Quinizarin characterization and quantification in aqueous media using UV-VIS spectrophotometry and cyclic voltammetry

This work reports for the first time, the acidity constant (pKa) values, in aqueous media, of four anthraquinones, namely: quinizarin (pKa(1) = 10.83 +/- 0.02, pKa(2) = 12.03 +/- 0.02); anthrarufin (pKa(1) = 10.93 +/- 0.02, pKa(2) = 12.49 +/- 0.02); chrysazine (pKa(1) = 8.73 +/- 0.01, pKa(2) = 12.37 +/- 0.01) and anthraflavin (pKa(1) = 7.47 +/- 0.03, pKa(2) = 8.48 +/- 0.03), and the quinizarin inclusion complex formation constant with 2-hydroxy-propyl-beta-cyclodextrin, pK(f) = 2.53 +/- 0.04, as well. Furthermore, it is shown that this supramolecular inclusion complex resulted convenient to perform quinizarin electrochemical quantification, in aqueous media at pH 7.00, by means of cyclic voltammetry attaining the following analytical performance: (3.129 +/- 1.200) mu M and (10.429 +/- 1.133) mu M were the detection and quantification limits, respectively, with a sensibility of (0.213 +/- 0.006) mu A mu M-1 and a (0-36) mu M linear interval. Analogously, UV-Vis spectrophotometry allowed determining quinizarin, under the same conditions, with the following analytic parameters: a detection limit of (1.411 +/- 0.586) mu M-1, quantification limit of (4.706 +/- 0.551) mu M-1, with (0.00547 +/- 0.0001) abs mu M-1 sensibility and a (0-29) mu M linear interval. Moreover, it is shown that cyclic voltammetry determined quinizarin in presence of anthrarufin, chrysazine and anthraflavin as interferents, contrary to the spectrophotometric method

Automated summary

This study reports the acidity constant values of four anthraquinones in aqueous media for the first time, and presents their application in electrochemical quantification and UV-Vis spectrophotometry when forming inclusion complexes with 2-hydroxy-propyl-beta-cyclodextrin. Cyclic voltammetry showed higher sensitivity and wider linear range for quinizarin detection and quantification compared to spectrophotometry, also demonstrating an advantage in the presence of interferents.