Lassaigne's Test for Nitrogen Detection

Lassaigne's test is used to detect the presence of Nitrogen, Sulphur, and Halogens in organic compounds. The formation of a specific colored precipitate upon adding reagents helps identify the element.

Prussian Blue Formation

When the sodium fusion extract (obtained after fusing the organic compound with sodium metal) is heated with iron(II) sulphate solution, the formation of a deep blue precipitate, known as Prussian blue, indicates the presence of Nitrogen.

Chemical Basis

The detection relies on the formation of sodium cyanide ($NaCN$) during the Lassaigne's test if Nitrogen is present in the organic compound. This cyanide then reacts with iron salts:

• The sodium fusion extract contains sodium cyanide ($NaCN$).
• Upon heating with iron(II) sulphate ($FeSO_4$), sodium hexacyanoferrate(II) is formed: $6 NaCN + FeSO_4 \rightarrow Na_4[Fe(CN)_6] + 2 Na_2SO_4$
• This intermediate complex, sodium hexacyanoferrate(II) ($Na_4[Fe(CN)_6]$), reacts with any present iron(III) ions ($Fe^{3+}$) (often from the iron(II) sulphate solution itself or impurities) to form the characteristic Prussian blue precipitate. The exact composition varies, but it's represented as ferric ferrocyanide: $Fe^{3+} + \frac{3}{4}[Fe(CN)_6]^{4-} \rightarrow \frac{1}{4} Fe_4[Fe(CN)_6]_3 \downarrow \text{ (Prussian Blue)}$

Therefore, the observation of a Prussian blue precipitate confirms the presence of Nitrogen in the original organic compound.