fluorescence intensity of quinine is pH dependent can be shown in figure 3. This is because pH affects the structure of the molecule which indirectly affects the fluorescence intensity. Quinine has two sites (N) of protonation. At low pH, it will exist as a diprotonated molecule. As the pH increases from 2.48 to 3.75, an increase in fluorescence intensity is observed. This is because as the pH increases, there is more quinine molecules in the solution has become monoprotonated. This implies that the monoprotonated form of quinine has a higher fluorescence intensity value than that of the diprotonated form. However, as the pH exceeds 3.75, an increase in pH value leads to a fall in fluorescence Intensity. This is because as the pH increases further, more and more monoprotonated quinine will become totally deprotonated. This implies that the deprotonated form has a lower fluorescence intensity value as compared to the monoprotonated form.
However, to better predict the fluorescence intensity of a fully deprotonated quinine, we could carry out the experiment at pH 14. Maximum fluorescence intensity was observed at around pH 3.71, when all of the quinine molecules exist in the monoprotonated form, since the monoprotonated form gives rise to the highest fluorescence Intensity value as compared to the diprotonated and the fully deprotonated form.