Test of nitrate ion

A nitrate test is a chemical test used to determine the presence of nitrate ion in solution.

A common nitrate test, known as the brown ring test can be performed by adding iron(II) sulfate to the solution, then slowly adding concentrated sulfuric acid (NOT nitric acid, since it gives nitrate ion which gives a positive result anyway) and watching for a brown ring on the test tube, which will indicate the presence of the nitrate ion. Note that the presence of nitrite ions will interfere with this test.

The test follows several phases; these have been written as balanced chemical equations:

1: 2 Zn(NO₃)₂ + 2 H₂SO₄ → 2 ZnSO₄ + 4 HNO₃

4 FeSO₄ + 2 H₂SO₄ → 2 Fe₂(SO₄)₃ + 2H₂

These two reactions happen simultaneously.

2: 4 HNO₃ → 2 H₂O + 3O₂ + 4 NO

The Nitric Acid decomposes in the intense heat produced by the high concentrate Sulphuric Acid used.

3: 2 Fe₂(SO₄)₃ + 2 H₂ + 4 NO → 4 [Fe(H₂O)5NO]SO₄ + 2 H₂SO₄

The [Fe(H₂O)5NO]SO₄ forms a brown ring in the middle of the solution produced by the reaction, making it easy to identify the presence of nitrates in the water.

An alternative is to use Devarda's alloy, (Cu/Al/Zn), a reducing agent, which when mixed with nitrate and sodium hydroxide liberates ammonia.