Effect Of Proton Diffusion On Voltammetric Behavior Of Hydroquinone

The electrochemical behavior of hydroquinone in unbuffered media was investigated by cyclic voltammetry.In the cathode scan after anodization,a new cathode peak appears at a more negative potential.In addition,similar new cathode peaks can also be observed for catechol.With the pH of the bulk solution,the concentration of hydroquinone,the maximum scanning potential increases,and the magnitude of the new cathode peak also increases.During the oxidation of hydroquinone,the pH of the solution near the electrode surface in the non-buffered solution decreased,and hydroquinone and p-benzoquinone were stable in acidic solution.Therefore,the generation of this new peak was not assigned to a new species.It was observed that when p-benzoquinone was added to the solution only the new cathode peak height increased while the other peaks remained constant.We propose the formation mechanism of the new cathode peak: Since the diffusion rate of protons in aqueous solution is much faster than that of other reactants by four orders of magnitude,a significant pH gradient is formed on the surface of the electrode when hydroquinone is oxidized in an unbuffered solution.Accelerates the diffusion of the protons released by hydroquinone into the bulk solution.In the reduction of p-benzoquinone,the proton concentration of the reaction layer is less than twice that of p-benzoquinone,so that the formation of a new cathode peak is the same as the cathode potential of p-benzoquinone in an unbuffered aqueous solution.This conclusion will help us understand the specificity of the voltammetric behavior of hydroquinone in unbuffered media.The diffusion rate of protons is dependent on the solvents.The diffusion rate of protons in water is greater than that of ethanol,and the diffusion rate in ethanol is much greater than that of acetonitrile and DMF.The formation of the new cathode peak is due to the ultrafast diffusion of protons near the electrode surface,adding hydroquinone to a mixed solvents,increasing the concentration of the solvent with a relatively fast proton diffusion rate,and a new cathode peak can not be observed while the magnitude of the original cathode peak increased.In acetonitrile solution,different concentrations ofinorganic acid perchloric acid and proline valine were added.Acetonitrile was an aprotic solvent and acid was a protic solvent.Increasing the acid concentration also increased the original cathode peak.In addition,the internal structure of the molecule also influences the diffusion of protons in the solution and thus affects the new cathode peak.The conclusions help us to understandof the effective of proton diffusion on the electrochemical behavior of proton-coupled electron transfer reactions in non-buffered solutions.