Other types of polarographic wave

Some depolarizers are observed to produce waves which are smaller than those predicted by the Ilkovic equation and whose heights show no variation with h. 

These waves are controlled by the rate of some chemical reaction preceding the electron transfer. A good example of such a system is provided by the behaviour of formaldehyde. This compound exists in aqueous solution largely as the hydrate which is electro-inactive and produces no reduction wave. The anhydrous molecule, which is reducible is formed from the hydrate only slowly. The overall reaction may be represented by 

Sometimes, currents may be found which are many times larger than would be expected for a diffusion-controlled process. These are found when a species is reduced at the dropping electrode to form a product which rapidly reacts chemically with some other species in the solution to regenerate the original depolarizer. Such behaviour is shown by ferric species in the presence of hydrogen peroxide, the reaction scheme being simply 

A further type of wave, whose height is a direct linear function of h, is characteristic of adsorption processes at the dropping mercury electrode. An adsorption wave is always associated with a larger wave and occurs either just prior to or just after the latter (Fig. 8.16). So-called pre-waves are caused by 

Superposition upon normal waves of pre- and post-adsorption waves. 

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You have already learnt that the height of a polarographic wave can be limited or controlled by the rate at which the analyte diffuses to the electrode. In this Part you will learn how to identify other types of polarographic waves and understand their role. These new types of polarographic current should generally be seen as potential interferences to be identified and avoided. 

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