Studies Using Organic Compounds as Depolarizers

Polarographic studies of organic compounds are very complicated. Many of the compounds behave as surfactants, most of them exhibit multiple-electron charge transfer, and very few are soluble in water. The measurement of the capacitance of the double layer, the cell resistance, and the impedance at the electrode/solution interface presents many difficulties. To examine the versatility of the FR polarographic technique, a few simple water-soluble compounds have been chosen for the study. The results obtained are somewhat exciting because the FR polarographic studies not only help in the elucidation of the mechanism of the reaction in different stages but also enable the determination of kinetic parameters for each step of reduction. 

The experimental cell, the measurement technique, and the method used for purification of the solutions are the same as given in Section IV (l(i)). The organic substances studied are vanillin, isatin, and 5-nitrobenzimidazole. Each of them will be dealt with in detail. 

The reduction of vanillin has been studied by the FR polarographic technique.107 The FR summit peaks are conspicuous up to 200-250 Hz at all pH s. With an increase in pH, the FR summit peak potential tends to be more cathodic. The FR wave is invariably obtained on the negative side of the abscissa, indicating that a is less than 0.5. The FR polarograms at pH 10.5 are shown in Fig. 31. 

The reduction of vanillin is accompanied by a two-electron charge transfer and the mechanism of the reaction can be explained as follows  

The values of a and the rate constant k°a at varying pH s have been determined107 and are given in Table 9. The values of a are generally found to decrease with increasing pH and lie between 0.23 and 0.34. The rate constant decreases with an increase in pH except in a neutral medium and it ranges from 3.63 x 10-2 cm/s to 2.42 x 10-2 cm/s. 

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