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Int. J. Electrochem. Sci., Vol. 14, 2019 5382 pKa3. Therefore, the most significant difference is over the first acidity constant, because the curcumin is largely degraded in acid solutions, as clearly shown before. In this sense, it is relevant to add that in this work instead of working with only one curcumin solution to modify its pH, there were prepared 32 curcumin solutions setting them under controlled conditions with different pHs, and then recording their respective absorption spectra, in order to minimize the period during which a particular curcumin sample was exposed to its respective acidity condition. This form of point analysis together with the use of the software SQUAD allows working with large quantities of absorbance and wavelength data (4975 data from 25 absorption spectra sampled every 4 nm), thus granting a higher degree of reliability to the pKa values reported. Table 2. Acidity constant values reported for curcumin in aqueous solution pKa1 Method H3Cur = H2Cur− + H+ pKa2 pKa3 H2Cur- = HCur2− + H+ HCur2− = Cur3− + H+ Ref. HPLC with florescence detector 7.75 - 7.80 8.55 ± 0.05 9.05 ± 0.05 15 UV Vis 8.10 ± N.R. 10.45 ± N.R. N. R. 19 UV-Vis 8.55 ± 0.05 10.41 ± 0.05 N. R. 28 Spectrophotometric titration 8.38 ± 0.04 9.88 ± 0.02 10.51 ± 0.01 29 Spectrophotometric 7.428 ± 0.015 9.552 ± 0.024 10.946 ± 0.034 This work N.R. Not reported. Figure 4b shows the curcumin absorptivity coefficient values for the 4 curcumin species present in this system and the comparison of the theoretical and experimental spectra, see Figure 4c, obtained through figure 3a experimental data analysis with SQUAD. Note from Figure 4c that the fitting of the theoretical data on the experimental ones is quite good, showing thus that the acidity constant values estimated through this methodology are plainly reliable. With the curcumin acidity constants data obtained in this work, a species distribution diagram was processed, see Figure 5a, and from the predominant zone diagram depicted in Figure 5b (see Rojas-Hernández et al. [30, 31]) it is possible to note that below pH 7.428 ± 0.015, the predominantPDF Image | Curcumin in Aqueous Media at Different pH
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