A level Chemistry · Edexcel 9CH0 · Topic 14C
Redox titrations
Key points
- Potassium manganate(VII) titrations: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. It is self-indicating: end point is the first permanent pale pink.
- Acidify with dilute sulfuric acid (not HCl, which would be oxidised to Cl₂; not nitric acid, an oxidiser).
- Key ratio: MnO₄⁻ reacts with Fe²⁺ in a 1:5 ratio; with ethanedioate in a 2:5 ratio.
- Iodine–thiosulfate titrations: an oxidising agent liberates I₂ from excess I⁻, then I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻.
- Add starch near the end point (when the solution is pale yellow); the blue-black colour disappears at the end point. Used to find copper in brass or chlorine in bleach.
- Calculate via moles of titrant → mole ratio → moles of analyte → concentration or % purity, and estimate measurement uncertainties.
Topic notesPremium
Redox titrations work like acid–base titrations, but the reaction is electron transfer. The two you must know are potassium manganate(VII), which acts as its own indicator, and iodine–thiosulfate, which uses starch.
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