मराठी

Analysis of Cations - Analysis of Group-III Cations

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Estimated time: 4 minutes
CBSE: Class 12

Key Points: Analysis of Group III Cations

Analysis of Group III Cations (Fe³⁺, Al³⁺, Cr³⁺):

  • Group reagent: NH₄OH in presence of NH₄Cl
  • Precipitates: Al(OH)₃ (white), Fe(OH)₃ (red-brown), Cr(OH)₃ (green, cotton-like)
  • Add H₂O + Na₂O₂ to filtrate: Cr³⁺ oxidised → CrO₄²⁻ (yellow filtrate) confirms Cr
  • Only Al(OH)₃ and Cr(OH)₃ are soluble in NaOH (amphoteric); Fe(OH)₃ is insoluble

Test for Fe³⁺ Ion:

  • KCNS (potassium thiocyanate): Fe³⁺ + SCN⁻ → [Fe(SCN)]²⁺ → blood red colour
  • K₄[Fe(CN)₆] (potassium ferrocyanide): 4FeCl₃ + 3K₄[Fe(CN)₆] → Fe₄[Fe(CN)₆]₃ → Prussian blue (insoluble in dil. acids; NaOH converts to Fe(OH)₃ red + [Fe(CN)₆]³⁻)

Test for Al³⁺ Ion:

  • Ammonium sulphide: 2Al³⁺ + 3S²⁻ + 6H₂O → 2Al(OH)₃↓ + 3H₂S (white ppt.) → litmus paper turns red (acidic H₂S)
  • Lake test: Al(OH)₃ + blue litmus indicator → adsorbs blue dye → blue floating mass in colourless liquid (called "lake")

NH₄NO₃ cannot replace NH₄Cl in Group III because NO₃⁻ ions will oxidise Mn²⁺ → Mn⁴⁺ → MnO₂ precipitate (Mn will then precipitate in Group III instead of IV)

(NH₄)₂SO₄ cannot replace NH₄Cl because SO₄²⁻ will precipitate barium as BaSO₄ in Group V

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