Flow injection spectrophotometric determination of nitrite

4-Nitroaniline, which gives intensely yellow solution on dissolution in dilute hydrochloric acid, has been found to react with nitrite almost instantaneously in acidic medium yielding a colourless product that has been identified as 4-nitrophenyl diazo cation. The measurement of decrease in colour intensity suggested a new, simple and one-step reaction procedure that has been used for the reversed flow injection determination of 5microg-5 mg/l. NO(2)-N. The limit of detection has been found to be 2 microg/l. NO(2)-N. A number of species such as copper(II) and lead(II), which interfere in other spectrophotometric procedures, do not affect the results. The method has been applied to determine nitrite in natural waters when the %RSD was in the range 1.8-3.5%.