Indigo Method

The method is based on the reaction of ozone with indigo blue dye. The indigo molecule contains 1 carbon double bond (C = C) that reacts with ozone and results in nearly colorless reaction products. During sample collection, 2 cellulose filters coated with 40 micro of 1.0 x 10(-3) M indigo blue were used. The determinations were done spectrophotometrically at 250 and 600 nm. 

The method is stoichiometric and extremely fast. The change of absorbance vs ozone added is −2.0 ± 0.1 × 104 M−1 cm−1 and is independent of the concentration of aqueous ozone in the range 0.005–30 mg 1−1. The precision of the analysis is 2% or 3 μg 1−1 for low concentrations if a spectrophotometer or a good filter instrument is used. Visual methods can be used to measure 0.01 mgl−1 ozone. Secondary oxidants produced by ozone in natural water, including hydrogen peroxide or chlorite, do not interfere; chlorine can be masked. The reagent solution is stable for 3 months. The method is recommended for kinetic measurements, for studies of ozonation processes and for visual field methods.