## 4 Discussion  
CV and DPV oxidation and reduction curves recorded at different concentrations of **BR** show DPV and CV currents for anodic and cathodic peaks that increase with the concentration of **BR** (at low concentrations the dependences are linear, as shown in their insets). The oxidation and reduction processes that occur during potential scans were denoted according to DPV peaks. The results show that peaks a1-a4 and c1-c5 are related to electron transfers to **BR**. The characteristic peak of oxygen reduction at the anion radical (O\({}_{2}\)/O\({}_{2}\)\({}^{-}\)) appears in the cathodic CV scans, at about \(-1.2\) V, due to the traces of water in the solvent. However, this peak was reduced as intensity because the experiments took place under argon overpressure.  
CV curves at different scan rates (Figure 3) and domains (Figure 4) allowed the estimation of the reversibility degree of the processes highlighted during anodic or cathodic scans (Table 2). Additional information was obtained from RDE curves at different concentrations and electrode rotation rates (Figure 5). Figure 4 shows that c1, c2, and c4 peaks are irreversible (no response peaks in the return sweeps). Cathodic peak c5 is quaireversible.  
Figure 14: UV-Vis spectra at t = 1 min for different [Pb]/[**BR**] ratios; insets: variation of the absorbance for the peak at 374 nm vs. [Pb]/[**BR**] (**S1** inset) and vs. [Pb]/([Pb]+ [**BR**]) (**S2** inset).  
Figure 13: (**A**)—UV-Vis spectra obtained for different concentrations of **BR** and (**B**)—dependence of absorbances on **BR** concentration ([**BR**]).  
Almost all processes in the anodic domain are irreversible. Only a3 had a small response in the return sweep, and it was mentioned as quasireversible in Table 2. Due to the complexity of functional groups and heteroatoms from the **BR** cycle, it was difficult to attribute the reactions for each of the peaks.  
It can be observed that the occurrence of the linear relationship of the peak currents versus the square root of the scan rate when analyzing the CV curves resulted in scan rates between 0.05 and 0.5 V/s (Figure 3B) on the potential domains of a1 and c1. These variations on the scan rate show absolute slopes of 7.868 \(\times\) 10\({}^{-5}\) and 5.871 \(\times\) 10\({}^{-5}\) A \(\cdot\) V\({}^{-1/2}\) \(\cdot\) s\({}^{1/2}\) for a1 and c1 peaks, respectively. From the obtained slopes, a ratio of 3:2 between the number of electrons participating in the first oxidation/reduction mechanism can be estimated.  
The RDE curves obtained at different **BR** concentrations and the RDE curves that resulted in various rotation rates of the electrode (Figure 5) illustrate the high degree of asymmetry between oxidation and reduction processes. Reduction processes reveal a rise in RDE currents with the concentration and rotation rate of the electrode. Cathodic RDE curves have normal behavior, while anodic ones have small currents only for processes a1-a2; for processes developed at more positive potentials the currents are close to those for the supporting electrolyte. The shape of anodic curves is in agreement with the formation of films by electrooxidation at potentials higher than 1.2 V.