मराठी

Detection of Elements: Lassaigne's Test - Detection of Nitrogen

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Estimated time: 3 minutes
CBSE: Class 12

Key Points: Detection of Nitrogen

  • SFE is boiled with freshly prepared FeSO₄ solution, then acidified with conc. HCl, then 2–3 drops of FeCl₃ added
  • Prussian blue colour confirms nitrogen

Reactions:

  • FeSO₄ + 2NaOH → Fe(OH)₂ + Na₂SO₄
  • 6NaCN + Fe(OH)₂ → Na₄[Fe(CN)₆] + 2NaOH
  • 3Na₄[Fe(CN)₆] + 4FeCl₃ → Fe₄[Fe(CN)₆]₃ (Prussian blue) + 12NaCl

Exceptions: Test NOT given by compounds lacking C atoms that contain N (e.g., NH₂NH₂, NH₂OH) because CN⁻ ion won't form. Also NOT given by diazonium compounds.

When BOTH N and S are present:

  • If Na is in insufficient amount → NaSCN (thiocyanate) forms instead of NaCN
  • SCN⁻ + Fe³⁺ → [Fe(SCN)]²⁺ → Blood red colour (instead of Prussian blue)
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