The electrode reaction sequence involving a homogeneous chemical reaction following a charge transfer step (EC mechanism) kf A±ne-~=~ B B+X ~ Z kb (I) (II) in which the electrode reaction product, B, decays via (pseudo) first-order kinetics to form Z which is electrochemically inert at the potentials where reaction (I) proceeds, is frequently encountered in theoretical and experimental studies t -z2. Recent theoretical contributions based on the integral equation method 23 and/or finite difference methods 24 have extended the theory of the EC mechanism to include reactions involving dimerization of the electrode reaction product and reaction (II) stoichiometries other than 1 : 1. Technique specific reviews are available which detail the application of a.c.'methods 2s, controlled potential coulometry in bulk solution 26, hydrodynamic electrodes/v, cyclic voltammetry zs, thin-layer electrochemistry 29'3°. chronopotentiometry at, and potentiostatic relaxation techniques 32 to the study of EC electrode mechanisms.
Schwarz and Shain 13 have described the chronoamperometric response for a double potential step experiment where the potential is stepped from some initial value, Ei, where no current flows to a potential. El, which causes the electrode reaction product. B, to be generated at a diffusion-controlled rate; after a given generation time, z, the potential is returned to Ei where the electrode reaction is reversed consuming B at a diffusion-controlled rate. For situations exhibiting the decay kinetics of reaction II with kb = 0, the current measured during the reverse step will be less than if B were stable indefinitely. It was shown that the ratio of the reversestep current measured at a time (t-r) to the forward step current measured at a corresponding time is an effective measure of the rate of decay of B and is independent of electrode area, bulk concentration of A, and the diffusion coefficients of all species. The theory for this type of experiment was verified using the "well studied" azobenzene-HC104 system 1°'1~'13'37. Double potential step chronoamperometric theory has recently been extended to include similar dimensionless current ratio expressions for dimerization 33 and disproportionation of B (ref. 34).
Christie 4 has proposed the use of double potential step chronocoulometry,