1. Preparation of Solutions:

• Oxalic Acid Solution: Accurately weigh a known quantity of oxalic acid (H₂C₂O₄•2H₂O) and dissolve it in distilled water to create a solution of known concentration. For example, you might dissolve around 0.630 g of oxalic acid in 100 mL of distilled water to make a 0.05 M solution.
• KMnO₄ Solution: Prepare an approximate solution of KMnO₄. The concentration does not need to be exact at this stage, as it will be standardized. A typical solution is around 0.02 M to 0.05 M. Since KMnO₄ solutions are not stable over long periods, standardization is crucial.

2. Titration Procedure:

• Preparation:
  • Pipette a known volume (e.g., 20 mL) of the oxalic acid solution into a conical flask.
  • Add about 20 mL of dilute sulfuric acid (1 M H₂SO₄) to the flask. The sulfuric acid provides the acidic medium necessary for the reaction.
  • Heat the solution to about 60-70°C. This helps in speeding up the reaction between KMnO₄ and oxalic acid.
• Titration:
  • Fill a burette with the KMnO₄ solution.
  • Slowly add the KMnO₄ solution from the burette into the conical flask containing the oxalic acid solution. Continuously stir the mixture.
  • Initially, the purple color of KMnO₄ will disappear as it reacts with the oxalic acid. As you approach the endpoint, the purple color will disappear more slowly.
  • Continue adding KMnO₄ dropwise until one drop of KMnO₄ solution imparts a faint, permanent pink color to the solution that persists for at least 30 seconds. This indicates that all the oxalic acid has reacted, and you have reached the endpoint.

3. Calculation:

• Reaction Stoichiometry: The balanced chemical equation for the reaction is:
  5 H₂C₂O₄ + 2 KMnO₄ + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 10 CO₂ + 8 H₂O
  From the equation, 2 moles of KMnO₄ react with 5 moles of oxalic acid.
• Calculations:
  • Calculate the number of moles of oxalic acid used:
  Moles of H₂C₂O₄ = (Volume of H₂C₂O₄ solution in Liters) × (Molarity of H₂C₂O₄ solution)
  • Use the stoichiometry of the reaction to find the number of moles of KMnO₄ that reacted:
  Moles of KMnO₄ = (Moles of H₂C₂O₄) × (2/5)
  • Calculate the molarity of the KMnO₄ solution:
  Molarity of KMnO₄ = (Moles of KMnO₄) / (Volume of KMnO₄ solution used in Liters)

Example Calculation:

Suppose you prepared a 0.05 M oxalic acid solution, and you used 20 mL of it in the titration. The volume of KMnO₄ required to reach the endpoint was 20.0 mL.

• Moles of H₂C₂O₄ = (20/1000) L × 0.05 mol/L = 0.001 mol
• Moles of KMnO₄ = 0.001 mol × (2/5) = 0.0004 mol
• Molarity of KMnO₄ = 0.0004 mol / (20/1000) L = 0.02 M

4. Repeat Titrations:

Repeat the titration at least three times to ensure accuracy and precision. Average the molarity values obtained from multiple titrations to get a more reliable standardized concentration of the KMnO₄ solution.

Important Considerations:

• Ensure all glassware is clean.
• Use distilled water to prepare solutions.
• Heat the solution to 60-70°C to facilitate the reaction, but do not overheat, as oxalic acid can decompose.
• Stir the solution continuously during the titration.
• Observe the endpoint carefully to avoid overshooting.

References:

• LibreTexts: Applications of Redox Titrations
• Vedantu: Standardisation of KMnO4