Column preconcentration of nitrite in water samples with retention of azo dyes on naphthalene with subsequent determination using spectrometry

Abstract

Column methods have been established for the preconcentration of nitrite with the retention of azo dyes formed from the diazotization and coupling reaction on naphthalene using a simple glass-tipped tube. Nitrite ion reacts with p-aminoacetophenone (AAP) in the pH range 2.0–4.0 and with p-nitroaniline (NA) in pH range 1.5–3.0 in a hydrochloric acid medium to form a water-soluble, colourless diazonium chloride, respectively. These diazonium ions form the uncharged azo dyes with 1-naphthol as a coupling agent in the pH ranges 1.0–8.0 for the AAP–1-naphthol method and 2.3–7.0 for NA–1-naphthol method, which are adsorbed on inactivated naphthalene packed in a column. The solid mass is dissolved out from the column with 5 ml of ethanol and the absorbance of the solution is measured with a spectrophotometer at 470 nm for the AAP–1-naphthol method and at 473 nm for the NA–1-naphthol method, respectively. The calibration curves were linear over the concentration ranges 0.5–6.6 µg of nitrite in 5 ml of ethanol in both methods. The molar absorptivity and Sandell's sensitivity were calculated to be 3.7 × 10⁴ l mol^–1 cm^–1 and 9.9 × 10^–4µg cm^–2 for the AAP–1-naphthol method and 4.4 × 10⁴ l mol^–1 cm^–1 and 1.3 × 10^–3µg cm^–2 for the NA–1-naphthol method, respectively. Ten replicate determinations of solution containing 3.3 µg of nitrite for the AAP–1-naphthol method and 2.6 µg of nitrite for the NA–1-naphthol method gave mean absorbances of 0.531 and 0.508, with relative standard deviations of 1.13 and 1.55%, respectively. Interferences due to various foreign ions have been studied and the methods have been applied to the determination of nitrite in river water.