Potentiometric and UV-Vis spectrophotometric titrations for evaluation of the antioxidant capacity of chicoric acid

The antioxidant capacity (AOC) of chicoric acid (ChA, the main antioxidant component of Echinacea) or an ethanol/water-extract of Echinacea flowers was determined by potentiometric and UV-Vis absorption spectrophotometric titrations with ABTS˙(+) radical cations as the oxidizing probe. The potentiometric and spectrophotometric titration results agreed well with each other. The trolox-equivalent antioxidant capacity (TEAC) of ChA was found to be 5.00 ± 0.07 (potentiometry) and 4.81 ± 0.06 (spectrophotometry) at pH 7.4, and the TEAC value has been proven to be actually equal to the ratio of the stoichiometric ratio of the ABTS˙(+)–ChA redox reaction to that of the ABTS˙(+)–trolox redox reaction. The TEAC of the ethanol/water-extract of Echinacea flowers, expressed in mM (trolox) per gram per liter (Echinacea extract), was found to be 0.241 ± 0.006 mmol g(−1) (potentiometry) and 0.240 ± 0.007 mmol g(−1) (spectrophotometry) at pH 7.4. The stoichiometric ratio of the ABTS˙(+)–ChA redox reaction varied from 10.8 to 3.2, depending on the solution pH and the final ABTS˙(+)–ChA concentration ratio. However, the stoichiometric ratio of the ABTS˙(+)–trolox redox reaction remained ca. 2.0 at various solution-pH values and final ABTS˙(+)–trolox concentration ratios. The unusual stoichiometric ratio of the ABTS˙(+)–ChA redox reaction is examined by potentiometric/spectrophotometric titrations and cyclic voltammetry, clearly revealing the new mechanism of a rapid redox process followed by a slow redox process at pH 7.4 and 9.0 when the ABTS˙(+)–ChA molar concentration ratio is greater than 4. The electrochemistry methods coupled with spectrophotometry can conveniently and reliably provide important quantitative and qualitative information on redox chemistry, and are expected to find wider applications in accurately evaluating the redox activities of many other natural/synthesized antioxidants and oxidants.