Spectrophotometric determination of nitrate and nitrite in soil and water samples with a diazotizable aromatic amine and coupling agent using column preconcentration on naphthalene supported with ion-pair of tetradecyldimethylbenzylammonium and iodide

Composite diazotization-coupling reagents containing sulfanilamide (SAM), sulfapyridine (SP) or sulfathiazole (ST) as the diazotizable aromatic amines and sodium 1-naphthol-4-sulfonate (NS) as the coupling agent using column preconcentration on naphthalene-tetradecyldimethylbenzylammonium(TDBA)-iodide adsorbent have been used for the spectrometric determination of trace nitrate and nitrite in soil and water samples. Nitrite ion reacts with SAM in the pH range 2.0–5.0, SP in the pH range 2.0–2.5 and ST in the pH range 2.0–3.3 in HCl medium to form water-soluble colourless diazonium cations. These cations were coupled with NS in the pH range 9.0–12.0 for the SAM system, 9.6–12.0 for the SP system and 8.5–12.0 for the ST system to be retained on naphthalene-TDBA-I material packed in a column. The solid mass is dissolved from the column with 5 ml of dimethylformamide and the absorbance is measured spectrometerically at 543 nm for SAM-NS, 533 nm for SP-NS and 535 nm for ST-NS. Nitrate is reduced to nitrite by a copper-coated cadmium reductor column and the nitrite is then treated with the diazotization-coupling reagent by column preconcentration. The absorbance due to the sum of nitrate and nitrite is measured and nitrate is determined by difference. The calibration graph was linear over the range 2–40 ng NO2−-N ml−1 and 1.5–30 ng NO3−-N ml−1 in aqueous samples for the SAM and ST systems and 2–48 ng NO2−-N ml−1 and 1.5–36 ng NO3−-N ml−1 in aqueous samples for the SP system, respectively. The sensitivity, accuracy and precision of the systems decreased in the order ST⪢SAM⪢SP. The detection limits were 1.4 ng NO2−-N ml−1 and 1.1 ng NO3−-N ml−1 for SAM, 1.6 ng NO2−-N ml−1 and 1.2 ng NO3−-N ml−1 for SP, and 1.0 ng NO2−-N ml−1 and 0.75 ng NO3−-N ml−1 for ST, respectively. The preconcentration factors are 8, 5 and 6 for SAM-NS, SP-NS and ST-NS, respectively. Interferences from various foreign ions have been studied and the methods have been applied to the determination of ng ml−1 levels of nitrite and nitrate in soil and water samples. The mean recovery was 95–102% for all three systems.