IMPEDANCE ANALYSIS OF THE REDUCTION OF PYRIMIDINE AT A DROPPING MERCURY-ELECTRODE .1. SEQUENTIAL MECHANISMS

The electrochemical reduction of the 3,4-N=C bond of pyrimidine was studied in the pH range 6.5-7.7 within which only one d.c. polarographic wave is observed. The rate constants were obtained as a function of potential both from polarography and from impedance analysis. Because the latter method provides data over a wider potential range, the existence of a sequential mechanism with more than one determining step could be shown. In fact, the potential dependence of the rate constants can be explained by a sequential CEE mechanism. However, the CEE mechanism cannot account for the more involved R(ct)-E curves. These curves and the influence of the pH on the rate constants indicate the presence of a parallel mechanism, in which species with different degrees of protonation are being reduced simultaneously.