The method describes how to determine the manganese content of water through oxidation to permanganate.
Manganese ions are catalytically oxidized into purple-colored permanganate ions. The purple color of the solution is determined using a comparator (or photometrically):
Mn2+ + 12 H2O → MnO4- + 8 H3O+ + 5 e-
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