The Investigation And Application For Electrochemical Behavior Of Benzoquinone And Hydroquinone In Nonaqueous Solvent

The electrochemical behaviors of benzoquinone and hydroquinone under different conditions which regulate the potentials and reaction pathways, can provide much information on PCET reaction mechanism. Consequently, the electrochemistry of hydroquinone and quinone has been widely studied. Compared with that in aqueous, the electrochemical behavior of hydroquinone is markedly different from that of quinone in aprotic solvent. It is accepted that in aqueous solution, quinones undergo reversible two-electron reduction in which the number of protons varies with pH and depends on their basicity; by contrast the quinones in neutral aprotic media undergo two successive one-electron reduction steps. The redox chemistry of hydroquinone in aprotic media is also best described as2e-,2H+reaction giving the quinone as the final product.Our results show that the electrochemical behaviors of benzoquinone and hydroquinone are also dependent on the buffer in aprotic media. Furthermore, the height of a new anodic peak is dependent on the concentration of proton acceptors and the potential of the anodic peak is dependent the basicity and pKb of proton acceptor when it is not enough to accept all protons produced by the electrochemical oxidation of hydroquinone and similar results also occur for proton donor in the electrochemical reduction of quinone. It is true that when the electrochemical redox of hydroquinone and quinone is performed in low buffer, the pH change near the surface of electrode in aprotoic media is more acute than that in aqueous because water itself is a donor or an accepter of proton. Therefore, we have reason to speculate that the rate of redox reactions of hydroquinone and benzoquinone also depends strongly on pH in aprotic solvent. As a result, it is suggested that the new peak in cyclic voltammograms is attributed to the acute pH changes near the surface of the electrode in acetonitrile.In this paper, we systematically investigate the effect of some proton donors and acceptors during the process of the oxidation of1,4-hydroquinone and the reduction of1,4-quinone in acetonitrile. We paid special attention to the "effective pH" near the surface of electrode in aprotic solvents and successfully interpreted the experimental results. A new mechanism of electrochemical redox of benzoquinone and hydroquinone in aprotic solvent is proposed. Furthermore, a new reduction peak was formed after adding acid to the quinone. Likewise, a new oxidation peak was found by the reaction between hydroquinone and base, and the current of new peak is proportional to the concentration of acid or base. This method is rapid and accurate for the direct determination of acid/base without adding any acid-base titration agent and the color indicator.