Ambient Air quality Monitoring Guidlines. - Maharashtra Pollution ...

5.1.1 Up to 100 fold ratio of sulfur dioxide to oxidant may be eliminated with out loss of
oxidant by incorporating a chromic acid paper absorber in the sampling train
upstream from the impinger (2).
5.1.2 The absorber removes sulfur dioxide without loss of oxidant but will also oxidize
nitric oxide to nitrogen dioxide.
5.1.3 When sulfur dioxide is less than 10% of the nitric oxide concentration, the use of chromic acid
paper is not recommended. In this case, the effect of sulfur dioxide on the oxidant reading can
be corrected for by concurrently analyzing for sulfur dioxide and adding this concentration to the
total oxidant value.
5.2 Nitrogen dioxide is known to give a response in 1% KI (1), equivalent to 10% of
that of an equimolar concentration of ozone. The contribution of nitrogen dioxide
to the oxidant reading can be eliminated by concurrently analyzing for nitrogen
dioxide by an appropriate method from this volume and subtracting one-tenth of
the nitrogen dioxide concentration from the total oxidant value.
5.3 Peroxyacetyl nitrite gives approximately a response equivalent to 50% of that of
an equimolar concentration of ozone (3). Concentrations in the atmosphere may
range up to 0.1 ppm.
5.4 Other oxidizing substances besides ozone will liberate iodine with this method :
e.g., halogens, proxy compounds, hydro-peroxides, organic nitrites and
hydrogen peroxide (4, 5).
5.5 Hydrogen sulfide, reducing dusts or droplets can act as negative interferences.
5.6 It has been shown that the amount of iodine formed increases with relative
humidity during sampling (6, 7). This effect is nearly linear. Increase of iodine
formation is 0-10% with RH values ranging from 0-60%. Insignificant effects
were observed by increases of RH from 60 to 75%.
6.0 SAMPLE PRESERVATION
6.1 Ozone liberates iodine through both a fast and a slow set of reactions. Some of
the organic oxidants also have been shown to cause slow formation of iodine (4,
5). Some indication of the presence of such oxidants and of gradual fading due
to reductant can be obtained by making several readings during an extended
period of time, e.g., every 20 min.
6.2 Occasionally mold may grow in the absorbing reagent. When this occurs discard
the reagent because reducing substances and a change in pH make it useless.
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