Dissociation rates of weak acids using sinusoidal hydrodynamic modulated rotating disk electrode employing Koutecky-Levich equation

The hydrogen evolution reaction of HCOOH, CH₃COOH, and C₂H₅COOH solutions consists of two different reduction processes depending on the evaluated potential region; (1) independent reduction of RCOOH and (2) simultaneous reduction of RCOOH and H₂O. The reduction of each carboxylic acid generates an apparent convective diffusion-controlled limiting current. The first achievement of the present study is that by using a rotating disk electrode (RDE) and a sinusoidal hydrodynamic modulated-rotating disk electrode (SHM), it was elucidated that the additive property of the reduction currents of RCOOH and H₂O was not effective, and the convective-diffusion current was successfully distinguished from the total current. The second achievement is the successful analysis of the rotation-speed dependency of the limiting current in RDE and SHM using a modified theory of the Koutecky-Levich equation. The slopes of the plots for each carboxylic acid increased in the following sequence: RDE, SHM (p = 0.05), and SHM (p = 0.24), which is consistent with the theory. The dissociation rates of the carboxylic acids and the reverse recombination rate were calculated.