Electrochemical Detection Of Weak Acid And Weak Base In Actual Samples Based On The Oxidation-reduction Of P-benzoquinone And Hydroquinone

In recent years,many consumers enjoy the consumption of yogurt because of its rich nutrition and unique health care function.Lactic acid bacteria in yogurt can maintain the micro ecological balance of intestinal flora,but also can reduce blood cholesterol levels.Moreover,during fermentation,lactic acid(LA)bacteria produce LA,which can adjust the p H of the intestinal tract,promote gastric secretion and improve digestion.Quinine is one of the most alkaloids for pharmaceutical purposes.Historically,it has been used as an important antimalarial drug for more than 300 years.It can also be used as an additive in some beverages.In view of the role of lactic acid and quinine,it is important to detect the content of lactic acid and quinine in monitoring the quality of yoghurt and beverages or drugs.Various techniques have been reported for the determination of lactic acid and quinine.These techniques include high-performance liquid chromatography(HPLC),gas chromatography(GC),capillary zone electrophoresis and electrochemical biosensors.However,some limitations,such as high cost,complex sample preparation,and analytical procedures are presented.Therefore,it is necessary to develop a new method for detecting weak acids and weak bases.P-benzoquinone and hydroquinone have been extensively studied.The mechanism of the redox is proton-coupled electron transfer(PCET).P-benzoquinone get two protons and electrons are reduced at the electrode surface;hydroquinone lose two protons and electrons are oxidized.When BQ is electrochemically reduced,proton donor is consumed and the pH near the electrode surface increases sharply in diluted buffered solutions.As pH increases,the reduction potential of BQ decreases rapidly and forms a new peak.In acetonitrile,the hydroquinone loses two prton.With the addition of the weak base,the proton transfer rate is accelerated and the proton is consumed.The pH near the electrode surface changes and produces new oxidation peak at the negative potential.QH2 forms the corresponding p-benzoquinone.The peak current of new peak is proportional to the concentration of the proton acceptor and the proton donor,and the peak potential is dependent on the p Ka of the proton donor and the pKb of the proton acceptor.In this paper,we study the effect of proton donor and acceptor during the process of the reduction of p-benzoquinone in aqueous solution and the oxidation of hydroquinone in nonaqueous solvent.We found that the peak current value of the new oxidation peak or reduction peak was in good linear relationship with the concentration of proton acceptor or proton donor,but not with the concentration of p-benzoquinone and hydroquinone.According to this result,we developed differential pulse voltammetry method to detect the content of lactic acid in different yogurt and the content of quinine in quinine water,respectively.The proposed method is simple and sensitive,and does not require any sample pretreatment.Results are consistent with those obtained by the traditional method.