हिंदी

Analysis of Cations - Analysis of Group-II Cations

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Estimated time: 5 minutes
CBSE: Class 12

Key Points: Analysis of Group II Cations

Group IIA (HgS, PbS, CuS, Bi₂S₃, CdS):

Dissolve in dil. HNO₃ → then dil. H₂SO₄ → Pb²⁺ precipitates as PbSO₄ (white)

Remaining sulphates of Bi³⁺, Cu²⁺, Cd²⁺ → NH₄OH (excess):

  • Bi³⁺ → white Bi(OH)₃ precipitate
  • Cu²⁺ → deep blue [Cu(NH₃)₄]SO₄ (tetramminecopper II) — confirms Cu²⁺
  • Cd²⁺ → colourless solution → add H₂S → yellow CdS precipitate

Test for Cu²⁺ Ion:

  • With NH₃ (excess): deep blue solution (tetrammine copper II complex)
  • With K₄[Fe(CN)₆]: chocolate brown ppt. of Cu₂[Fe(CN)₆]
  • With KCN (sparingly): yellow ppt. of Cu(CN)₂ (quickly decomposes → Cu₂(CN)₂; excess KCN → colourless [CuCN₄]³⁻)
  • With NaOH (cold): blue ppt. of Cu(OH)₂ → heated → black CuO
  • With KI: white CuI ppt. (intensely brown due to I₃⁻ formation)

Group IIB (As₂S₃, As₂S₅, Sb₂S₃, SnS, SnS₂):

  • Soluble in yellow ammonium sulphide [(NH₄)₂Sx]
  • Distinction: As₂S₃ insoluble in conc. HCl; Sb₂S₃ and SnS₂ soluble in conc. HCl
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