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Brown ring test :

In brown ring test dilute FeSO4 is addedto an aqueous solution of nitrate ion.Concentrated H2SO4 is then added along the sided of the test tube. A brown ring is observed at the interface between the solution and H2SO4 layers indicates the presence of nitrate ion in the solution

NO3?  + 3Fe^+2 + 4H+ -----> NO + 3Fe^3+ + 2H2O

[Fe(H2O)6]^2+ + NO ------> [Fe(H2O)5(NO)]^2+ + H2O