Detection of Elements: Lassaigne’s Test - MeitY OLabs - YouTube

Detection of Elements: Lassaigne's Test

Preparation of Lassaigne's Extract

We require china dish, organic compound, funnel, tripod stand, wire gauze, distilled water,

Bunsen burner, fusion tubes, test tube, sodium metal, small spatula, forceps etc.

Take a small piece of sodium metal using the forceps.

Place the sodium metal piece on a filter paper and dry it by pressing it with another filter

paper.

Take dry sodium using the forceps.

Hold a fusion tube using a tongs and put the dry sodium in it.

Heat the fusion tube slightly over the Bunsen burner, so that the sodium melts to a shining

globule.

Take a pinch of organic compound using the spatula.

Add the organic compound into the fusion tube.

Replace the spatula.

Heat the fusion tube over the Bunsen burner till it becomes red hot.

Plunge the red hot tube into the china dish containing distilled water.

The tube will break.

Crush the tube with a glass rod.

Repeat these processes 2-3 times and heat the contents of the china dish to boiling

point for few minutes.

Stop heating when the volume of the liquid in the china dish is reduced to 1/3rd of its

initial volume.

Take a filter paper.

Place the filter paper in the funnel, which is fixed to a stand, and filter the contents

of the china dish through the funnel to a clean test tube placed under the funnel.

The filtrate is called Lassaigne's Extract.

Detection of Nitrogen

We require, freshly prepared ferrous sulphate solution, ferric chloride solution, concentrated

hydrochloric acid, test tube, Lassaigne's extract and droppers.

Take a small amount of Lassaigne's extract using a dropper and pour it into a clean test

tube.

Using another dropper, take a small amount of freshly prepared ferrous sulphate solution.

Pour ferrous sulphate solution into the test tube containing Lassaigne's extract and hold

the test tube with a test tube holder.

Heat the test tube over the Bunsen burner.

The sodium cyanide formed during the preparation of Lassaigne's extracts is converted to sodium

ferrocyanide, on treating with ferrous sulphate.

Place the test tube in the test tube rack.

Take a small amount of ferric chloride solution using another dropper.

Add ferric chloride solution into the contents of the test tube.

Using another dropper, take small amount of concentrated hydrochloric acid.

Add concentrated hydrochloric acid into the contents of the test tube.

On further treating with ferric chloride and concentrated hydrochloric acid, sodium ferrocyanide

forms a Prussian blue complex, ferric ferrocyanide.

Detection of Sulphur

Sodium Nitroprusside Test

We require sodium nitroprusside solution, test tube, Lassaigne's extract and droppers.

Take a small amount of Lassaigne's extract using a dropper and pour it into a clean test

tube.

Using another dropper, take a small amount of sodium nitroprusside solution.

Add few drops of sodium nitroprusside solution into the test tube containing Lassaigne's

extract.

The sodium sulphide formed during the preparation of Lassaigne's extract gives a purple/violet

colour with sodium nitroprusside, due to the formation of sodium thionitroprusside,

Lead Acetate Test We require, acetic acid, lead acetate solution,

test tube, Lassaigne's extract and droppers.

Take a small amount of Lassaigne's extract using a dropper and pour into the clean test

tube.

Using another dropper, take a small quantity of acetic acid.

Add acetic acid into the test tube containing Lassaigne's extract.

Take small amount of lead acetate solution using another dropper.

Add few drops of lead acetate solution into the contents of the test tube.

Sodium sulphide formed during the preparation of Lassaigne's extract reacts with lead acetate

and forms a black precipitate of lead sulphide.

Detection of Halogens

Silver nitrate test

We require, concentrated nitric acid, silver nitrate solution, ammonium hydroxide solution,

test tube, Lassaigne's extract, droppers etc.

Take a small amount of Lassaigne's extract using a dropper and pour it into the clean

test tube.

Using another dropper, take small amount of concentrated nitric acid.

Hold the test tube using a test tube holder and add concentrated nitric acid into the

Lassaigne's extract.

Boil the content of the test tube over the Bunsen burner for some time.

Cool the contents of the test tube.

Take small amount of silver nitrate solution using another dropper.

Add silver nitrate into the contents of the test tube.

If the organic compound contains chlorine, the sodium chloride formed during the preparation

of Lassaigne's extract reacts with silver nitrate and forms a white precipitate of silver

chloride.

If the organic compound contains bromine, the sodium bromide formed during the preparation

of Lassaigne's extract reacts with silver nitrate to form pale yellow precipitate of

silver bromide.

If the organic compound contains iodine, the sodium iodide formed during the preparation

of Lassaigne's extract reacts with silver nitrate to form yellow precipitate of silver

iodide.

Take ammonium hydroxide solution using another dropper.

Take the test tube containing white precipitate of silver chloride.

Add excess of ammonium hydroxide into the silver chloride precipitate and shake the

test tube well.

Silver chloride is soluble in excess of ammonium hydroxide due to the formation of a soluble

silver complex.

Take ammonium hydroxide solution using the dropper.

Take the test tube containing pale yellow precipitate of silver bromide.

Add excess of ammonium hydroxide into the silver bromide precipitate and shake the test

tube well.

Silver bromide is sparingly soluble in excess of ammonium hydroxide.

Again take ammonium hydroxide using the dropper.

Take the test tube containing yellow precipitate of silver iodide.

Add excess of ammonium hydroxide into the silver iodide precipitate and shake the test

tube well.

Silver iodide is insoluble in ammonium hydroxide solution.

Carbon Disulphide Test

We require, chlorine water, carbon disulphide, dilute hydrochloric acid, test tube, Lassaigne's

extract and droppers.

Take small amount of Lassaigne's extract using a dropper and pour it into a clean test tube.

Using another dropper, take small amount of dilute hydrochloric acid.

Add dilute hydrochloric acid into the test tube containing Lassaigne's extract.

Take small amount of carbon disulphide using another dropper.

Add carbon disulphide to the test tube.

Carbon disulphide forms a separate layer above the Lassaigne's extract.

Take chlorine water using another dropper.

Take the test tube containing Lassaigne's extract.

Pour chlorine water to the test tube and shake it well.

When sodium bromide formed in the Lassaigne's extract is treated with chlorine water, the

bromide is oxidised to bromine.

Bromine dissolves in carbon disulphide and imparts orange colour in carbon disulphide

layer.

When sodium iodide formed in the Lassaigne's extract is treated with chlorine water, the

iodide is oxidised to iodine.

Iodine dissolves in carbon disulphide and imparts violet colour in carbon disulphide

layer.