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.

References

1. A. Henriksen and A. R. Selmer-Olsen, Analyst, 1970, 95, 514.
2. A. C. Ariza, P. Linares, M. D. Luque de Castro and M. Valcarcel, J. Autom. Chem., 1992, 14, 181.
3. M. Yaqoob, M. A. Siddiqui and M. Massoom, J. Chem. Soc. Pak., 1991, 13, 248.
4. M. A. Koupparis, K. M. Walczak and H. V. Malmstad, Anal. Chim. Acta, 1982, 142, 119.
5. Z. Marczenko, Spectrophotometric Determination of Elements, Ellis Horwood, Chichester, 1976, pp. 397–399.
6. J. F. Van Staden, Anal. Chim. Acta, 1982, 138, 403.
7. Japanese Industrial Standard, JIS K0101, 1971, p. 123.
8. J. Nair and V. Gupta, Anal Chim Acta, 1979, 111, 311.
9. S. A. Rahim, N. A. Fakhri and W. A. Bashir, Microchem. J., 1983, 28, 479.
10. A. K. Baveja, J. Nair and V. K. Gupta, Analyst, 1981, 106, 955.
11. A. Foris and T. R. Sweet, Anal. Chem., 1965, 37, 701.
12. M. Pandurangappa and N. Balasubramanian, Mikrochim. Acta, 1996, 124, 137.