A Method for the Determination of Weak Acid Concentration Based on Electrochemical Reduction of Benzoquinone

Abstract

With the addition of a weak acid, a new prepeak occurs at a positive potential when quinone is reduced in an unbuffered solution. This prepeak was initially attributed to the protonation of a quinone anion radical. However, two prepeaks of the reduced quinone are detected in a solution containing dilute acetic acid and sodium dihydrogen phosphate. Evidently, the two prepeaks could not be assigned to the protonation of the quinone anion radical. In the unbuffered solution, the pH near the electrode surface increases drastically during the electrochemical reduction of p-benzoquinone (BQ). Our previous work revealed that a steep change in pH near the electrode surface produces a new voltammetric wave in a dilute buffer solution. We suggest that the new prepeaks are generated by a drastic change in pH near the electrode surface. Weak acids can release protons and suppress the increase in pH on the electrode surface. Therefore, the height of the prepeak is proportional to the concentration of the weak acid but is independent of BQ. We then establish a differential pulse voltammetric method to measure the weak acid and acetic acid contents of two kinds of vinegars. Our results agree well with those measured through potentiometric titration.

Voltammetric approach to measuring free acid in anhydride and ester based on benzoquinone electrochemical reduction

“…Scheme 1 implies that the amount of the reduced BQ decides by that of proton donors (free acid) when the pH near the working electrode surface increases markedly [13]. We observed that the height of the new peak is proportional to the concentration of free acids but is independent of the BQ concentration [14]. Therefore, the weak acid concentration can be measured.…”

“…The addition of HAc to the solution results in a new peak at about + 0.15 V. The new peak is caused by a drastic pH decrease near the electrode surface compared to the BQ solution at pH 6.98. Based on this, we have developed methods for the determination of acetic acid [14]. When the concentration of HAc is less than twice the concentration of BQ, and the value of the new peak current is linear with the concentration of HAc.…”

Determination Method of the ATP, ADP, and CTP Concentrations by Electrochemical Reduction of Benzoquinone

“…When the protons provided by ATP, ADP and CTP were depleted, the pH of the electrode surface increased abruptly, which caused the formation of the peaks A 2 , A 3 , B 2 , B 3 , C 2 , and C 3 . Wang 27,28,34 revealed that the peak current was dependent on the proton donor concentration.…”

“…The benzoquinones (BQ)/hydroquinone (BQH 2 ) couple is often studied in the context of proton-coupled electron transfer. 26 Some researchers [27][28][29] have utilized BQ/BQH 2 couples to determine acids by BQ reduction in an aqueous solution (Scheme 1). We constructed a new electrochemical method using bare glassy carbon electrodes to determine acid.…”

“…We constructed a new electrochemical method using bare glassy carbon electrodes to determine acid. 27,28 Given that the hydrolysis of ATP, ADP, and CTP generates proton gradients, 30 we utilized their features and developed a voltammetric method to determine ATP, ADP, and CTP levels at low potential.…”