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Int. J. Electrochem. Sci., Vol. 14, 2019 5380 electrochemically generated are immediately oxidized (stage I in Reac.(1)). At higher potentials (stage II in Reac. (1)) also occurs due to an oxidation, after hydroxylation at position 1 and/or 7. I II REAC. (1) R The CVs depicted in Figure 3 (a and b) recorded after 60 min can be associated with the electrochemical oxidation of ferulic acid (3-methoxy-4-hydroxycinnamic acid), one of the curcumin degradation products that have been observed [16] and the electrochemical reaction associated (see React. (2)) have been proposed by Kallel Trabelsi et al. [26] -1e- REAC. (2) - H+ 3.2 Spectrophotometric estimation of the curcumin acidity constants under controlled experimental conditions Figure 4a shows a family of absorption spectra recorded from aqueous curcumin solutions at different pH values. A hypsochromic shift occurs in the absorption maximum located at 471 nm as the pH turns more acid. Further, insofar as the pH becomes more acid, the absorption spectra develop a new, well defined band at 357 nm. Likewise, note several isosbestic points at 306, 327, 382, 421, 428 and 530 nm, which suggest that more than one curcumin acid-base equilibrium occur in solution. Their acid-base nature corresponds to successive dissociations of the acid protons of the curcumin molecule, which in agreement with its enol form structure they could be three. In order to consider the curcumin three deprotonations in aqueous media, these are input to SQUAD [24] along with information of the 25 absorption spectra as a function of pH every 4 nm (from Figure 4): for further details of such procedure see Palomar-Pardavé et al. [27] Table 2 shows the refined values of the acidity constant from the SQUAD software for curcumin, that are compared with those reported in the literature.PDF Image | Curcumin in Aqueous Media at Different pH
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