Q. Brown ring test is used for the detection of:
Answer: nitrate
Notes: Brown Ring test is a common qualitative test for the nitrate ion. The test is performed by adding iron(II) sulfate to a solution of a nitrate, then slowly adding concentrated sulfuric acid such that the acid forms a layer below the aqueous solution. A brown-ring forms at the junction of the two layers, indicating the presence of the nitrate ion. The overall reaction is the reduction of the nitrate ion by iron(II) which is oxidised to iron(III) and formation of a nitrosonium complex where nitric oxide is oxidised to NO+.

• 2HNO3 + 3H2SO4 + 6FeSO4 —> 3Fe2(SO4)3 + 2NO + 4H2O
• [Fe(H2O)6]SO4 + NO = [Fe(H2O)5(NO)]SO4 + H2O