Differential Pulse Polarography for a First‐Order EC Process and Its Diagnostic Parameters

Theoretical expressions for differential pulse polarography (DPP) for a reversible electron transfer coupled with an irreversible follow‐up first‐order chemical reaction (E_rC_i) is derived approximately. The peaks as given by the current expressions are analyzed in terms of several parameters such as a ratio of anodic‐to‐cathodic peak‐currents (i_p^a/i_p^c), a separation of peak‐potentials (E_p^c?E_p^a), and a ratio of anodic‐to‐cathodic half‐peak‐widths (W_1/2^a/W_1/2^c) in order to characterize the E_rC_i process and distinguish it from other types of electrode processes. The anodic peak is found to be more susceptible to the post kinetics than the cathodic peak. The new parameter of W_1/2^a/W_1/2^c ratio is much more sensitive to the post kinetics than the peak separation (E_p^c?E_p^a). The peak current ratio (i_p^a/i_p^c) and the peak‐width ratio (W_1/2^a/W_1/2^c) have comparable sensitivities to the kinetics. Hence, W_1/2^a/W_1/2^c ratio is a better diagnostic parameters than (E_p^c?E_p^a) which has a poor sensitivity. This phenomenon is different from cyclic voltammetry (CV) in which E_p^c?E_p^a is as sensitive as i_p^a/i_p^c. The new criteria for EC with DPV is tested and successfully applied to several Co(III) complex systems, including coenzyme B₁₂. The homogeneous rate constant (k) for the follow‐up step is estimated from the measurements of the experimental values of the parameters. The present treatment is valid quantitatively at lower values of k, yielding relatively larger errors for higher k values (k>10 s^?1).