Iodine overpotential

This interpretation is tentative in the sense that it has not been proved directly. There is, however, indirect evidence that this kind of behavior is possible and even plausible, from the study of the oxidation of iodide in thin-layer cells. Tliere it was shown experimentally that the first layer of iodide ions adsorbed on the surface of platinum is not electroactive and iodine is formed from ions in solution, which presumably can be adsorbed on top of the first layer. It was also found that the first adsorbed layer can be oxidized at higher overpotentials, as proposed here for the h.e.r. 

Another example of the limitations of the data in Figure 17.14 is the inference that O2 should oxidize I and Br at pH 0. Further, the fact that CI2 rather than O2 is evolved when hydrochloric acid is electrolysed is a consequence of the high overpotential for O2 evolution at most surfaces (see worked example 17.3). Despite some limitations. Figure 17.14 does provide some useful information for example, the more powerful oxidizing properties of periodate and perbromate than of perchlorate when these species are being reduced to halate ions, and the more weakly oxidizing powers of iodate and iodine than of the other halates or halogens respectively. 

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