# The First Stages of Chemical and Electrochemical Aniline Oxidation--Spectroscopic Comparative Study
## 4 Discussion  
The chemical oxidation of aniline at starting pH 1 and 7 was followed by temperature, pH, OCP, FTIR in situ; UV-Vis and SERS ex situ. The oxidation of aniline in acidic medium is already quite well known [24]. After mixing the aniline and APS solutions, an induction period of ca 3 min takes place. Formation of linear AOs in the oxidized state is observed by FTIR and SERS. Then the reaction rate increases, and the propagation of the pernigraniline chain is observed by all methods. After ca 15 min, pernigraniline is reduced to emeraldine, as again observed by all methods. What is important, a well-adhering PANI film is formed, but no \(N\)-phenylphenazinium or benzoquinone-based structures are observed at any stage of the chemical aniline oxidation started at pH 1. These structures are not necessary for the PANI film formation, and they are also not an inherent part of the process of chemical oxidation of aniline. The aniline couples strictly in the _para_ positions, resulting in a linear polymer at low pH, as proposed earlier [16; 17; 19; 22]. The oxidation of aniline at neutral starting pH was studied before in a couple of studies [24; 37]. In this case, sodium sulfate salt was added to the reaction medium to ensure conductivity for the comparable electrochemical experiment. The reaction started immediately, as can be seen from temperature, pH and OCP kinetics. UV--Vis spectroscopy detected only \(N\)-phenylphenazinium-based  
Figure 13: Potential dependencies of the normalized intensities of selected bands of semiquinonoid-charged linear AO (**a**), quinonoid-charged linear AO (**b**), and N-phenyl phenazinium-based AO (**c**) during the spectroelectrochemical study at pH 1 of the aniline oligomer (AO) formed by electrochemical oxidation of aniline in aqueous medium of pH 7. Excitation 633 nm. AOs after 100 s of reaction and later on. FTIR spectra of the reaction products remained unchanged during the reaction, and phenazine-like AOs were observed throughout the whole reaction time. Their protonation state cannot be identified from the FTIR spectra. Only in the first seconds, carbonyl groups were observed. SERS, on the other hand, detected enormous changes during the aniline oxidation in the neutral medium: In the first seconds, linear AOs were observed. After 100 s, \(N\)-phenylphenazinium cation and benzoquinone units absolutely dominated the spectrum. Later on, linear chain growth was observed in addition to the \(N\)-phenylphenazinium cation-based AOs formation. It needs to be mentioned that neutral phenazine-like AOs do not produce well-resolved Raman spectra. A fluorescence background makes them virtually impossible to analyse with Raman spectroscopy. Once they become protonated, forming the \(N\)-phenylphenazinium cation, SERS enhancement of these units becomes operative. From our observations we can conclude that \(N\)-phenylphenazinium-based AOs are by far the most dominant products of the reaction. However, they do not become protonated straight away. The protonation takes place after ca 100 s, which corresponds to pH 5.5, according to the pH kinetics. Linear chain and benzoquinone-based AOs are minor components in the AO mixture. The comparison between the chemical and electrochemical oxidation of aniline is based on SERS experiments. The in-situ SERS spectroelectrochemistry at pH 1 was published earlier [18]. Here, we present an in-situ SERS spectroelectrochemical study of the aniline oxidation at pH 7; and behavior of the formed product in a medium of pH 1. The material formed on the electrode at pH 7 is not very electroactive, wherein only weak redox features of the linear AO fraction was observed. The SERS spectra of the material lack any distinguishable/significant features. The spectra also did not change very much with potential; the changes can be assigned to the linear AO fraction. In addition, the spectra were obscured by fluorescence, which is typical for the neutral form of the \(N\)-phenyl phenazine structure.