Oxidation Reduction Titration

Reaction between Sodium Oxalate and Potassium Permanganate:
An Oxidation-Reduction Titration

TASK: To determine the amount of oxalate in an impure sample through the use
of a titration with potassium permanganate, after standardization with
pure sodium oxalate. The titration is based on oxidation-reduction
properties of the reactants.

BACKGROUND INFORMATION:

The technique of titration has been used previously in acid-base reactions to detect the amount of acid using a known base (or the reverse). It can also be used in situations in which the reaction involves oxidation and reduction.

Oxidation is defined as the loss of electrons (on right of arrow) or increase in oxidation state (each C changes from +3 to +4) as shown below by the half-reaction involving oxalate ion.
C2O42- ? 2 CO2 + 2 e-

Reduction is shown by the permanganate ion as it gains electrons and Mn decreases in oxidation state from +7 to +2.
8 H+ + MnO4- + 5 e- ? Mn2+ + 4 H2O

Oxidation and reduction must occur together and are often designated as redox processes to emphasize this; the number of electrons lost by one substance must equal the number of electrons gained by the other substance. If we combine the two half-reactions above, we would end with a balanced net ionic equation if we have a total of 10 electrons exchanged.

16 H+ + 2 MnO4- + 5 C2O42- ? 2 Mn2+ + 10 CO2 + 8 H2O

Because the materials we weigh and measure occur as compounds, it is often useful to have the balanced molecular equation (sometimes called the balanced total equation).

8 H2SO4 + 2 KMnO4 + 5 Na2C2O4 ? 2 MnSO4 + 10 CO2 + K2SO4 + 5 Na2SO4 + 8 H2O

Potassium permanganate is reduced because it contains the permanganate ion; we can also say that it behaves as an oxidizing agent because it causes something else to become oxidized (the oxalate). Sodium oxalate is oxidized because it has the oxalate ion and we could also specify it as the reducing agent because it causes the permanganate to become reduced. It can also be seen from both the net ionic equation or molecular equation that acid is required for this reaction to occur; i.e., H+ or H2SO4 show up in the balanced equations. We also know from the previous experience that a temperature higher than room temperature makes a reaction more rapid.