Kinetic limitations in oxidation of dithionite and sulphite

As described in the theoretical section of this book, an electrochemical reaction consists of different steps, and each of these steps (transport and/or charge-transfer steps) can be rate determining. In this section, it is explained why it is not possible to obtain a purely transport-controlled oxidation reaction for sulphite as outlined in the previous section. This is caused by the platinum electrode surface condition that has a large influence on the electron-transfer rate. Therefore, the electrochemical behaviour of the electrode surface itself is described first and limited to observations made during oxidation of dithionite and sulphite. 

the electrochemical behaviour of the substrate material, which is platinum, was investigated as a function of applied potential by cyclic voltammetry. For that, the potential was swept in positive direction from -0.5 V vs. AglAgCl to a first vertex potential of 0.9V vs. AglAgCl, followed by a scan in reversed direction to a second vertex potential of -0.8 V and back to -0.5 V vs. AglAgCl. The curves obtained in solutions of different pH values are shown in Fig. 6.5. Similarly shaped curves were obtained by variation of 

6 Six successively recorded current-potential curves at a platinum rotating-disc electrode with A/=16.67Hz and pH = 12.20, after pretreatment of the surface by polarisation at -0.5V vs. AglAgCI for 5 min. (Reprinted from Electrochemistry Communications, Vol 2, No 10, Gasana ef a/., Influence of changes of... pp 727-732, Copyright 2000, with permission from Elsevier.) 

It can be concluded from the curves in Fig. 6.6 that a reproducible platinum 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. 

In section6.3.1, 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 oxidation reaction of sodium dithionite. 

---