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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2026 · Paper I

Q8(c)(iii). Separation of the lanthanides by complex formation (ion exchange)

  • 5 marks
  • Answered
Question

How are lanthanides separated by a complex formation method? Give description.

Answer in brief

Ion exchange: Ln³⁺ ions are held on a cation-exchange resin and washed off with a buffered complexing agent. Smaller, heavier ions form more stable complexes and leave first: Lu first, La last.

Key points

  • 3R-SO₃H + Ln³⁺ ⇌ (R-SO₃)₃Ln + 3H⁺; all the ions sit in a narrow band at the top.
  • the ammonium salt of a complexing agent is passed down, and each ion is shared between resin and solution: Ln³⁺(resin) + Lⁿ⁻ ⇌ [LnL]⁽ⁿ⁻³⁾⁻(aq).
  • the stability constant rises from La to Lu as the ion shrinks, so heavier ions spend more time in solution and move faster.
  • each ion is taken up and released many times, so a small difference builds into separate bands. Fractions are precipitated as oxalate and ignited to Ln₂O₃. Order: Lu first, La last.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. Loading: 3R-SO₃H + Ln³⁺ ⇌ (R-SO₃)₃Ln + 3H⁺; all the ions sit in a narrow band at the top.
  2. Elution: the ammonium salt of a complexing agent is passed down, and each ion is shared between resin and solution: Ln³⁺(resin) + Lⁿ⁻ ⇌ [LnL]⁽ⁿ⁻³⁾⁻(aq).
  3. Why they separate: the stability constant rises from La to Lu as the ion shrinks, so heavier ions spend more time in solution and move faster.
  4. Many steps: each ion is taken up and released many times, so a small difference builds into separate bands. Fractions are precipitated as oxalate and ignited to Ln₂O₃. Order: Lu first, La last.

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Model answer to Q8(c)(iii), 2026 Paper I: solution page 54
Solution page 54 of 55
Read the whole 2026 solution2026 question paper

All parts of Question 8