A method has been developed for the determination of ozone in air in the presence of nitrogen dioxide. Samples of the test atmosphere are drawn, in parallel, through neutral, buffered potassium iodide solutions at 2 litres per minute for 20 minutes. One sample, prior to its passage through the iodide solution, is drawn through a cotton-wool plug. This removes the ozone, and the iodine liberated is a measure of the nitrogen dioxide present, expressed in terms of ozone equivalents. The iodine liberated by the other sample is a measure of the total oxidant, ozone plus nitrogen dioxide, also expressed as ozone equivalents. After addition of starch the iodine liberated by each sample is determined visually against prepared colour standards. The difference between the two readings obtained is a measure of the ozone present. The method enables from 0·1 mg m–3(0·05 p.p.m. v/v) of ozone in air, in the presence of up to 9 mg m–3(5 p.p.m.) of nitrogen dioxide, to 0·8 mg m–3(0·4 p.p.m.) of ozone in air, in the absence of nitrogen dioxide, to be determined. The procedure is relatively simple and the time required for a complete analysis is about 30 minutes.
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