Acid-Base Reactions Affecting the Limiting Current

Acid-Base Reactions Affecting the Limiting Current 

To affect the limiting current the acid-base equilibria must either precede the electrode process or occur between two electrode processes. In the former case the simplest system involves a monobasic acid (HA), the anion of which (A-) is protonated2 only by hydroxonium ions (18)  

2 The symbols HA, A- etc. represent only conjugate acid and base. The number of charges involved is not indicated. Hence HA can be a cation, neutral molecule, dipolar ion, or anion. 

There seems to be a general rule that in otherwise analogous molecules, the protonated cation is reduced at more positive potentials than the uncharged molecule and this again at more positive potentials than the corresponding anion. Consequently, it can be expected that the acid HA will be reduced at a more positive potential E than the potential E% at which the conjugate base is reduced. 

When Ei differs sufficiently from E%, then with the change in pH two wave appear on polarographic curves. Furthermore, if n = n%, the change in the wave-heights is similar to that shown in Fig. 2. The difference between the thermodynamic and activation-controlled systems is that in the region in which the two waves are observed for the equilibrium case, described in Chapter 2.1, both waves are diffusion-controlled, whereas for the activation-controlled system discussed here, the more positive wave at i 0.15 ia possesses a kinetic character. 

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