Potassium peroxodisulfate k2s2o8

All chemicals used were of analytical reagent grade and all solutions were prepared in deionized water (18 MΩ•cm, Barnstead, Thermo Scientific, USA). 2,2′-Azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and potassium peroxodisulfate (K2S2O8) were purchased from Sigma (St. Louis, USA). 2,4,6-Tripyridyl-S-triazine (TPTZ) and iron(III) chloride (FeCl3) were purchased from Sigma-Aldrich (Germany). L-Ascorbic acid was obtained from UNILAB (Philippines). A stock solution of 100 mM ascorbic acid was freshly prepared by dissolving 0.4400 g of the ascorbic acid in 25.00 mL deionized water. To construct the calibration curve, standard solutions were prepared by diluting the stock ascorbic acid solution to concentrations of 10, 20, 30, 40 and 50 μM, respectively. The ABTS radical cation (ABTS •+ ) reagent was prepared by reacting ABTS (0.0380 g, 7.00 mM) with potassium persulfate (0.0060 g, 2.45 mM) in distilled water with the total volume of 10.00 mL and the solution stored in the dark at room temperature for 12 -16 h. The solution was then diluted with deionized water until its absorbance was 0.70 at 734 nm. 6 The FRAP reagent was prepared by mixing solutions of 0.30 M acetate buffer (pH 3.6), 10 mM TPTZ (in 40 mM HCl) and 20 mM FeCl3, in the ratio 10:1:1 (v/v/v). 11