3.3. Experimental pKa Determination

Buffer solutions with different pH values were prepared using varying KH₂PO₄, K₂HPO₄ and K₃PO₄ concentrations. The pH of each solution was checked with a glass electrode. A 10⁻² M stock solution was prepared in methanol for the analyzed phenols (4-bromothymol and 4-nitrothymol). A volume of 30 μL of the stock solution was diluted in 3 mL of the buffer solutions at varying pH values, and the UV-vis absorption spectra were recorded. For all the tested compounds, the final concentration in the cuvette was 10⁻⁴ M. For data analysis, the absorbance values were reported as a function of pH at the appropriate wavelength; then, sigmoidal fitting was used to obtain the inflection point (Figures S1–S2), pKa_exp (4-bromothymol) = 9.92 ± 0.04; pKa_exp (4-nitrothymol) = 7.38 ± 0.06.