Spectroscopic determination of the dissociation constants of 2,4- and 2,5-dihydroxybenzaldehydes and relationships to their antioxidant activities

UV-visible spectra of 2,4- and 2,5-dihydroxy-benzaldehydes (2,4DHB and 2,5DHB) are recorded in a wide range of pH. The dissociation pK values obtained from these measurements were 6.94 ± 0.03 and 9.28 ± 0.03 for 2,4DHB and 8.42 ± 0.03 and 10.93 ± 0.03 for 2,5DHB. The results indicate that the pH at which the assays for antioxidant capacity measurements are made is very important in light of the hydroxyl group dissociation, because of the different dissociation constants of the different isomers. The percentage of dissociation of each group is essential, the positions of these groups in the ring appearing as a secondary factor.