Theory & Procedure, Detection of Elements (Lassaigne Test) - Class 11 PDF Download

Objective:

Our objective is to detect the presence of nitrogen, sulphur, chlorine, bromine and iodine in organic compounds by Lassaigne's test.

The Theory:

Carbon is the main constituent element of all organic compounds. Hydrogen is also present in most of the organic compounds, but there are few exceptions, such as: CCl₄, CS₂ , etc. Elements other than these elements such as nitrogen, sulphur and halogens (chlorine, bromine and iodine) may also be present in organic compounds. These extra elements are usually detected by Lassaigne's Test that was developed by the French Chemist J.L Lassaigne. In this test, the organic compound is fused with metallic sodium to convert these elements into water soluble sodium salt. Usual qualitative tests are performed on this extract for the detection of respective elements.

Detection of Nitrogen

If nitrogen is present in the compound, the Lassaigne's extract would contain sodium cyanide formed during fusion. Sodium cyanide is converted to sodium ferrocyanide on treating with ferrous sulphate. On further treating it with ferric chloride, a prussian blue complex, ferricferrocyanide is formed.

Detection of Sulphur

1. Sodium nitroprusside test

During the preparation of Lassaigne's extract, sulphur from the organic compound reacts with sodium to form sodium sulphide. It gives a purple colour with sodium nitroprusside due to the formation of sodium thionitroprusside.

2. Lead acetate test

Sodium sulphide formed during the preparation of Lassaigne's extract reacts with lead acetate to yield lead sulphide as black precipitate.

Detection of halogens

1. Silver nitrate test

a. Chlorine

During the preparation of Lassaigne's extract, chlorine form the organic compound reacts with sodium to form sodium chloride. Sodium chloride gives a white precipitate of silver chloride with silver nitrate solution. The precipitate is soluble in ammonium hydroxide.

b. Bromine

Sodium bromide formed during the preparation of Lassaigne's extract reacts with silver nitrate to form pale yellow precipitate of silver bromide, which is sparingly soluble in ammonium hydroxide.

c. Iodine

Sodium iodide formed during the preparation of sodium fusion extract reacts with silver nitrate solution to form yellow precipitate of silver iodide, which is insoluble in ammonium hydroxide.

2. Carbon disulphide test

When sodium bromide and sodium iodide in the Lassaigne's extract is treated with chlorine water, the bromide and iodide oxidise to the corresponding halogens. This halogen dissolves in carbon disulphide. Bromine imparts orange colour and iodine imparts violet colour in carbon disulphide layer.

Learning Outcomes:

• Students acquire the skill to prepare Lassaigne's extract.
• Students understand various tests to detect the presence of nitrogen, sulphur and halogens in an organic compound.
• Students understand the chemical reactions and their balanced equations that takes place during each test.
• Students acquire the skill to perform the experiment in the real lab once they understand different steps in the procedure.

Preparation of Sodium Fusion Extract (Lassaigne’s Extract)

Materials Required

Real Lab Procedure

• Take a small piece of dry sodium  in a fusion tube.
• Heat the tube slightly on a Bunsen burner so that the sodium melts to a shining globule.
• Add a pinch of the organic compound.
• Heat it slowly to start with so that the compound reacts with sodium metal.
• Now heat the tube strongly till it becomes red hot.
• Plunge the red hot tube into a china dish containing distilled water.
• Crush the contents with a glass rod and heat to boiling point.
• Stop heating and remove the insoluble matter by filtration.
• The filtrate is called Lassaigne's Extract.

Detection of Nitrogen

Materials Required

Real Lab Procedure 

Experiment Observation Inference

Detection of Sulphur

Materials Required

Real Lab Procedure 

Test Experiment Observation Inference

Detection of Halogens

Materials Required

Real Lab Procedure 

Test Experiment Observation Inference

Simulator Procedure (as performed through the Online Labs)

Detection of Nitrogen

• You can select the unknown element from the ‘Select the unknown element’ drop down list.
• Select the identification test from the ‘Select the identification test’ drop down list.
• To pour ferrous sulphate (FeSO₄) solution into the Lassaigne’s extract, drag the test tube towards it.
• Click on the knob of the burner to turn it on.
• Drag the test tube over the Bunsen burner to heat it.
• Drag the test tube to place it back.
• To pour conc. HCl into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
• To pour ferric chloride (FeCl₃) solution into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
• To see the inference, click on the inference icon.
• You can choose the correct answer from ‘Select the element’ drop down list.
• Click on the ‘Reset’ button to redo the experiment.

Detection of Sulphur

• You can select the unknown element from the ‘Select the unknown element’ drop down list.
• You can select the identification test from the ‘Select the identification test’ drop down list.
• You can select the identification method from the ‘Select the method’ drop down list.

a) Sodium nitroprusside test:

• Lassaigne’s extract is taken in a test tube.
• Drag the dropper towards the test tube to pour the sodium nitroprusside solution into it.
• To see the inference, click on the inference icon.
• You can choose the correct answer from ‘Select the element’ drop down list.
• Click on the ‘Reset’ button to redo the experiment.

b) Lead acetate test:

• To pour acetic acid into the test tube containing Lassaigne’s extract, drag the test tube containing acetic acid towards it.
• To pour lead acetate solution into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
• To see the inference, click on the inference icon.
• You can choose the correct answer from ‘Select the element’ drop down list.
• You can redo the experiment by clicking on the ‘Reset’ button.

Detection of Chlorine

Silver nitrate test

• You can select the unknown element from the ‘Select the unknown element’ drop down list.
• You can select the identification test from the ‘Select the identification test’ drop down list.
• To pour conc. HNO₃ into the test tube containing Lassaigne’s extract, drag the test tube towards it.
• Click on the knob of the burner to turn it on.
• Drag the test tube over the Bunsen burner to heat it.
• Once done, drag the test tube to place it back.
• To pour silver nitrate (AgNO₃) solution into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
• To pour ammonium hydroxide (NH₄OH) solution into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
• To see the inference, click on the inference icon.
• You can choose the correct answer from ‘Select the element’ drop down list.
• You can click on the ‘Reset’ button to redo the experiment.

Detection of Bromine and Iodine

• You can select the unknown element from ‘Select the unknown element’ drop down list.
• You can select the identification test from ‘Select the identification test’ drop down list.
• You can select the identification method from ‘Select the method’ drop down list.

a) Silver nitrate test:

• To pour conc. HNO₃ into the test tube containing Lassaigne’s extract, drag the test tube towards it.
• Click on the knob of the burner to turn it on.
• Drag the test tube over the Bunsen burner to heat it.
• Drag the test tube to place it back.
• To pour silver nitrate (AgNO₃) solution into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
• To pour ammonium hydroxide (NH₄OH) solution into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
• To see the inference, click on the inference icon.
• You can choose the correct answer from ‘Select the element’ drop down list.
• You can click on the ‘Reset’ button to redo the experiment.

b) Carbon disulphide test:

• To pour dil. HCl into the test tube containing Lassaigne’s extract , drag the dropper towards the test tube.
• To pour Carbon disulphide into the test tube containing Lassaigne’s extract , drag the dropper towards the test tube.
• To pour chlorine water into the test tube containing Lassaigne’s extract, drag the test tube towards it.
• Drag the test tube to take it from the test tube stand and to shake it well.
• To see the inference, click on the inference icon.
• You can choose the correct answer from ‘Select the element’ drop down list.
• You can click on the ‘Reset’ button to redo the experiment.

Note: Click on the ‘HELP’ button to see the instructions.

Precautions:

• Sodium metal is highly reactive. When exposed to air, it reacts even with the moisture present in the atmosphere. It also reacts with the sweat of hands. Hence do not hold it with hands. Always use forceps while handling sodium.
• Before using sodium, press it within the folds of the filter paper to remove kerosene oil.
• Use dry ignition tubes for sodium fusion. Sodium reacts with water violently.
• Put the unused sodium piece back in the bottle containing kerosene oil. Do not throw it into the sink.
• Repeat the process of sodium fusion with three ignition tubes. This is to ensure that the fusion has taken place.
• After immersing the red hot ignition tube in water, break it with a glass rod gently. Boil the contents for 2-3 minutes so as to extract the soluble sodium salts in water.