Column Preconcentration Analysis–Spectrophotometric Determination of Nitrate and Nitrite by a Diazotization–Coupling Reaction

Abstract

Composite diazotization–coupling reagents containing sulfanilic acid (SA) as the diazotisable aromatic amine and sodium 1-naphthol-4-sulfonate (NS), 1-naphthol or phenol as the coupling agent were used for the spectrophotometric determination of trace amounts of nitrate and nitrite. Column preconcentration on a naphthalene–tetradecyldimethylbenzylammonium (TDBA)–iodide (I) or biphenyl–trioctylmethylammonium chloride (TOMA)–bromate (BrO₃) adsorbent was used. Nitrate was determined by use of a Cd–Cu column which converts nitrate into nitrite. Nitrite ion reacted with SA in the pH range 2.1–2.8 for the SA–NS system, 2.0–2.75 for the SA– 1-naphthol system and 2.2–2.6 for the SA–phenol system in hydrochloric acid medium to form water-soluble, colourless diazonium cations. These cations were coupled with NS in the pH range 10.0–13.0, 1-naphthol in the pH range 1.6–4.6 and phenol in the pH range 9.05–13.0 to be retained on naphthalene–TDBA–I for SA–NS and SA–1-naphthol and on biphenyl–TOMA–BrO₃ for SA–phenol. The solid mass was dissolved from the column with 5 ml of the final dimethylformamide (DMF) solution and the absorbance was measured with a spectrophotometer at 526 nm for SA–NS, 418 nm for SA–1-naphthol and 480 nm for SA–phenol. The calibration curves were linear over the concentration ranges 0.14–2.86 × 10^–6 mol l^–1 NO₂-N and 0.11–2.14 × 10^–6 mol l^–1 NO₃-N for SA–NS, 0.71–18.86 × 10^–6 mol l^–1 NO₂-N and 0.54–14.00 × 10^–6 mol l^–1 NO₃-N for SA–1-naphthol and 0.21–7.21 × 10^–6 mol l^–1 NO₂-N and 0.16–5.36 × 10^–6 mol l^–1 NO₃-N for SA–phenol in aqueous samples. The molar absorptivity decreased in the order SA–NS > SA–phenol > SA–1-naphthol. In SA–NS, an unexpectedly high sensitivity was obtained because of the increased molar absorptivity in adsorption-DMF (9.75 × 10⁴ l mol^–1 cm^–1, λ_max = 526 nm), approximately five times larger than that in water (1.82 × 10⁴ l mol^–1 cm^–1, λ_max = 492 nm). The concentration factor decreased in the order SA–phenol > SA–NS > SA–1-naphthol. Interferences by various foreign ions were studied and the method was applied to the determination of low levels of nitrite and nitrate in water samples.