Sensitive Spectrophotometric Determination of Nitrite in Human Saliva and Rain Water and of Nitrogen Dioxide in the Atmosphere

A new simple, sensitive, and selective spectrophotometric method was developed for the determination of nitrite. The method is based on the reaction of nitrite with sulfathiazole in acidic medium to form a diazonium cation, which is subsequently coupled with N-(1-naphthyl)ethylenediamine dihydrochloride to form a highly stable, violet azo dye. The reaction product has an absorption maximum at 546 nm and obeys Beer's law over a nitrite range of 0.054–0.816 μg/mL. The molar absorptivity of the colored compound is 4.61 × 104 L/(mol·cm). The detection limit is 12.1 μg/L. The relative standard deviation is 0.85% for 5 determinations of nitrite at 0.27 μg/mL. The reproducibility and validity of the proposed method are discussed in the present paper. The simplicity of the method is demonstrated by the high stability of the azo-dye product as well as the short time required for its complete formation in a reaction at room temperature without pH control or extra extraction. The sensitivity of the method is shown by the successful determination of nitrite in human saliva and rain water, and of nitrogen dioxide in the atmosphere. The results compare favorably with those obtained by the reference method. The selectivity of the method is indicated by its freedom from most interferences, even at high concentrations of nitrate (500 μg/mL).