The electrochemical behaviors of the OMC/GCE at different pH values in 0.04 M B-R solution at a scan rate of 50 mV/s are shown in Fig. 5. The cathodic peak potential shifts negatively gradually with the increasing pH, representing the participation of the protons in the electrochemical reduction process of FA. Furthermore, the peak potential shifts with the pH values following the linear equation: Ep = 0.25–0.064 pH. The slope of 64 mV/pH is close to the theoretical value (59 mV/pH), so it can be deduced that
the proton number is equal to that of the number of transferred electrons. And as can be seen from the plot of Ip-pH, the cathodic peak current of FA increases from pH 3.0 and reaches top value at pH 7.0, then the response decreases slightly. The results may be ascribed to the unstability of FA under acidic conditions and in alkaline solution, the –COOH groups of FA turns to –COO, which gives rise to electrostatic repulsion to make FA difficult to diffuse to the surface of the electrode [37,38]. Meanwhile, the pH 7.0 is a physiological pH value, so it was selected for the determination of FA at OMC/GCE.