To measure ozone concentration, distilled water was substituted for
apple juice, as will be explained in theDiscussionsection. The ozone
concentration of distilled water treated with ozone in the same way as
apple juice was measured by the indigo method (Bader and Hoigni,
1981). An indigo stock reagent was prepared by the following method
(Gordon and Bubnis, 2002). Indigo stock solution was prepared by dissolving 770 mg of potassium indigotrisulfonate (Sigma Aldrich Co., LLC)
into a 1 lflask containing 500 ml of distilled water and 1 ml of phosphoric acid (85%) and diluting to volume (1 l) with distilled water. Indigo reagent II was prepared in a 1 lflask by adding 100 ml of indigo stock
solution, 10 g of sodium dihydrogen phosphate (NaH2PO4)and7mlof
phosphoric acid (85%). This was stirred and diluted to volume (1 l) with
distilled water. The solution was stored in the dark. After each treatment, the treated sample was cooled to about 15 °C and then 90 ml of
the sample was transferred to aflask containing 10 ml of indigo reagent
II. The absorbance was measured by a spectrophotometer at 600 nm.
Residual ozone in treated samples was obtained by the equation