Reference method for measuring emissions of nitrogen oxides from stationary sources: chapter 2

2 Summary of method

2.1 The gas sample is admitted into an evacuated flask containing an oxidizing absorbing solution of hydrogen peroxide in dilute sulphuric acid. The oxides of nitrogen are converted to nitric acid by gas phase oxidation due to oxygen in the sample, and by the absorbing solution. The resulting nitrate ion is reacted with phenol-disulphonic acid to produce a yellow compound (the tri-ammonium salt of 6-nitro-l-phenol-2, 4-isulphonic acid) which is measured colourimetrically. Calibration curves, prepared from samples of known nitrate content, are used to determine the amount of nitrate in the sample with results expressed as nitrogen dioxide.