# Glassy Carbon Electrochemical Sensor for Gallic and Vanillic Acid Detection in Aqueous Solutions
## 3 Results and Discussion
The values of \(E_{1,peak}\) and \(E_{2,peak}\) were taken from Figure 1. The results from the first peak (\(E_{1}\) and \(E_{1,peak}\)) can be described by the same equation, \(j\) (mA\(\cdot\)cm\({}^{-2}\)) = 6.4 \(\times\) 10\({}^{-4}\) c (mg\(\cdot\)L\({}^{-1}\)). A similar equation with a slightly different slope and an intercept value can be applied in the case of the second oxidation peak, \(j\) (mA\(\cdot\)cm\({}^{-2}\)) = 5.1 \(\times\) 10\({}^{-4}\) c (mg\(\cdot\)L\({}^{-1}\)) + 7 \(\times\) 10\({}^{-3}\). This non-zero intercept (7 \(\times\) 10\({}^{-3}\) mA\(\cdot\)cm\({}^{-2}\)) is the value of the current density observed in the CV taken without acid (i.e., blank experiment). Similar bar chart plots have also been constructed for the case of vanillic acid which are presented in Figure 4. In contrast to gallic acid (Figure 3), where all the results can be described by only one equation, the data for vanillic acid could be separated into two concentration ranges. Regardless of the oxidation potential (\(E_{1}\) or \(E_{2}\)), the results could be divided into two regions: in a low (0-50 mg\(\cdot\)L\({}^{-1}\)) and a high (70-200 mg\(\cdot\)L\({}^{-1}\)) concentration range. According to the fitting, each concentration range resulted in a different dependence on the observed oxidation current, indicated by the change of the line's slope. A possible explanation for this behavior is the obvious potential shift to higher applied potentials by increasing the concentration of vanillic acid in the aqueous solution (as shown in Figure 2). Figures 5 and 6 present the effect of the scan rate on the relationship between the phenol concentration and oxidation peak. In the case of gallic acid (Figure 5), there was not any special change of the interpretation of the data. Similar to Figure 3, a linear relationship between the intensity of the anodic peak and gallic acid concentration was also observed, with a minor change on the slope. On the contrary, the scan rate seemed to significantly affect the calibration curves of vanillin acid (Figure 6) and more specifically caused an increase in the slope as the scan rate value increased. The peak potential in Figures 1 and 2 together with the equations written in Figures 3-6 can be utilized to qualitatively and quantitatively determine the presence of gallic and vanillic acid in an aqueous solution. Our results can be rationalized with those reported in the literature. [PERSON] et al. (2019) using CV on a bare glassy carbon electrode found that the oxidation peak current was proportional to the phenol concentration up to 5 \(\upmu\)M. Another study of [PERSON] et al. (2020) over a bare glassy carbon electrode revealed the need for an alkaline environment (pH \(>\) 7), where more active -OH radicals were produced to oxidize phenol to intermediates, enhancing the overall efficiency of the process. Furthermore, using carbon paste electrodes, [PERSON] et al. (2020) illustrated that the obtained calibration curve of vanillic acid could be separated into two regions, as in low concentration values the slope of the intensity of anodic peak versus the vanillic acid concentration is changed, similar to our results. A possible explanation is attributed to the potential shift upon varying the vanillic acid concentration, as well as the observed low current density values at concentrations below 70 mg\(\cdot\)L\({}^{-1}\), suggesting a decrease in the electrode's sensitivity. Figure 3: Effect of gallic acid concentration on the current density value at \(E_{1}\) and \(E_{1,\text{peak}}\) (**left**) and \(E_{2}\) and \(E_{2,\text{peak}}\) (**right**) at a constant scan rate of 25 mV\(\cdot\)s\({}^{-1}\). Figure 4: Effect of vanillic acid concentration on the current density value at \(E_{1}\) and \(E_{1,\text{peak}}\) (**left**) and \(E_{2}\) and \(E_{2,\text{peak}}\) (**right**) at a constant scan rate of 25 mV.s\({}^{-1}\). In an attempt to simulate a real solution, a mixture of the above probe phenolic compounds was prepared.