Studies of Chemical Reactions at the Electrode Surface

Polarography is valuable not only for studies of reactions which take place in the bulk of the solution, but also for the determination of both equilibrium and rate constants of fast reactions that occur in the vicinity of the electrode. Nevertheless, the study of kinetics is practically restricted to the study of reversible reactions, whereas in bulk reactions irreversible processes can also be followed. The study of fast reactions is in principle a perturbation method the system is displaced from equilibrium by electrolysis and the re-establishment of equilibrium is followed. Methodologically, the approach is also different for rapidly established equilibria the shift of the half-wave potential is followed to obtain approximate information on the value of the equilibrium constant. The rate constants of reactions in the vicinity of the electrode surface can be determined for such reactions in which the re-establishment of the equilibria is fast and comparable with the drop-time (3 s) but not for extremely fast reactions. For the calculation, it is important to measure the value of the limiting current ( ) under conditions when the reestablishment of the equilibrium is not extremely fast, and to measure the diffusion current (id) under conditions when the chemical reaction is extremely fast finally, it is important to have access to a value of the equilibrium constant measured by an independent method. 

The principles of the treatment of such systems and the development of the mathematical treatment are described in textbooks (37—39) and reviews (40—43) accordingly, only the principles of some practical applications will be described here. 

Experimentally, the kinetic current (i ), i.e. the current under conditions when its limiting value is governed by the rate of a chemical reaction, is measured. To prove that the current is limited by the rate of 

In practice the current is measured under varying conditions. Because the rate of most of the reactions studied so far depends on pH, the current is measured in buffers of various composition and of controlled ionic strength and temperature. After the diffusion-governed value id has been determined, the ratio ik/ia or /( — ) is plotted as a function of pH. In some of the treatments mentioned below, it proved useful to determine the pH value at which the kinetic current attained half the value of the diffusion-controlled current. The numerical pH value at which t = taj2, which corresponds to the inflexion point of the ik/ia — pH plot, is denoted as the polarographic dissociation constant, pK . 

The reactions taking place in the vicinity of the electrode surface can be classified as acid-base reactions, hydration-dehydration equilibria, interactions of negatively charged species with metal cations, and ringopening reactions. Some of these reactions precede the electrode process proper, some are interposed between two electrode processes and some are consecutive to the electrochemical step. 

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