Chemistry Reference
In-Depth Information
of the linear parts of these curves to 1/
I
w
1/2
= 0 results in a current controlled
by the kinetics of electron transfer. Plotting the logarithm of these currents
versus the applied potential results in the Tafel slope and a value for the
charge-transfer coefficient. A value of 0.45 ± 0.03 was obtained, which is in
agreement with the values obtained by the other methods.
Finally, the pH dependency of the current signals was investigated.
Voltammetric curves were recorded obtained at a platinum rotating-disc
electrode for different pH values in the 11.65-12.95 range at constant elec-
trode-rotation rate. These experiments were repeated at other sodium
dithionite concentrations. It was found that the measured current in all
three regions of the voltammetric waves did not vary with pH.
6.4.2
Determination of the mechanism of dithionite
oxidation reaction
Before considering and proposing mechanisms that consist of several ele-
mentary steps, some assumptions need to be made. From a statistical point
of view, two general rules were taken into account:
• No more than two particles react with each other in one elementary
reaction step at the same time.
•A maximum of one electron is exchanged in each elementary reaction
step.
Specifically for the oxidation of dithionite, three additional rules were
defined:
• No elementary reduction step(s) was included in the mechanism
because no experimental evidence was found for it.
• In the first voltammetric wave of the reaction (see Fig. 6.1), sodium
dithionite is oxidised to sulphite at a platinum electrode, and two elec-
trons are exchanged in this reaction (see section 6.2).
•The condition of the platinum electrode surface can play an important
role in the kinetics of electrochemical reactions, but for the kinetics of
dithionite oxidation it was found that the platinum surface condition did
not seriously interfere (see section 6.3).
Proposing a mechanism also necessitates knowledge of the compounds that
can participate in the overall reaction. For an electrochemical reaction at
the surface of an electrode, adsorbed species not present in the bulk of the
solution should also be taken into account as a possible (intermediate)
species. Species potentially present or formed in solution and/or at the
surface of the electrode in the global oxidation reaction of dithionite to sul-
phate are summarised in Table 6.3.
