Test for Nitrogen in Organic Compounds

Nitrogen present in an organic compound is detected by Lassaigne’s test. The element present in the compound is converted from covalent form into the ionic form by fusing the compound with sodium metal.

Cyanide of sodium so formed on sodium fusion is extracted from the fused mass by boiling it with distilled water. This extract is known as sodium fusion extract.

The sodium fusion extract is boiled with iron(II) sulphate and then acidified with concentrated sulphuric acid. The formation of Prussian blue colour confirms the presence of nitrogen.

Sodium cyanide first reacts with iron(II) sulphate and forms sodium hexacyanoferrate(II). On heating with concentrated sulphuric acid some iron(II) ions are oxidised to iron(III) ions which react with sodium hexacyanoferrate(II) to produce iron(III) hexacyanoferrate(II) (ferriferrocyanide) which is Prussian blue in colour.

Nitrogen and Sulphur

In case, nitrogen and sulphur both are present in an organic compound, sodium thiocyanate is formed. It gives blood red colour and no Prussian blue since there are no free cyanide ions.