BROWN RING TEST
Aim
To determine the presence of nitrate ion in any solution
How It works
The test is based on the fact that nitrate ion acts as an oxidizing agent. In the
reaction mixture, reduction of nitrate ion takes place by iron (II) and iron(II) gets
oxidized to iron (III). Nitric oxide is reduced to NO- and forms a nitrosonium
complex, which forms a brown ring at the junction of two layers.
Brown Ring Test Procedure
1. Take the test solution in a clean, dry test tube.
2. Add a small amount of iron(II) sulfate to the test tube, using a spatula or a
small spoon. The amount of iron(II) sulfate added should be just enough to
cover the bottom of the test tube.
3. Mix the contents of the test tube by shaking it gently.
4. Carefully add concentrated sulfuric acid to the test tube so that it forms a
concentrated layer at the bottom of the tube. To do this, tilt the test tube at
an angle and slowly pour the sulfuric acid down the side of the tube, taking
care to avoid splashing or mixing the two layers.
5. Allow the test tube to stand for a few minutes, to allow the layers to
separate.
6. Observe the test tube for the formation of a brown ring at the junction of
the two layers. If a brown ring is observed, it indicates the presence of
nitrates in the test solution
Observation
A brown ring is formed at the junction of two layers (one layer of a mixture of iron
sulphate solution + sample solution and the second layer of concentrated
sulphuric acid solution).
Result
Brown ring confirms the presence of nitrate in the sample.
Brown Ring Test Reactions
Following reactions take place during brown ring test – reduction of the nitrate
ion, oxidation of iron(II) and reduction of nitric oxide.
2HNO3 + 3H2SO4 + 6FeSO4 → 3Fe2(SO4)3 + NO + 4H2O
The nitrate, if present in the given sample, reacts with ferrous sulphate solution and
gets reduced to nitric oxide. Ferrous (Fe2+) ions are oxidised to ferric (Fe3+) ions.
The nitric oxide that got formed as a product of the reduction, now reacts with the
remaining ferrous (Fe2+) ions to give a nitrosyl complex.
Where [Fe(H2O)5NO]SO4 is the nitrosyl complex.
The nitric oxide is reduced to give NO-. This leads to the formation of a
brown-coloured ring at the junction of the two layers. The brown ring thus
formed is the confirmation of the presence of nitrate ion in the given solution.
Applications of Brown Ring Test
Brown ring test is mainly used for the detection of nitrate. It is used to detect the
presence of nitrate in many food samples, soil, and water. As high quantities of
nitrate in food cause food poisoning and high quantities of nitrate in the soil make
it very acidic, which is harmful to plants and farming. If nitrate compounds are
present in water more than 10 milligrams per litre, then it's not drinkable.