Determination of Permanganate Index by Acidic Potassium Permanganate Titration

The acidic potassium permanganate titration method is a common technique used to assess the level of oxidizable organic and inorganic substances in water. This measurement is referred to as the permanganate index (CODMn), which serves as an indicator of organic matter pollution.

Principle:
The method is based on the oxidation reaction of potassium permanganate (KMnO₄) in an acidic medium. A known volume of water sample is acidified with sulfuric acid (H₂SO₄) and heated with a known excess of standard potassium permanganate solution. During heating, KMnO₄ (which is purple) oxidizes the reducing substances in the water sample and is itself reduced to colorless manganese(II) ions (Mn²⁺).

Reaction:
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

After the oxidation is complete, a standard solution of sodium oxalate (Na₂C₂O₄) is added to titrate the remaining, unreacted potassium permanganate. The sodium oxalate reduces the remaining KMnO₄.

Titration Reaction:
2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O

The endpoint of the titration is the disappearance of the faint pink color of potassium permanganate, indicating that all the excess permanganate has been reacted.

Calculation:
The permanganate index is calculated based on the volume of potassium permanganate consumed in oxidizing the substances in the water sample. By subtracting the amount of KMnO₄ equivalent to the added sodium oxalate from the total KMnO₄ added initially, the amount of KMnO₄ used to oxidize the pollutants is determined. The result is typically expressed in milligrams of oxygen consumed per liter of water (mg/L O₂), providing a standardized measure of water quality.