To determine this index, a water sample is mixed with a powerful oxidizer: potassium permanganate KMNO₄. The conditions of temperature and time are strictly controlled.

The dosage is based on three successive steps:

1. Oxidization of the organic matter with an excess of potassium permanganate (pink color), in an alkaline medium, at boiling temperature, for 10 minutes.
   MnO₄^- is reduced to MnO₂ in a basic medium
   
2. Addition of an excess of Mohr salt (in an acidic medium) to reduce the excess of permanganate that has not reacted with the organic matter (discoloration).
   MnO₄^- + 5 Fe²⁺ + 8 H⁺ → 5 Fe³⁺ + Mn²⁺ + 4 H₂O
   
3. Back titration of the excess of Mohr salt (in an acidic medium) with the potassium permanganate (the pink color reappears, due to the excess of MnO₄^- at equivalence).
   MnO₄^- + 5 Fe²⁺ + 8 H⁺ → 5 Fe³⁺ + Mn²⁺ + 4 H₂O