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.
(Image will be Uploaded Soon)
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.
(Image will be Uploaded Soon)
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. What is the primary purpose of the Brown Ring Test in chemistry?
The Brown Ring Test is a qualitative chemical test used to confirm the presence of the nitrate ion (NO₃⁻) in an aqueous solution. It is a common and reliable method used in salt analysis, particularly in the CBSE Class 11 and 12 curriculum for the academic year 2025-26.
2. What is the fundamental principle behind the formation of the brown ring?
The principle involves a two-step redox reaction. First, the nitrate ion (NO₃⁻) is reduced to nitric oxide (NO) by ferrous ions (Fe²⁺) in an acidic medium provided by concentrated sulphuric acid. The ferrous ions are oxidised to ferric ions (Fe³⁺). The nitric oxide produced then combines with excess ferrous ions to form a brown-coloured coordination complex, which is visible as a ring.
3. Can you describe the step-by-step procedure for conducting the Brown Ring Test?
The procedure for the Brown Ring Test involves the following steps:
- Take a small amount of the sample solution in a clean test tube.
- Add an equal volume of freshly prepared ferrous sulphate (FeSO₄) solution and mix it well.
- Incline the test tube and carefully pour concentrated sulphuric acid (H₂SO₄) along its inner side, allowing the acid to settle at the bottom as a distinct layer.
- A dark brown ring will form at the junction of the two layers if nitrate ions are present.
4. What is the chemical formula and IUPAC name of the compound that forms the brown ring?
The brown ring is formed by a coordination complex with the chemical formula [Fe(H₂O)₅NO]SO₄. According to IUPAC nomenclature, its name is Pentaaquanitrosyliron(I) sulphate. The characteristic brown colour is due to a charge transfer phenomenon within the complex.
5. Why is it essential to use a freshly prepared solution of ferrous sulphate for this test?
It is crucial to use a freshly prepared ferrous sulphate solution because ferrous ions (Fe²⁺) are readily oxidised by atmospheric oxygen to form ferric ions (Fe³⁺) over time. Ferric ions cannot form the required brown complex with nitric oxide. Using an old, oxidised solution would result in an insufficient concentration of Fe²⁺, potentially leading to a false negative result.
6. What is the oxidation state of iron in the final brown ring complex?
In the brown ring complex, [Fe(H₂O)₅NO]²⁺, the oxidation state of the central iron atom is +1. Although the reaction starts with Fe(II), the formation of the complex with the nitric oxide ligand results in this specific, and often misunderstood, oxidation state for iron.
7. What are some common limitations or interferences in the Brown Ring Test?
The test can be affected by the presence of other ions. For example:
- Nitrite ions (NO₂⁻) react with the acid to form a brown solution throughout, which can mask the ring.
- Bromide (Br⁻) and Iodide (I⁻) ions can be oxidised by the concentrated H₂SO₄ to produce brown (Br₂) or violet/brown (I₂) colours, interfering with the observation.
Therefore, these interfering ions must be removed before performing the test for nitrates.
8. Why does the Brown Ring Test not work for compounds like Barium Nitrate?
The test fails for Barium Nitrate (Ba(NO₃)₂) because adding the ferrous sulphate (FeSO₄) reagent immediately causes a precipitation reaction. The sulphate ions (SO₄²⁻) from the reagent react with the barium ions (Ba²⁺) in the sample to form Barium Sulphate (BaSO₄), which is a thick, white, insoluble precipitate. This precipitate obscures the junction between the layers, making it impossible to observe the formation of the brown ring.
