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

A simple and sensitive spectrophotometric method for the determination of n itrogen dioxide in air and nitrite in water, soil. some analytical grade ch emicals and tooth paste has been developed. Nitrogen dioxide in air is fixe d as nitrite ion in alkaline sodium arsenite or triethanolamine absorber so lution. The method is based on the reaction of nitrite in aqueous medium wi th 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 d ecrease in colour intensity due to diazotization, followed by deamination. Addition of bromide ion enhances the rate of diazotization, and the reactio n goes to completion almost instantaneously. Beer's law is obeyed over the range 0 - 20 mu g nitrite and the molar absorptivity is 2.5 x 10(4) 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 methanoli c sulphuric acid restores the dye colour. It showed molar absorptivity of 4 .3 x 10(4) L mol(-1) cm(-1). Beer's law is observed over the range 0 - 1.6 mu g of nitrite, having a detection limit of 0.15 mu g.