Permanganometry Titration (Redox Titration) – Class 12 Chemistry

Introduction

Permanganometry is a type of redox titration where potassium permanganate (KMnO₄) is used as a titrant due to its strong oxidizing properties and self-indicating nature.

Example: Estimation of Ferrous Ion (Fe²⁺) by titrating with KMnO₄ solution.

Principle

KMnO₄ in acidic medium is a powerful oxidizing agent. It oxidizes Fe²⁺ ions to Fe³⁺ ions, and KMnO₄ itself is reduced to Mn²⁺ ions. The reaction produces a color change used to identify the endpoint.

Chemical Reaction

In acidic medium:

MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
    

Apparatus and Chemicals Required

• Standard KMnO₄ solution (approximately 0.02 M)
• Ferrous sulphate (FeSO₄) solution (impure)
• Sulfuric acid (dilute H₂SO₄), acidifies the medium
• Burette, conical flask, pipette
• Beaker, funnel, distilled water

Standardization of KMnO₄ Solution

Since KMnO₄ solutions decompose over time, standardization is done before use with a primary standard like sodium oxalate (Na₂C₂O₄).

Procedure

1. Rinse the burette with KMnO₄ solution and fill it carefully.
2. Using a pipette, transfer 25 mL of ferrous sulphate solution into a clean conical flask.
3. Add about 25 mL of dilute sulfuric acid (not concentrated to avoid side reactions) to the flask.
4. Place the flask on a white tile and start titration by adding KMnO₄ solution from the burette dropwise while swirling continuously.
5. Initially, the purple color of KMnO₄ disappears instantly as it reacts with Fe²⁺ ions.
6. Near the endpoint, add KMnO₄ dropwise slowly. The endpoint is reached when a faint pink color persists for 30 seconds, indicating all Fe²⁺ is oxidized and excess KMnO₄ is present.
7. Note the volume of KMnO₄ used.
8. Repeat titration for 3 consistent readings and calculate average volume.

Calculations

Using the reaction equation, 1 mole MnO₄⁻ reacts with 5 moles Fe²⁺.

Formula:

M₁V₁ / n₁ = M₂V₂ / n₂

Where,
M₁ = Molarity of KMnO₄,
V₁ = Volume of KMnO₄ used,
n₁ = Number of electrons involved in KMnO₄ (5),
M₂ = Molarity of FeSO₄,
V₂ = Volume of FeSO₄ taken,
n₂ = Number of electrons in FeSO₄ (1)
    

Result

The molarity of ferrous ion in the given solution is calculated using the volume of KMnO₄ consumed.

Precautions

• Add KMnO₄ slowly near the endpoint to avoid overshooting.
• Use dilute H₂SO₄, not concentrated.
• Avoid exposure of KMnO₄ solution to sunlight (store in dark bottle).
• Swirl constantly during titration for proper mixing.
• Perform multiple titrations for accurate results.