A simple and sensitive spectrophotometric method for the determination of
nitrogen dioxide in ambient air and nitrite/nitrate in water and soil samples has been
developed. Nitrogen dioxide in air has been fixed as nitrite ion using alkaline sodium
arsenite as absorbing medium. The method is based on the reaction of nitrite with aminophenyl
benzimidazole in acid medium to form diazonium ion, which is coupled with
N-(1-naphthyl)ethylenediamine dihydrochloride to form an azo dye with an absorption
maximum at 555 nm in aqueous phase. The method obeys Beer’s law in the concentration
range 0–10 mg of nitrite in 25 ml solution. The molar absorptivity has been
found to be 6.3 104 l mol21 cm21. The dye can be extracted quantitatively into
isoamyl alcohol under alkaline condition and the addition of methanolic hydrochloric
acid restores the original dye colour. Beer’s law is obeyed in the concentration range
0–2 mg of nitrite with a detection limit of 0.009 mg. The effect of interfering species
has been studied and the developed method has been applied to determine trace levels
of nitrogen dioxide in ambient air and the results have been compared with the
standard method. It is also applied to measure the nitrite/nitrate levels of surface and
ground water samples collected from lakes, tube wells as well as soil samples.
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