The pH Changes at the Electrode Surface Caused the New Voltammetric Waves in PCET Reactions

Abstract

Proton-coupled electron transfer (PCET) reactions are ubiquitous in chemical and biological processes. However, the role of pH has been a source of confusion, hindering our understanding of the PCET mechanism. In dilute buffered solutions, a new anodic hydroquinone wave depends on the proton acceptor, and two cathodic quinone waves also depend on the pH and the proton donor. New complexes corresponding to the new waves are ruled out because the UV/vis spectra of hydroquinone and quinone did not vary with the concentration of buffer solution at the same pH. The reaction rate of PCET is dependent on the pH, and it is suggested that the new waves are produced by the steep pH decrease or increase near the electrode surface. This conclusion made it easy to interpret the redox chemistry of quinone in unbuffered solution.