Lassigne test for nitrogen

Detection of nitrogenA small quantity of the sodium extract is taken in a test tube. It is made alkaline by adding 2-3 drops of sodium hydroxide (NaOH) solution. 1 mL of freshly prepared solution of ferrous sulphate is added to this solution. The mixture of the two solutions is boiled and then acidified with dilute sulphuric acid. The appearance of prussian blue or green colouration of the precipitate confirms the presence of nitrogen in the given organic compound.The carbon and nitrogen present in the organic compound on fusion with sodium metal give sodium cyanide (NaCN) soluble in water. So, the sodium extract contains sodium cyanide which, on reaction with ferrous sulphate, gives sodium ferrocyanide. Some of the ferrous salt is oxidised to the ferric salt on heating and this reacts with sodium ferrocyanide to form ferric ferrocyanide.

Note: When nitrogen and sulphur both are present in any organic compound, sodium thiocyanate is formed during fusion. When extracted with water sodium thiocyanate goes into the sodium extract and gives 'blood red coloration' with ferric ions due to the formation of ferric thiocyanate.

A method of testing for the presence of a halogen, nitrogen, or sulphur in an organic compound. A sample is heated in a test tube with a pellet of sodium. The hot tube is dropped into pure water and the fragments ground up in a mortar. The presence of a halogen (now in the form of a sodium halide) is detected by precipitation with silver nitrate solution. Nitrogen is revealed by the formation of a precipitate of Prussian blue on heating part of the solution with iron(II) sulphate solution containing hydrochloric acid and a trace of iron(III) ions. Lead ethanoate or sodium nitroprusside gives a precipitate with any sulphur present