Brown Ring Test

Brown ring test is used for the detection of nitrate ions in a solution. As the name suggests it detects presence of nitrate ions by formation of brown ring at the junction of two layers of the solution. This brown colored ring is formed because of the formation of brown colored iron complex which is called brown ring complex. This test is useful or sensitive 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 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 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 containing 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 mixture of iron sulphate solution + sample solution and 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 + 2NO + 4H2

[Fe(H2O)6]SO4        +            NO            🡪         [Fe(H2O)6]SO4             +  H2

                                                                          Nitrosonium complex 

Applications of Brown Ring Test 

Brown ring test is mainly used for the detection of nitrate. It is used to detect 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 soil makes it very acidic which is harmful for plants and farming. If nitrate compounds are present in water more than 10 milligrams per liter then it's not drinkable.