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6.2.2Unit 6. Practical Skills in Chemistry II

Redox Titration -Thiosulfate and Iodine — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 11 of 14 Unit quiz

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Part 1 of 6The Chemistry

6.2.2 Redox Titration -Thiosulfate and Iodine

Iodine produced from iodide is measured by thiosulfate.

Titration reaction
  • I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻, so 1 mol of iodine reacts with 2 mol of thiosulfate.
Generating iodine
  • An excess of potassium iodide is added to an oxidising agent, releasing iodine in an amount equal to the oxidant used.
Copper(II) example
  • 2Cu²⁺ + 4I⁻ → 2CuI + I₂, so 2 mol of Cu²⁺ produce 1 mol of I₂.
Overall ratio
  • Combining the equations shows 1 mol of Cu²⁺ is equivalent to 1 mol of thiosulfate.

Part 2 of 6Procedure

1
Pipette 25.0 cm³ of the copper(II) solution into a conical flask and add excess potassium iodide, forming a brown mixture with white solid.
2
Fill a burette with standard sodium thiosulfate and titrate until the brown colour fades to pale yellow.
3
Add a few drops of starch solution, which turns the mixture blue-black.
4
Continue adding thiosulfate dropwise until the blue-black colour disappears and leaves a cream suspension.
5
Repeat until concordant results are obtained.

Part 3 of 6Key Points

Key Points
Why add starch late
  • Adding starch when much iodine remains forms a strong complex that releases iodine slowly and blurs the end point.
End-point colour
  • The sharp change from blue-black to colourless shows all iodine has reacted.
Excess iodide
  • Excess I⁻ ensures all the oxidising agent reacts and keeps I₂ dissolved as I₃⁻.
Standard solution
  • Thiosulfate is standardised because it is not a primary standard.

Part 4 of 6Sources of Error

Iodine loss
  • Iodine is volatile, so the flask should not be left open for long.
Light and air
  • Iodide is oxidised by air, so the titration should be done soon after adding iodide.
End point judgement
  • The white copper(I) iodide makes the colour change slightly less sharp.
Burette reading
  • Read at the bottom of the meniscus of the clear thiosulfate solution.

Part 5 of 6Worked Example

Question

  • 25.0 cm³ of Cu²⁺(aq) with excess KI needs 20.00 cm³ of 0.100 mol dm⁻³ Na₂S₂O₃.
  • Find [Cu²⁺].

  1. Moles of thiosulfate = 0.100 × 0.02000 = 2.00 × 10⁻³ mol.
  2. Moles of I₂ = 2.00 × 10⁻³ ÷ 2 = 1.00 × 10⁻³ mol.
  3. Moles of Cu²⁺ = 2 × 1.00 × 10⁻³ = 2.00 × 10⁻³ mol; concentration = 2.00 × 10⁻³ ÷ 0.0250 = 0.0800 mol dm⁻³.

Answer [Cu²⁺] = 0.0800 mol dm⁻³

Part 6 of 6Exam Checklist & Tips

6.2.2 – Key Points for the Exam

  • Thiosulfate S₂O₃²⁻ ions, used as a reducing agent in iodine titrations.
  • Starch indicator A solution that gives a blue-black colour with iodine.
  • Tetrathionate The S₄O₆²⁻ ion formed when thiosulfate is oxidised.
  • Back calculation Using a titration to work out the amount of the original oxidising agent.

Common Mistakes

  • Adding starch at the start, so the end point is slow and unclear.
  • Using the ratio 1 I₂ : 1 S₂O₃²⁻ instead of 1 : 2.
  • Expecting a completely clear solution at the end point; the white copper(I) iodide leaves a cream suspension.
  • Forgetting that iodide must be in excess.

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