Spectrophotometric determination of ascorbic acid in foods with the use of vortex-assisted liquid-liquid microextraction

A novel, simple, sensitive and selective method for the spectrophotometric determination of ascorbic acid (Asc) has been developed. The developed method is based on the redox-reaction between Asc and triiodide ions (I3−) and vortex-assisted liquid-liquid microextraction (VALLME) of excess I3− as ion associate (IA) with polymethine dye Astra Phloxine FF (AP), with subsequent determination of IA spectrophotometrically at 560 nm. In the presence of Asc, discoloration of the extract of IA occurs. The influence of different factors was studied, and optimized conditions for the spectrophotometric determination of Asc were found: 4.0 · 10−6 mol L−1 I3−, 4.0 · 10−4 mol L−1 KI, 4.0 · 10−5 mol L−1 AP, 500 μL of carbon tetrachloride (ratio of volumes of aqueous and organic phases 10:1), vortexing for 15 s at 3000 rpm and centrifugation for 2 min at 2000 rpm. The determination of Asc is possible in the presence of many interferants: 20,000-fold amounts of tartaric, citric and malic acids and 10,000-fold amount of oxalic acid, over 2000-fold amounts of HCO3−, CH3COO−, HPO42−, SO42−, Cl−, F−, Na+, K+, Li+, Ca2+, Sr2+, Ni2+, Co2+ and Zn2+; and 5-fold amounts of Fe3+ and Cu2+. The organic acids (oxalic, tartaric, citric and malic acids) eliminate the effect of >100-fold amounts of Fe3+ and Cu2+. The calibration curve was linear in the range from 0.003-0.53 mg L−1 of Asc (R2 = 0.997); the limit of detection (n = 10, P = 0.95) was 0.89 μg L−1 of Asc, and the limit of determination (n = 10, P = 0.95) was 2.94 μg L−1 of Asc. The suggested procedure was successfully applied for the determination of Asc in food samples and model mixtures (RSD 2.1-3.9%, recovery 98.0-102.3%). The accuracy of the procedure was confirmed by analysis with a reference method.