A common nitrate test, known as the brown ring test can be 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 will form at the junction of the two layers, indicating the presence of the nitrate ion.  Note that the presence of nitrite ions will interfere with this test.
The overall reaction is the reduction of the nitrate ion to nitric oxide by iron(II), which is oxidised to iron(III), followed by the formation of nitrosyl ferrous sulfate between the nitric oxide and the remaining iron(II), where nitric oxide is reduced to NO.
:2HNO + 3HSO + 6FeSO  → 3Fe(SO) + 2NO + 4HO
:[Fe(HO)]SO + NO → [Fe(HO)(NO)]SO + HO
This test is sensitive up to 2.5 micrograms and a concentration of 1 in 25,000 parts.