Diaminobenzene as a Novel Reagent for Nitrite Assay in Environmental Samples: Evidence for Its Mechanistic Aspects

Abstract

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 µg in aqueous medium and 0–50 µg in organic medium with molar absorptivity of 4.09 × 10⁴ L mol⁻¹ cm⁻¹ and 4.3 × 10⁴ L mol⁻¹ cm⁻¹ respectively. Nitrate is determined by reducing it to nitrite after passing through the copperized cadmium reductor column. The developed method has been applied to determine nitrite/nitrate levels in water, soil, and biological samples.

Keywords:

• Ambient nitrogen dioxide
• cyclization
• nitrate
• nitrite
• o-phenylenediamine
• reductor column
• sodium arsenite
• solvent extraction
• spectrophotometry

Acknowledgments

The authors acknowledge the UGC-DRS program of the Department of Chemistry, Bangalore University, for financial assistance.

Notes

^a Air was sampled on different days.

^b at 25°C (298 K) and 101.3 kpa where V is the volume of air sampled, 0.82 is the factor of collection efficiency for sodium arsenite as trapping medium, and 532 is the conversion factor to convert µg l⁻¹ of NO₂ to ppb of nitrogen dioxide at 298 K and 101.3 kpa.

Notes. Trapping solution: 25 ml of alkaline sodium arsenite. Sampling rate: 0.3 Lmin⁻¹. The volume of solution taken for analysis was 10 ml from 50 ml made-up solution.

Note. Total originally present formed by the reduction of nitrate (µg).

^a,b Water samples were collected from different locations around Bangalore city.

^a Nitrite was not detected in these soil samples.

^b Total nitrite (µg) = nitrite originally present (µg) + nitrite formed by the reduction of nitrate (µg).

^c Nitrate in soil (µg/g) = [Total nitrite (µg)−Nitrite originally present (µg)]/ Weight of soil (g) × 62/46.

^d,e Soil samples were collected from agricultural fields in the rural areas of Bangalore.

Total nitrite (µg) = nitrite originally present (µg) + nitrite formed by the reduction of Nitrate (µg).

NO₃ ⁻ (mg L⁻¹) = {[Total NO₂ ⁻ (µg)−NO₂ ⁻ originally present (µg)]/10}× 62/46.

^a Milk sample was collected from milk dairy, Bangalore.

^a Nitrate was not found.

NO₃ ⁻ (mg L⁻¹) = [{Total NO₂ ⁻ (µg)−NO₂ ⁻ originally present (µg)]/10}× 62/46.

^b Coolant sample was taken from an automobile garage.

^a Based on the solubility of organic solvent in aqueous phase, different volumes were used. In all cases the extract was collected into a 5-ml standard flask and made up to mark with methanolic HCl to restore the original color.

^b 1:1 by volume ratio.

^a Treated with 1 ml of 0.01% lead acetate solution: it was centrifuged, the residue was washed, and the centrifugate and washings were mixed and used for color development.

^b Treated with 2 ml of 0.05% formaldehyde solution before the addition of coupling agent.

^c Treated with 1 ml of 1 M NaOH solution: it was centrifuged, the residue was washed, and the centrifugate and washings were mixed and used for color development.

^d Treated with 2 ml of 0.05 M EDTA solution.