Polarographic Studies of pH Effects and Reaction Mechanisms

A further case in the above category arises when the protonation rate depends on pH and becomes slow under certain pH conditions. If both the acidic and basic forms of the organic substance in solution can be reduced, a common wave may arise in the pH range where slow protonation occurs. Since the total amount of the substance available for reduction is the same, independent of pH a the fraction in the form HA and 1 - a in the form 

A ) iiim will remain constant. In all these cases, protonation precedes electron transfer (H, e, e or e, e, H ). 

For a system in which an equilibrium protonation precedes electron transfer to the acidic form HA, the variation of 1/2 with pH near the pK for the protonation equilibrium (constant K) is given by 

In the case of a protonation-llmited rate, the ratio of currents due to Ha reduction and that of inm at low pH is given by 

Reduction of the tropylium ion investigated by Zuman et provides an example of the above type of behavior. In the case of a chemical step dependent on pH, e.g., hydration or acid-catalyzed rearrangement, the relation analogous to (90) above is 

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