This method describes how to determine the permanganate index of water. The permanganate index is used to ascertain the concentration of organic matter and oxidizable inorganic matter in the water. It is used to evaluate the quality of drinking water and untreated water. Water samples containing higher levels of contamination can be diluted for analysis.
Water intended for use as an ingredient in the production of beer (brewing liquor) or other foods
The oxidative capacity provides information regarding the amount of oxidizable, organic matter in water. Oxidizable, inorganic matter can also be present and must be taken into consideration for this analysis. Due to the various oxidants in the water and the specific reaction conditions, the results are not comparable in every respect.
Potassium permanganate oxidizes many organic and certain inorganic substances more or less completely in acidic, neutral or alkaline solutions. The volume of potassium permanganate required in the analysis is determined potentiometrically. Since oxidation depends on the type of solution, on its temperature and on the reaction time, the procedure described below must be followed precisely.
In acidic solutions, permanganate ions are typically reduced to manganese(II) ions:
MnO4- + 5 e- + 8 H3O+ → Mn2+ + 12 H2O
In alkaline solutions, the reduction results in tetravalent manganese only:
MnO4- + 3 e- + 4 H3O+ → MnO2 + 6 H2O
Since in both cases the titration takes place in an acidic solution, this is irrelevant for the calculation. By adding oxalic acid, both the excess permanganate ions as well as the tetravalent manganese are reduced to manganese(II) ions:
2 MnO4- + 5 C2O42- + 16 H3O+ → 2 Mn2+ + 24 H2O + 10 CO2
MnO2 + C2O42- + 4 H3O+ → Mn2+ + 6 H2O + 2 CO2