.
6.4
Mechanism of the charge-transfer kinetics of
dithionite oxidation
As mentioned in the previous section, the onset of the first oxidation wave
of sodium dithionite is determined by kinetic parameters, and therefore it
is possible to study the reaction rate and the mechanism of this oxidation
reaction. In practice, a hypothesis will be formulated and compared with
the experimental evidence. As long as there is no contradiction between the
experimental evidence and the results predicted by the hypothesis, the
defined hypothesis remains valid.
6.4.1
Experimental evidence
In Fig. 6.1, current-potential curves are shown of the oxidation of sodium
dithionite at a platinum rotating-disc electrode in alkaline solution for dif-
