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Int. J. Electrochem. Sci., Vol. 14, 2019 5381 a) b) c) Figure 4. a) UV-Vis spectra recorded in aqueous solution containing 33 M curcumin at different pH values shown in the figure. Experimental conditions were controlled (protection from light, N2(g) bubbling and temperature control (21.0 ± 0.1) °C). b) Molar absorptivity coefficients for the curcumin species in aqueous media: (Δ) H3Cur (Х) H2Cur− (□) HCur2− (Ο) Cur3−, estimated from analysis of the spectra in Figure 3a using the SQUAD software. c) Comparison of the theoretical spectra (lines) with the corresponding experimental spectra (points) for the different pH: ( ) 10.526 ± 0.001, ( ) 7.025 ± 0.001 and ( ) 3.576 ± 0.001. From the comparison of the values obtained for the curcumin acidity constant in this work, it becomes apparent that there is a difference of two units with respect to the other two values. The difference between the values of pKa2 and pKa3 is only 1.4 units approximately. Such closeness of the last two pKa values indicates that pKa2 and pKa3 correspond to deprotonation of the phenols located at each end of the molecule (due to the symmetry of the curcumin molecule), whereas that of pKa1 is associated with a keto-enol equilibrium. From the comparison of this acidity constant value with those shown in Table 2 the closeness between them becomes clearly apparent with only a difference not larger than a unit for pKa2 and pH 11.450 ± 0.001 pH 3.626 ± 0.001PDF Image | Curcumin in Aqueous Media at Different pH
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