## 3 Results and Discussion
### The Effect of pH
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**First-Order Model**} & \multicolumn{3}{c}{**Sigmoidal Model**} \\ \cline{2-7}
**pH** & \(k\) **(s\({}^{-1}\))** & \(\mathbf{f_{\frac{1}{2}}}\) **(s)** & \(\mathbf{R^{2}}\) & \(\mathbf{k}\) **(s\({}^{-1}\))** & \(\mathbf{t_{\frac{1}{2}}}\) **(s)** & \(\mathbf{R^{2}}\) \\  & **(\(\times\)10\({}^{-3}\))** & \(\mathbf{f_{\frac{1}{2}}}\) **(s)** & \(\mathbf{R^{2}}\) & **(\(\times\)10\({}^{-3}\))** & \(\mathbf{f_{\frac{1}{2}}}\) **(s)** & \(\mathbf{R^{2}}\) \\ \hline
6.5 & 0.30 & 2310 & 0.94 & 0.76 & 2189 & 0.99 \\
7.0 & 0.40 & 1733 & 0.97 & 0.92 & 1470 & 0.99 \\
7.5 & 1.60 & 433 & 0.93 & 3.65 & 673 & 1.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Kinetic data generated after implementing first-order and sigmoidal kinetic models. Figure 2: Adjustment of the sigmoidal model on the kinetic data obtained for Qt thermal degradation at various pH. observed when kinetic parameters were determined with the sigmoidal model instead of the first-order model. The determination of \(k\) with either model demonstrated an approximate 5-fold increase from pH 6.5 to 7.5, illustrating the effect of pH. Although information on thermal degradation kinetics of quercetin at 95 \({}^{\circ}\)C was previously provided to some extent [23], no data regarding the effect of pH at similar temperatures have been reported. Early examinations demonstrated the dependence of quercetin oxidative degradation on pH within the range 4-7, suggesting a logarithmic increase in first-order kinetics constant as a function of pH, at 50 \({}^{\circ}\)C [24]. In the same line, more recent studies showed an increase in first-order decomposition of quercetin as pH increased from 6.0 to 7.5 [25]. It was proposed that quercetin deprotonation at alkaline pH accelerates its oxidation, affecting the C-ring and the \(o\)-diphenol structure of the B-ring, and leading to the formation of a benzofuranone derivative [26; 27]. The LC-MS/MS analysis of a Qt solution after treatment at 90 \({}^{\circ}\)C for 50 min (Figure 4) enabled the tentative identification of typical degradation products, including protocate-chuic acid (PCA) and phloroglucinol carboxylic acid (PGCA) (Table 2; Figure 3). Figure 3: Structures of Qt degradation/oxidation/reaction products tentatively identified in the model solutions tested. Assignments: (**1**) Protocatechuic acid (\(m/z=151\)); (**2**) phloroglucinol carboxylic acid (\(m/z=169\)); (**3**) Qt quinone (\(m/z=299\)); (**4**), 2,3,4,-chalcan-trione \(o\)-quinone; (**5**) 3,4-dione-2-oxo-phenylacetic acid (\(m/z=179\)); (**6**) Qt 3-sulfate (\(m/z=381\)); (**7**) Qt 3-sulfate quinone (\(m/z=379\)). Identification of both PCA and PGCA was based on mass spectral data previously reported [28]. Qt quinone was also detected, which confirmed its formation as an intermediate in Qt degradation. The typical oxidation product 2-(hydroxybenzoyl)-2-hydroxybenzoyl uran-3(2\(H\))-one (benzofuranone), which may arise as a result of Qt oxidation by various means [28], was not detected. This fact might be attributed to the relatively high temperature of the treatment, which most probably triggered benzofuranone thermal decomposition. On the other hand, alternative oxidation pathways might have taken place as previously proposed [18; 29], resulting in PCA and PGCA formation. It should be noted that Qt oxidation in the absence of H2O2 did not yield PCA and PGCA [30; 31]. Therefore, the degradation pathway(s) involved would be likely to include formation of H2O2 too, as intermediate. This could arise through reaction of O2 with 2H atoms offered by Qt, hence the formation of Qt quinone. Such a reaction would be facilitated at the mildly alkaline pH used (7.5). Then H2O2 nucleophilic attack at the C2 position would lead to cross ring cyclic peroxidation, resulting an intermediate depside. Decomposition of this depside would eventually yield PCA and PGCA [31]. Such a reaction would also be feasible through radical formation, as previously suggested [29].