A novel method has been developed for the photometric determination of nitrogen oxides (NOx; NO and NO2 gases). The sample gas is shaken with an NaOHalkaline H2O2 solution in a container so that all the NOx is converted to the NO-2 form. After decomposition of the H2O2 by heating, the colour is developed using naphthylethylenediamine and the absorbance at 545 nm is determined. The reference calibration gases consisting of NO and NO2 are determined within 20 min by adding catalase (enzyme) as a rapid decomposition reagent for H2O2 instead of the heating decomposition. This simple and rapid photometric method can be used to determine NOx down to 0.5 ppm (v/v) (0.244 mg/m3 as NO2) in the case where a 1-1 sampling flask is used.
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