Electrochemical oxidation of gallic acid: A reexamination of the reaction mechanism in aqueous medium.

• Oxidation of gallic acid (GA) on gold electrode is examined in a broad pH range. • Oxidation mechanism of GA based on bi-cubic scheme is proposed. • pKa value of the final ortho-quinone product is experimentally determined. • Spectroelectrochemical studies identify the different intermediates and the products. • The kinetics of gallic acid redox process involves a disproportionation reaction. A clear understanding of redox mechanism of antioxidants, like gallic acid (GA) is important to decipher their radical scavenging functioning in the biochemical environment of the body. Herein, electrooxidation process of GA is reexamined in an aqueous medium through a combination of cyclic voltammetric and spectroelectrochemical studies on gold electrode in wide pH and concentration ranges. These studies reveal redox process of GA as an irreversible two electrons process, resulting in the formation of ortho-quinone, where the second electron transfer is the rate limiting step. The redox process of GA at different pH is presented in the form of a bicubic scheme and the associated thermodynamic parameters are analyzed using Laviron's theory. It allows for the first time the determination of the acidity constant of the corresponding ortho-quinone (pK a = 5.33). The number of exchanged electrons and protons depends on the pH conditions, such that (2e−, 2H+) are exchanged at pH < 4.24 and pH > 5.33, while (2e−, 1H+) are exchanged at 4.24 ≤ pH ≤ 5.33. A careful analysis using the theoretical treatment proposed by Amatore and Saveant for an ECE-DISP mechanism demonstrates the existence of a DISP2 mechanism leading to the formation of ortho-quinone through disproportionation of the semiquinone radicals, made possible thanks to their stabilization. [Display omitted] [ABSTRACT FROM AUTHOR]