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This International Standard specifies a photometric method for the determination of the mass concentration of nitrogen oxides in exhaust gases from ducts or chimneys. NOTE — Nitrogen oxides (NOx) are practically defined as the sum of NO and NO2. The mass concentration of NOx is expressed as the equivalent NO2 concentration in milligrams per cubic metre. The method is applicable to the determination of the mass concentration of nitrogen oxides in exhaust gases from combustion processes, processes for surface treatment of metals and from organic chemical reactions, if necessary after removal by washing, in-furnace reduction and/or catalytic denitration, prior to their dispersion into the atmosphere. This International Standard is applicable to the concentration range 5 mg/m3 to 1000 mg/m3 as NO2 for a sample gas volume of 1000 ml. For gas concentrations greater than 1000 mg/m3, the determination of sample gas concentrations up to 5000 mg/m3 can be performed by diluting the sample solution or by taking an aliquot of the sample solution. This International Standard is not applicable to the determination of dinitrogen monoxide (N2O).