Procedure

Preparation of Lassaigne’s Extract

A small pellet of metallic sodium together with a little of the substance is heated to red hot in an ignition tube. It is then suddenly plunged into about 10 mL of distilled water in a china dish. The mixture is boiled well and filtered. The filtrate is used for doing the following tests.

Detection of Nitrogen

To a little of the filtrate, add equal volume of freshly prepared FeSO₄ solution that is boiled and cooled. Then add a few drops of FeCl₃ solution and acidify with dilute HCl.

Observation: Blue coloured precipitate indicates the presence of nitrogen.

Detection of Chlorine

To a little of the filtrate, add a few drops of dilute HNO₃, then boil and cool it. Add AgNO₃ solution.

Observation: A white precipitate soluble in ammonium hydroxide confirms the presence of chlorine.

Detection of Sulphur

To a little of the filtrate, add a few drops of freshly prepared sodium nitroprusside solution.

Observation: Appearance of brilliant violet colour indicates the presence of sulphur.