Redox Titrations

Redox titrations use electron-transfer reactions instead of acid–base reactions to determine unknown concentrations. The most commonly examined redox titrations at A-Level involve potassium manganate(VII) and iodine–thiosulfate systems. Both rely on clear colour changes to indicate the endpoint.

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Potassium Manganate(VII) Titrations

Potassium manganate(VII) (KMnO₄) is a strong oxidising agent. In acidic solution, the purple MnO₄⁻ ion is reduced to the almost colourless Mn²⁺ ion:

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

This reaction is self-indicating: the MnO₄⁻ is intensely purple, while Mn²⁺ is very pale pink (effectively colourless in dilute solution). During the titration, each drop of KMnO₄ solution is decolourised by the reducing agent in the flask. The endpoint is reached when one drop of MnO₄⁻ is no longer decolourised — the solution turns a permanent pale pink/purple colour.

No separate indicator is needed.

The Acidic Conditions

The H⁺ ions are provided by adding excess dilute sulfuric acid to the flask before titrating. Hydrochloric acid is not used because Cl⁻ would be oxidised by MnO₄⁻, interfering with the results. Nitric acid is not used because it is itself an oxidising agent.

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Iron(II) Determination with KMnO₄