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 10ml 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 few drops of FeCl3 solution and acidify with dil HCl. Blue colour precipitate indicates the presence of nitrogen.
Detection of Chlorine:
To a little of the filtrate add few drops of dil HNO3 that is boiled and cooled. Then add AgNO3 solution to it. A white precipitate soluble in ammonium hydroxide confirms the presence of chlorine.
Detection of Sulphur:
To a little of the filtrate add few drops of freshly prepared sodium nitroprusside solution. Appearance of brilliant violet colour indicates the presence of sulphur.