# Glassy Carbon Electrochemical Sensor for Gallic and Vanillic Acid Detection in Aqueous Solutions
## 3 Results and Discussion  
Cyclic voltammetry was performed to study the redox behavior of the phenolic compounds (i.e., gallic and vanillic acid). For stabilization purposes, the second scan of cyclic voltammograms is presented. Figure 1 displays the electrooxidation of gallic acid upon potential variation under two different scan rates (i.e., 25 and 50 mV\(\cdot\)s\({}^{-1}\)). The cyclic voltammetry results (Figure 1) displayed the presence of two oxidation peaks upon anodic polarization at around +0.5 V (labeled as \(E_{1}\)) and +0.8 V (labeled as \(E_{2}\)), respectively. Nevertheless, a small shift of both oxidation peaks to higher potentials was observed by increasing the concentration of gallic acid in the reaction solution. According to the literature [30], the electrooxidation mechanism of gallic acid in an acidic environment can be described in four steps. The first peak represents the electrooxidation of gallic acid to the semiquinone radical cation. Then this radical cation loses a proton to form the semiquinone radical. The second oxidation peak is associated with further oxidation of semiquinone radical to the quinone cation. Finally, deprotonation of the quinone cation completes the overall two-electron process to form the final quinone [30]. It is worth noting that pure KNO\({}_{3}\) electrolyte (blank experiment) showed negligible oxidation current densities (up to 7 \(\times\) 10\({}^{-3}\) mA\(\cdot\)cm\({}^{-2}\)). It is clear from Figure 1 that higher concentrations  
Figure 1: Second scan of the cyclic voltammograms for gallic acid in various concentrations at a constant scan rate of 25 (**top**) and 50 (**bottom**) mV\(\cdot\)s\({}^{-1}\). Analogous electrochemical experiments were carried out also for vanillic acid. Figure 2 demonstrates the second scan of the cyclic voltammetry results for various concentrations of vanillic acid. The cyclic voltammograms of vanillic acid (Figure 2) show three distinct peaks: two oxidation peaks (labeled as \(E_{1}\) and \(E_{2}\)) and one reduction peak (labeled as \(E_{3}\)). Various smaller peaks (wave-like features) can also be seen prior to the oxidation and reduction peaks. The \(E_{1}\) and \(E_{2}\) anodic peaks are localized at around +0.1 V and +0.4 V, respectively, whereas the \(E_{3}\) cathodic peak appeared at approximately 0 V. A small reduction wave feature was observed at around +0.25 V during the cathodic scan. It is important to mention that the first cycle (not shown here) was different than the following one (presented in Figure 2) since it consisted of only one anodic peak (\(E_{2}\)) and a small cathodic peak (\(E_{3}\)). The presence of the new (\(E_{1}\)) anodic peak in the second scan can be attributed to the OCH\({}_{3}\) groups present in vanillic acid that are oxidized in parallel with OH groups in the initial scan sweep of the electrode [31]. Thus, new compounds were formed that can be subsequently reduced to new catechol-containing polyphenols. This behavior may also indicate the formation of a thin polymeric film on the glassy carbon electrode [31]. Furthermore, the \(E_{2}\) oxidation peak corresponds to the oxidation of the hydroxyl groups on the B-ring of vanillic acid [32]. The appearance of the reduction peak is due to the reduction of multiple oxidation products, some of which could remain adsorbed on the working electrode surface as a carbonaceous film [33]. Similar to the case of gallic acid, increasing the vanillic acid concentration results in a remarkable potential shift to higher values. According to the literature [30] a two-electron oxidation mechanism can describe the vanillic acid results, as the gallic acid ones. Figure 3 presents a bar chart plot of the intensity of the anodic peak as a function of the gallic acid concentration at a fixed potential (\(E_{1}\) = +0.49 V or \(E_{2}\) = +0.81 V) and at the peak potential \(E_{1,peak}\) or \(E_{2,peak}\) (taking into account the small shift of the two peaks).