Spectrophotometric determination of nitrogen dioxide, nitrite and nitrate with Neutral Red

A simple and sensitive spectrophotometric method for the determination of nitrogen dioxide in air and nitrite in water, soil, some analytical grade chemicals and tooth paste has been developed. Nitrogen dioxide in air is fixed as nitrite ion in alkaline sodium arsenite or triethanolamine absorber solution. The method is based on the reaction of nitrite in aqueous medium with known excess of Neutral Red (C.I. 50040), an azine dye having a primary amino group with absorption maxima at 530 nm. In acid medium, it showed a decrease in colour intensity due to diazotization, followed by deamination. Addition of bromide ion enhances the rate of diazotization, and the reaction goes to completion almost instantaneously. Beer’s law is obeyed over the range 0 – 20 μg nitrite and the molar absorptivity is 2.5 × 104 L mol–1 cm–1. The colour system is stable for 2 days. The dye can be extracted under alkaline condition in iso-amyl alcohol and the addition of methanolic sulphuric acid restores the dye colour. It showed molar absorptivity of 4.3 × 104 L mol–1 cm–1. Beer’s law is obeyed over the range 0 – 1.6 μg of nitrite, having a detection limit of 0.15 μg.