o-Phenylenediamine has been used as a reagent to quantify nitrites= nitrates in a variety of sample matrices. The method is based on the cyclization reaction between o-phenylenediamine and nitrite in acid medium. The amine undergoes diazotization with nitrite in the presence of acid to form the diazonium ion, which subsequently cyclizes to yield yellowish orange benzotriazole at room temperature with an absorption maximum at 450 nm. The formed dye has been separated, purified, and characterized by IR, NMR, and spectroscopy techniques. The parameters of the reaction between amine and nitrite have been optimized. The effect of interfering ions on the determination of nitrites=nitrates has been described. The developed method has been applied for the determination of residual NO2 gas present in the ambient air after fixing it as a nitrite ion using sodium arsenite as a trapping medium. The dye formed has been extracted into organic solvent to improve the detection limit during the measurement of low levels of ambient NO2 in air. The method obeyed Beer’s law in the concentration range 0–250 mg in aqueous medium and 0–50 mg in organic medium with molar absorptivity of 4.09
104L mol1 cm1 and 4.3
104 L mol1 cm1 respectively. Nitrate is determined by reducing it to nitrite after passing through the copperized