Brown ring test is used for the detection of nitrate ions in a solution. As the name suggests, it detects the presence of nitrate ions by the formation of a brown ring at the junction of two layers of the solution. This brown coloured ring is formed because of the formation of the brown-coloured iron complex, which is called the brown ring complex. This test is useful or sensitive to up to 2.5micrograms and a concentration of 1 in 25,000 parts.
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Brown Ring Test
Aim
To detect nitrate in the given sample.
Requirements
Test tubes, freshly prepared iron sulphate solution, concentrated sulphuric acid.
Principle
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: Test for Nitrate Ion
Brown ring test experiment for nitrate ions can be performed by using the following steps –
Take a sample in which you suspect nitrate is present in a test tube.
Prepare fresh iron sulphate solution.
Now mix freshly prepared iron sulphate solution in the sample.
Add concentrated sulphuric acid solution along the side to the bottom of the test tube with the test tube tilted, which contains the mixture of sample and freshly prepared iron sulphate solution. Concentrated sulphuric acid is added in such a way that it forms a layer below the aqueous solution.
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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 –
2HNO3 + 3H2SO4 + 6FeSO4 → 3Fe2(SO4)3 + NO + 4H2O
(Fe(H2O)6)SO4 + NO → (Fe(H2O)5NO)SO4 + H2O
Where, (Fe(H2O)5NO)SO4 is given as a nitrosyl complex.
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.
FAQs on Brown Ring Test
1. Why is there no Ring test for Barium Nitrate?
We mix the supposedly Nitrate-containing salt with freshly made Iron(II) Sulfate for the Ring Test. At room temperature, Barium Sulfate has very poor solubility and will precipitate out as a white precipitate. The test cannot be completed with the precipitate in the same test tube.
2. Which salts don't show up on a brown ring test?
The oxidation of iron (II) ions from an iron (II) sulphate solution is used in the brown ring test. As nitrate anions are oxidizers, this test is commonly used to detect nitrates. However, any solution containing an oxidant may cause the brown ring to appear.
Any salt that does not produce a strong enough oxidizing ion will, for the most part, fail the test.
Salts like sulphates, carbonates, and chlorides are likely to show negative results.
3. Explain the presence of the Brown Ring Test.
The brown ring test is a common nitrate test. This test can be used to show the presence of nitrate in a combination or solution. Take the nitrate solution. a grain of iron(II) sulphate, Pour the mixture in strong sulphuric acid until it creates a layer beneath the watery solution. At the intersection of the two layers, a brown ring will appear. This shows that the nitrate ion is present. In this process, the nitrate is reduced and the iron is oxidized.
