The Effect And Application Of Acids On The Electrochemical Behavior Of P-benzoquinone And TCNQ

Proton coupling electron transfer (PCET) is important and universal in biology and chemistry field. The reduction process of P-benzoquinone (Q) and 7,7,8,8-Tetracyanoquinodimethane (TCNQ) often undergo along with proton coupling electron transfer. In this paper, the reducation of P-benzoquinone(Q) is uesed to measure weak acids concentration in aqueous solution and aprotic solvent with electrochemical methods. And I also research electrochemical behavior of 7,7,8,8-Tetracyanoquinodimethane(TCNQ). The experiments showed that acids have effects on the electrochemical behavior of TCNQ.Using cyclic voltammetry and differential pulse voltammetry, we obseved that At a more positive potential, a new reduction peak of Q occured, which is hydroquinone(QH2). It is interpreted that Q can accept proton from acids during reducing Q in aqueous solution or aprotic solvent. However, in non-aqueous solvent, water content would have effect on the peak potential.Adding the water, the peak will shift to positive potential.According to these results, we can measure acid concentration in aqueous solution and acetonitrile. At more positive potential, Q can form a new reducing peak. The current of new peak increases with increase the concentration of acids. Under optimum conditions, we can make the standard curve with peak current and concentration of acid. Then we can get the formula with this curve. We can measure acids content. And it was applied to measure acetic acid in vinegar. In the similar way, we established a method to measure mixture acids in acetonitrile. pKa. The new peak potentials are dependent on the pKa of acid. If the pKa of acid is bigger, the peak potential will appear at less positive potential. If the pKa of acid is small,the peak potential will occurr at more positive potential. According the height of the new peak which caused by acid, we can calculate acid concentration. This method does not require any complicated pretreatment, and it has high sensitivity. It can be applied to measure the acid content of practical samples.In addition, we also use cyclic voltammetry to study the electrochemical behaviors of TCNQ with adding different kinds of acid. We found that strong acids can protonize TCNQ·, and it can make reduction peak of TCNQ merge, oxidation peak disappeard. Dium-strong acids have effects on the negative peak of TCNQ, this is because dium-strong acids can protonize TCNQ2-; weak acids have no effect on TCNQ. So we got some reasonable conclusions about proton-coupled electron transfer(PCET) of TCNQ.