When sodium fusion extract of an organic compound is boiled with iron (II) sulphate solution followed by addition of concentrated \(\text{H}_2\text{SO}_4\), gives Prussian blue colour. This confirms the presence of the element
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If both Nitrogen and Sulphur are present, Sodium Thiocyanate (\(\text{NaSCN}\)) is formed. This gives a blood-red colour with \(\text{Fe}^{3+}\) due to the formation of \([\text{Fe(SCN)}]^{2+}\), not Prussian blue.
Step 1: Understanding Lassaigne's Test:
The test described is the standard qualitative analysis test for Nitrogen in organic compounds, known as Lassaigne's test. The organic compound is fused with sodium metal to convert covalent nitrogen into ionic sodium cyanide.
Step 2: Chemical Reactions Involved:
1. Fusion: Carbon and Nitrogen from the organic compound react with Sodium metal.
\[ \text{Na} + \text{C} + \text{N} \xrightarrow{\Delta} \text{NaCN} \]
2. Reaction with Iron(II) Sulphate: The extract containing cyanide ions is boiled with ferrous sulphate (\(\text{FeSO}_4\)). It forms sodium ferrocyanide (Sodium hexacyanoferrate(II)).
\[ \text{FeSO}_4 + 2\text{NaOH} \rightarrow \text{Fe(OH)}_2 + \text{Na}_2\text{SO}_4 \]
\[ \text{Fe(OH)}_2 + 6\text{NaCN} \rightarrow \text{Na}_4[\text{Fe(CN)}_6] + 2\text{NaOH} \]
3. Formation of Prussian Blue: Upon adding concentrated sulphuric acid, some ferrous ions (\(\text{Fe}^{2+}\)) are oxidised to ferric ions (\(\text{Fe}^{3+}\)). These ferric ions react with the ferrocyanide complex to form a blue precipitate of Iron(III) hexacyanoferrate(II), known as Prussian Blue.
\[ 3\text{Na}_4[\text{Fe(CN)}_6] + 4\text{Fe}^{3+} \rightarrow \text{Fe}_4[\text{Fe(CN)}_6]_3 \cdot x\text{H}_2\text{O} \ (\text{Prussian Blue}) + 12\text{Na}^+ \]
Conclusion:
The appearance of the characteristic Prussian blue colour confirms the presence of the cyanide ion, which in turn confirms the presence of Nitrogen in the original organic compound.
