Question

Explain about lassaigne’s test for detection of nitrogen in an organic compound.

Answer 1

I step:

Preparation of sodium fusion extract:

A small piece of Na dried by pressing between the folds of filter paper is taken in a fusion tube and it is heated. When it melts to a shining globule, a pinch of organic compound is added to it. The tube is then heated till the reaction ceases and becomes red hot.

Then the test tube is plunged in about 50 ml of distilled water taken in a china dish and break the bottom of the tube by striking against the dish. The contents of the dish is boiled for about 10 minutes and then filtered. This filtrate is known as lassaigne’s extract (or) sodium fusion extract.

II step :

Test for Nitrogen:

If Nitrogen is present, it gets converted to sodium cyanide which reacts with freshly prepared ferrous sulphate and ferric ion followed by cone. HCl and gives a Prussian blue colour (or) green coloured precipitate, it confirms the presence of nitrogen. HCl is added to dissolve the greenish precipitate of ferrous hydroxide produced by the action of NaOH on FeSO₄ which would otherwise mark the Prussian blue precipitate.

Reactions involved:

Na + C + N →  NaCN

FeSO₄ + 2NaOH → Fe(OH)₂ + Na₂SO₄

if both N & S are present. a blood red colour is obtained due to the following reactions