Question

If the standardization of permanganate by sodium oxalate happens in a solution which is not sufficiently acidic, a side reaction will occur and a brown precipitate will be produced. How will this interference affect the redox titration?

Answer 1

The standardization of permanganate (MnO₄^-) with oxalate takes place in acidic medium. MnO₄^- is reduced to the lowest possible oxidation of Mn, Mn²⁺ in acidic medium. The reduction reaction is

Reduction in acidic medium:

MnO₄^- + 8 H⁺ + 5 e^- --------> Mn²⁺ + 4 H₂O

These 5 electrons are obtained from the oxidation of oxalate (C₂O₄^2-) to carbon dioxide, where carbon has the highest possible oxidation state, +4.

In basic medium, MnO₄^- is reduced to brown colored MnO₂ where manganese has + 4 oxidation state. The reaction taking place is

Reduction in neutral medium:

MnO₄^- + 2 H₂O + 3 e^- ------> MnO₂ + 4 OH^-

The reduction involves the addition of 3 electrons to Mn(VII) in permanganate. However, under these conditions, oxalate may not be reduced to CO₂, but may form some intermediate oxidation product of carbon. Therefore, some of the available oxalate will not be employed in reducing MnO₄^- to Mn²⁺ and hence the reaction stoichiometry fails. Moreover, due to the formation of a brown precipitate, the detection of end point becomes exceedingly difficult.