Spectrophotometric determination of ozone in air with indigo disulphonate

Abstract

A simple, inexpensive and reliable manual spectrophotometric method for the determination of ozone in ambient air is evaluated. The air is passed through the blue solution of indigo disulphonate (IDS) in a special double sintered disc absorber. The resulting partly bleached solution is very stable. The discoloration is measured at 610 nm. The stoicheiometry of the reaction is independent of sample flow-rate and the amount of ozone absorbed. The collection efficiency is 100%, even at 2000 ml min^–1. The decrease of absorbance caused by ozone is 21960 l mol^–1 cm^–1. Probably this value is largely independent of the brand and batch of IDS. Therefore, the method can be used for simple determinations without calibration when an inaccuracy up to about 20% is admissible. Otherwise, a calibration against a reference method is necessary.

In an intercomparison study referenced against gas phase titration an apparent stoicheiometry factor of 1.13 was obtained. Probably this deviation from 1.00 was caused by non-reactive impurities in the IDS. Nitrogen dioxide gives a slightly positive interference (6% of its mass) but the other common air pollutants, in ambient concentration, do not interfere. The relative standard deviation for determinations of ozone concentrations between 50 and 1000 µg m^–3 is less than 5%. The limit of detection is about 8 µg m^–3.