Determination of Fe²⁺ by KMnO₄ titration

eALOM Practical · AL Units 6 + 12 (transition metals, redox)

Aim

To determine the concentration of Fe²⁺ ions in a given solution by titration with standardised potassium permanganate.

Principle

Acidified KMnO₄ oxidises Fe²⁺ to Fe³⁺ while itself being reduced to Mn²⁺. The intense purple colour of MnO₄⁻ acts as its own indicator — the first persistent pink tinge marks the end-point.

Ionic equation:
MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
Stoichiometry: 1 mol MnO₄⁻ reacts with 5 mol Fe²⁺.

Apparatus

Procedure

Rinse the burette with a small portion of KMnO₄ solution; discard the rinse. Fill the burette and record the initial reading (read the upper meniscus — solution is opaque purple).
Pipette 25.00 cm³ of the Fe²⁺ solution into a clean conical flask.
Add about 25 cm³ of dilute H₂SO₄ (this provides the H⁺ needed by the half-equation).
Place the flask on a white tile under the burette. Titrate, swirling continuously. The pink colour disappears rapidly at first.
End-point is reached when one drop produces a persistent pale pink colour that does not fade within ~30 s.
Repeat for concordant titres (agreeing within 0.10 cm³).

Observation table

	Rough	1	2	3
Final burette reading / cm³
Initial burette reading / cm³
Volume KMnO₄ used / cm³

Calculation helper

Mean titre V(KMnO₄) / cm³ [KMnO₄] / mol dm⁻³ Pipetted volume Fe²⁺ / cm³

Common errors examiners flag

Reading the lower meniscus by mistake — KMnO₄ is opaque, use the upper.
Using HCl instead of H₂SO₄ — chloride is oxidised by MnO₄⁻, giving false high titres.
Adding the KMnO₄ too fast at the start — local concentration spikes give MnO₂ (brown), invalidating the titration.
Forgetting to swirl — gives premature pink colour, false low titre.

Past-paper questions on this practical

(Will be topic-tagged in Phase 2.) Most years between 2015 and 2024 include at least one structured Paper II question involving KMnO₄ titration of Fe²⁺ or related redox.