Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

6 ELECTROCHEMICAL METHODS 157tant step was the transfer of H+ to the alcohol. The reaction is also significant inbeing the first very fast dissociation reaction of a strong acid whose kinetics havebeen measured.6. Electrochemical methodsIn suitable cases one of this family of techniques may be used to follow fastreactions in solution. They may be appropriate if one of the reactants is electrochemicallyreduced under conditions where the others are not. In the normal electrolyticreduction process a potential difference is applied between two electrodesplaced in the conducting solution, resulting in the migration of ions towards theelectrodes and the discharge at the cathode of that species which is most easilyreduced-i.e.,X+e- -+ A (in this section, the electroreduced species will be giventhe symbol X and other species will be represented by A, B, . . .). Because of thedischarge of X at the electrode a concentration gradient is set up between the bulkof the solution and the surface of the cathode. The current flowing is proportionalto the rate at which X is reduced. This can be expressed mathematically in terms ofmeasurable quantities-an exercise which is greatly simplified if certain conditionsare obeyed. In the first place, the species X, which may be an ion, a molecule or afree radical, must be discharged “immediately” on arriving at the cathode; in otherwords, the discharge process should not be the rate-limiting step. Secondly, it is ahelp if the cathode is of a shape whose surface area may be simply expressed mathematically-e.g.,a sphere or an “infinite” plane. The third simplifying conditionis that the mass transfer from the bulk of the solution to the electrode surface beof one or, at most, two types. Three types may be considered: diffusion down aconcentration gradient, general convectional motion and movement down or up apotential gradient. The latter is no problem for uncharged X, but it can also beeffectively eliminated in the case of ions by using a large excess of an “inert” electrolytesuch as potassium chloride to carry the current in the bulk of the solution.These “inert” ions are not discharged at the electrodes until most of the specieswith smaller electrode potentid has been discharged. Typically, the conditions arechosen so that the limiting physical process in the electrolysis is diffusion down theconcentration gradient. A current which is characteristic of the system is measured,called the “diffusion current”.Now, suppose that the species which is discharged (the depolarizer) is involvedin an equilibrium of the typeX+BsC (7)of which the equilibrium constant is considerably greater than unity. Again a concentrationgradient is set up but, if the dissociation of C into X and B is slowerReferences pp. 176-1 79