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In Lassaigne's extract of an organic compound, both nitrogen and sulphur are present, which gives blood red colour with Fe³⁺ due to the formation of -
Both N and S present ⟹ Na + C + N + S ⟶ NaSCN. Adding Fe³⁺ yields blood-red [Fe(SCN)]²⁺ in 5 seconds! (No Prussian blue forms because no free CN⁻ is present).
During the preparation of Lassaigne’s extract, the organic compound is fused with molten metallic sodium. When both nitrogen and sulphur are present in the organic framework, they react jointly with carbon and sodium to form sodium thiocyanate rather than separate sodium cyanide and sodium sulphide:
The fused mass is boiled with distilled water and filtered. The resulting alkaline sodium fusion extract contains soluble sodium thiocyanate (NaSCN), dissociating into sodium cations and thiocyanate anions:
When acidified with dilute hydrochloric acid and treated with a few drops of ferric chloride solution, ferric ions (Fe³⁺) coordinate with thiocyanate ions to form the soluble blood-red colored coordination complex ion:
Prussian blue, Fe₄[Fe(CN)₆]₃ · xH₂O, requires free cyanide ions (CN⁻). Because nitrogen is tied up as SCN⁻, no Prussian blue forms unless an excess of sodium is used to break thiocyanate into cyanide and sulphide.
Diagnose every choice: Know exactly why the right option works and why distractors fail.
Accurate formula of the monothiocyanatoiron(III) complex responsible for the blood-red color.
Prussian blue complex formed when ONLY nitrogen is present (free CN⁻ reacting with Fe²⁺/Fe³⁺).
NaSCN is the colorless intermediate in the extract, NOT the colored complex formed upon adding Fe³⁺.
Violet-colored sodium thionitroprusside complex formed in the sodium nitroprusside test for SULPHUR alone.
The question asks what chemical species imparts the BLOOD RED color upon reaction with Fe³⁺. NaSCN is the colorless salt in the extract; the colored species is the coordination complex ion [Fe(SCN)]²⁺.
"If both N and S are present, NaSCN is formed which gives a blood-red coloration with Fe³⁺ due to [Fe(SCN)]²⁺."
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