A.C. polarography and faradaic impedance of strongly adsorbed electroactive species

The expression of the faradaic impedance is calculated in the case of a quasi-reversible system O+ne R under the following conditions: (a) both the oxidized and the reduced forms are strongly adsorbed; (b) the adsorption rate is large, and does not control the kinetics of the system; (c) the adsorption of both O and R obeys a Langmuir isotherm. The results show that the tangent of the phase angle is proportional to κs/ω (κs=rate constant of the electrochemical reaction). When ω→o, the phase angle tends towards 90°: the faradaic impedance becomes purely capacitive. The equation of the a.c. polarogram has been derived; whatever κs/ω, the peak height is proportional to the bulk concentration of the reactant, to τ7/6 (τ=drop time), and to h−1/2 (h=height of the mercury reservoir). When κs/ω→∞, the shape of the a.c. wave is identical to that of a "diffusion-controlled" a.c. wave. The experimental results obtained by a.c. polarography for the three systems azo-hydrazobenzene, benzo(c)cinnoline-dihydrobenzo(c)cinnoline and phenazine-dihydrophenazine are in excellent agreement with the theoretical predictions.