Reference method for measuring emissions of nitrogen oxides from stationary sources: chapter 9

9 Calibration

9.1 Sample Collection Flask Volume

Fill the sample flask and stopcock assembly with water up to the stopcock plug. Determine the volume to ±10 ml by measuring either the volume or weight of the water contained in the assembly. Number and record the volume on the flask.

9.2 Spectrophotometer Calibration

Prepare a calibration curve to cover a range of about 0 to 125 ppm NO₂, based on 2000-ml samples of dry gas at 25°C and 760 mm Hg. Using a microburet or pipets, transfer 0.0, 1.0, 2.0, 3.0, 4.0 and 5.0 ml of the working standard KNO₃ (1 ml = 100 µg NO₂) into the 200-ml evaporating dishes and add 25.0 ml of absorbing solution to each. Proceed in accordance with 8.3 and 8.4, Construct a calibration curve by plotting the absorbancies of the solutions at 405 nm against the micrograms of NO₂.

Note 7 - Higher sample concentrations may be analyzed using this curve by taking smaller aliquots of the absorbing solution and/or by diluting the colour-developed solution before reading on the spectrophotometer.