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Brown Ring Test

Brown Ring Test

The brown ring test, known as the nitrate test, is a chemical method for detecting nitrate ions in a sample. This standard qualitative method confirms the presence of nitrate ions by producing a distinctive brown-coloured ring at the interface of two layers in the reaction mixture.

1.0Introduction

The brown ring test is used to detect nitrate ions in a solution. It forms a brown ring at the interface of two layers in the solution to create a brown-coloured iron complex known as the brown ring complex. This brown ring indicates the presence of nitrate ions in the solution. The test is sensitive and can detect nitrate ions in concentrations as low as 2.5 micrograms or 1 in 25,000 parts.

2.0Brown Ring Test

Procedure:

  • Add Ferrous Sulfate: Ferrous sulfate is added to the solution suspected of containing nitrate ions.
  • Acidify with Sulfuric Acid: The mixture is carefully acidified with concentrated sulfuric acid.

Observation:

  • A brown-coloured ring forms at the interface of the ferrous sulfate and sulfuric acid layers. This ring indicates the presence of nitrate ions in the solution.

Chemical Reaction:

  • Nitrate ions are reduced to nitric oxide (NO) by ferrous (II) ions from ferrous sulfate, which are oxidised to ferric (III) ions.
  • Nitric oxide reacts with any remaining ferrous (II) ions to form a nitrosyl complex, which appears as the brown ring. This complex is formed when nitric oxide is reduced to NO⁺.

Overall Reaction involved : 

The overall reaction involved in the Brown Ring Test includes the reduction of the nitrate ion, the oxidation of iron(II), and the reduction of nitric oxide.

  • 2HNO3 + 3H2SO4 + 6FeSO4 → 3Fe2(SO4)3 + 2NO + 4H2O

Formation of the Nitrosyl Complex: (Remaining)

  • [Fe(H2O)6]SO4  + NO → [Fe(H2O)5(NO)]SO4 + H2O

3.0Experimental Procedure: Detection of Nitrate Ions

Aim: To detect the presence of nitrate ions in the given sample.

Apparatus Required:

  • Test tubes
  • Freshly prepared ferrous sulfate solution
  • Concentrated sulfuric acid solution

Theory: The brown ring test relies on the reaction between ferrous sulfate and concentrated sulfuric acid. The reaction is a reduction process where nitrate ions, if present, react with ferrous sulfate solution and are reduced to nitric oxide. Ferrous (Fe²⁺) ions are oxidised to ferric (Fe³⁺) ions. The nitric oxide then reacts with the remaining ferrous ions to form a nitrosyl complex, [Fe(H2O)5(NO)]SO4, resulting in a brown-coloured ring at the interface of the two layers. This ring confirms the presence of nitrate ions in the sample.

Procedure:

  • Take the sample containing nitrate ions in a test tube.
  • Prepare a fresh solution of ferrous sulfate in another test tube.
  • Mix the freshly prepared ferrous sulfate solution with the test tube containing the sample.
  • Carefully add concentrated sulfuric acid along the sides of the same test tube.
  • While adding the acid, tilt the test tube slightly to ensure it reaches the bottom, forming two distinct layers.
  • Observe the junction of the two layers for the formation of a brown-coloured ring.
  • Do not shake the test tube, which may cause the brown ring to disappear into the solution.

Nitrate Ion test - Brown Ring Test

Observation: A brown-coloured ring appears at the interface between the two layers formed by the mixture of the sample, ferrous sulfate solution, and sulfuric acid. This brown ring indicates the presence of nitrate ions in the sample.

Note: The brown ring test is sensitive to nitrate concentrations as low as 2.5 micrograms, or 1 part in 25,000

4.0Uses of the Brown Ring Test

  • Nitrates added to the soil through fertilisation can be used by plants for growth or absorbed by the soil. However, high levels of nitrates can leach into groundwater, posing a risk to human health. Testing is essential to ensure that nitrate levels in healthy water are within safe limits.
  • Nitrates in water are tasteless, odourless, and colourless, making them difficult to detect without proper testing. In contrast, high sulfur levels in water are usually noticeable due to their unpleasant taste and smell. Such tests are necessary for the presence of nitrates in drinking water to be noticed, potentially leading to health hazards.

Frequently Asked Questions

Sulfuric acid is used in the brown ring test because: It creates a highly acidic environment for reducing nitrate ions to nitric oxide. Its density allows it to form a distinct layer below the ferrous sulfate solution, which is crucial for visual detection of the brown ring. It acts as a dehydrating agent, facilitating the formation of the nitrosyl complex. It can oxidise ferrous ions to ferric ions, aiding the reaction.

The formation of a nitrosyl complex causes the brown ring in the brown ring test [Fe(H2O)5(NO)]SO4. The brown ring indicates the presence of nitrate ions NO₃⁻ in the solution.

Freshly prepared ferrous sulfate (FeSO₄) is used in the brown ring test to: (1). Prevent Oxidation: Ensures Fe²⁺ remains in its reduced form, which is crucial for accuracy. (2). Ensure Effective Reduction: Fe²⁺ is needed to reduce nitrate ions to nitric oxide. (3). Ensure Reliable Results: Active Fe²⁺ ions are essential for consistent test results.

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