Qualitative Analysis of Nitrite Ion
Nitrite ion (NO2–) can be detected by treating a salt with dilute H2SO4.
Observation
A brown coloured gas is evolved.
2 MNO2 + H2SO4 → M2SO4 + 2 HNO2 (nitrous acid, colourless)
3 MNO2 → H2O + HNO3 + NO
NO + O2 → 2 NO2 (Nitrogen dioxide, Reddish brown)
Confirmatory Test
When a filter paper soaked in a mixture of KI + starch solution is exposed to the gas, it turns blue or violet in colour.
2 NO2 + 2 KI → 2 KNO2 + I2
I2 + Starch - Starch iodide complex, deep blue or violet.