Basic electrochemical reactions of dithionite and sulphite

A necessary pre-requisite to starting the development of a sensor system is knowledge of the reactions that occur at the surface of the sensor electrode and result in the response signal delivered by the sensor system. In this section, the pathway of how the sensor reaction is found and studied is described. 

In a first set of experiments, the voltammetric behaviour of sodium dithionite was investigated in alkaline solution (pH around 12.5), by variation of the rotation rate of the platinum-disc electrode for different concentrations of sodium dithionite. In Fig. 6.1, current-potential curves are shown, obtained at different rotation rates of the electrode in a solution with constant sodium dithionite concentration. Two anodic waves are observed. In principle, sodium dithionite is the only electroactive species in solution, therefore it is supposed that both well-separated waves can be attributed to the oxidation of sodium dithionite with formation of a relatively stable intermediate product in the first wave. 

1 Current-potential curves recorded in a solution containing 5.747 x 10 3moll 1 sodium dithionite at pH = 12.5 (NaOH) at a platinum rotating-disc electrode with rotation rates of (1) 100, (2) 400, (3) 900, (4) 1600, (5) 2500, (6) 3600 and (7) 4900rpm 7=298K. 

Assuming that the first oxidation wave in Fig. 6.1 corresponds to the oxidation of sodium dithionite to sulphite, two possible oxidation reactions can be considered in alkaline solution  

Other particles like S2062 and S4062- are not considered, because in the literature no evidence can be found of them being stable in alkaline solution31. In the second oxidation wave, sulphite is further oxidised according to  

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