Determination of Total Nitrogen Oxides as Nitrate (Phenoldisulphonic Acid Method)*

Nitric oxide (NO), nitrous anhydride (N₂O₃), nitrogen dioxide (NO₂), nitrogen tetroxide (N₂O₄), also vapor or mist of nitric acid (HNO₃) and nitrous acid (HNO₂), but not nitrous oxide (N₂O), may be collected and oxidized to the nitrate ion in an evacuated flask containing sulfuric acid and hydrogen peroxide. The yellow compound resulting from reaction of the nitrate ion with phenoldisulfonic acid is measured colorimetrically at 400 nm.