ANALYSIS OF THE FARADAIC ADMITTANCE FOR AN ECE MECHANISM IN THE CASE OF NON-RANDLES BEHAVIOR WITH FREQUENCY AND ITS APPLICATION TO NITROMETHANE REDUCTION

The analysis of admittance and impedance data in the case of a particular ECE mechanism in which a stable intermediate is assumed to undergo a unidirectional chemical reaction followed by an infinitely fast electron transfer is evaluated critically. It is shown that situations can prevail which might be misinterpreted as "pseudo-Randles" behaviour with frequency. To avoid this risk some procedures are proposed for obtaining the kinetic parameters from the frequency dependence of the impedance data, using the complete equation deduced previously for this mechanism. These procedures are applied to the reduction of nitromethane on mercury at two extreme pH values: at pH 9 where a "pseudo-Randles" circuit holds and at pH 1 where the "non-Randles" behaviour requires the application of the new procedures. In the latter conditions the more complex potential dependence of the impedance parameters contradicts the mechanism proposed previously on the basis of polarographic data.