Titration of Oxalic Acid with Potassium Permanganate (KMnO₄) – Class 12 Chemistry

Introduction

This titration is a classic example of redox titration (permanganometry) where oxalic acid, an organic reducing agent, is titrated against potassium permanganate, a strong oxidizing agent in acidic medium.

Chemicals Involved

• Oxalic Acid (H₂C₂O₄·2H₂O): The analyte to be estimated; reduces KMnO₄ to Mn²⁺
• Potassium Permanganate (KMnO₄): The titrant; oxidizes oxalic acid and acts as its own indicator
• Sulfuric Acid (H₂SO₄): Provides acidic medium required for the reaction

Principle

Under acidic conditions, KMnO₄ oxidizes oxalic acid (oxalate ions) to carbon dioxide, while itself being reduced from Mn⁷⁺ (purple) to Mn²⁺ (colorless). The endpoint of the titration is indicated by the persistence of a faint pink color due to excess KMnO₄.

Chemical Reaction

5H₂C₂O₄ + 2MnO₄⁻ + 6H⁺ → 10CO₂ + 2Mn²⁺ + 8H₂O
    

Apparatus Required

• Burette (filled with KMnO₄ solution)
• Pipette (to measure oxalic acid solution)
• Conical flask
• Beaker
• Measuring cylinder
• Distilled water
• Funnel, clamp stand

Preparation of Solutions

• KMnO₄ Solution: Usually prepared approximately and standardized before use.
• Oxalic Acid Solution: Accurately weighed pure oxalic acid dissolved in distilled water.
• Sulfuric Acid (Dilute): Required to acidify the reaction mixture.

Procedure

1. Rinse the burette with KMnO₄ solution and fill it.
2. Using a pipette, transfer a known volume (e.g., 25 mL) of oxalic acid solution into a conical flask.
3. Add about 25 mL of dilute sulfuric acid to the flask to acidify the solution.
4. Place the flask on a white tile for better observation.
5. Heat the flask gently to about 60-70°C. (Important: Reaction is slow at room temperature.)
6. Start adding KMnO₄ slowly from the burette while swirling the flask continuously.
7. The purple color of KMnO₄ disappears immediately as it reacts with oxalic acid.
8. Continue the titration until a light pink color persists for 30 seconds, indicating the endpoint.
9. Record the volume of KMnO₄ used.
10. Repeat the titration to get concordant results (within 0.1 mL).

Calculations

From the balanced equation:

• 2 moles of KMnO₄ react with 5 moles of oxalic acid.
• Using the formula:
  M₁V₁ / n₁ = M₂V₂ / n₂
  Where:
  M₁ = Molarity of KMnO₄, V₁ = Volume of KMnO₄ used,
  n₁ = electrons involved in KMnO₄ (5),
  M₂ = Molarity of oxalic acid, V₂ = Volume of oxalic acid taken,
  n₂ = electrons for oxalic acid (2 × 5 = 10 electrons total but simplified to moles ratio)

Result

The molarity of the oxalic acid solution is calculated using the volume of KMnO₄ consumed during titration.

Precautions

• Use dilute sulfuric acid, not concentrated or nitric acid.
• Heat the reaction mixture to speed the reaction but avoid boiling.
• Add KMnO₄ slowly near the endpoint.
• Swirl the flask continuously to mix the reagents properly.
• Store KMnO₄ solution in a dark bottle to prevent decomposition.
• Perform multiple titrations for accuracy.