A brown ring is formed in the ring test for NO3 – ion. It is due to the formation of (i) [Fe(H2O)5 (NO)]2+ (ii) FeSO4.NO2 (iii) [Fe(H2O)4 (NO)]2+ (iv) FeSO4.HNO3
The identification and conformation test of nitrate ion ( NO3–) using conc. H_2SO_4 and freshly prepared FeSO4 , is called as 'Brown Ring Test. In a test tube take the NO3– containing sample .Then add conc.H2SO4 dropwise with gentle shaking and then heat it shortly using Bunsen burner & add freshly prepared greenish FeSO4. Then, we can observe a brown ring formed in that test tube due to the formation of brown colored [Fe(H2O)5(NO)]SO4 complex.
