Qualitative Analysis of Nitrate Ion
On adding concentrated H2SO4 to a salt, if a brown coloured gas with pungent smell is evolved and the gas intensified on adding copper turning, NO3– may be present.
2MNO3 + H2SO4 → M2SO4 + 2HNO3
4HNO3 → 2H2O + O2 + 4NO2 (Nitrogen dioxide, light brown gas)
3Cu + 8HNO3 → 2Cu(NO3)2 + 2NO + 4H2O (Nitorgen oxide, colourless)
2NO + O2 → 2NO2 (Brown gas)
Confirmatory Test
Ring Test
On adding concentrated H2SO4 gradually along the side of the test tube to a mixture of salt solution in water or its sodium carbonate extract, acidified with dil. H2SO4, and freshly prepared ferrous sulphate solution, a brown ring is obtained at the junction of two layers.
M(NO3)2 + H2SO4 → MSO4 + 2HNO3
6FeSO4 + 2HNO3 + 3H2SO4 → 3Fe2(SO4)3 + 4H2O
X FeSO2 + Y NO → (FeSO4)x (NO)y
Nitrosoferrous sulphate (brown ring)
Fe2+ + 5H2O + NO → [Fe(H2O)5NO]2+