The titration of potassium permanganate (KMnO4) against oxalic acid (C2H2O4) is an example of redox titration. Potassium permanganate is a strong oxidizing agent and in the presence of sulfuric acid it acts as a powerful oxidizing agent.
Solution containing MnO4- ions are purple in color and the solution containing Mn2+ ions are colorless and hence permanganate solution is decolorized when added to a solution of a reducing agent. The moment there is an excess of potassium permanganate present the solution becomes purple. Thus, KMnO4 serves as self indicator in acidic solution.
Potassium permanganate is standardized against pure oxalic acid. It involves a redox reaction. Oxalic acid is oxidized to carbon dioxide by KMnO4, which itself gets reduced to MnSO4.
(a) Preparation of 0.1N standard solution of oxalic acid:
The quantity of oxalic acid required for the 250ml of the solution having a normality of 0.1N can be calculated as: Equivalent weight of oxalic acid = Molecular weight/No of electrons lost by one molecule
Equivalent weight of oxalic acid = 126/2 = 63
Strength = Normality x Equivalent weight
Strength = 1/10 x 63 = 6.3 g/l
Quantity required for 250 mL = 6.3/4 =1.575 g
1. Weigh an empty watch glass using a digital balance (up to 4 decimal in g).
2. Weigh 1.575g of oxalic acid accurately in the watch glass.
3. With the help of a funnel / butter paper transfer the oxalic acid into the measuring flask.
4. Now wash the funnel / butter paper with distilled water without removing from the flask.
5. Make the solution up to the marked point (250 mL) with distilled water and make sure the oxalic acid is fully dissolved.
6. This solution is 0.1N standard solution of oxalic acid.
(b) Preparation of 0.1N (Approx.) solution of potassium permanganate:
1. Dissolve 3.3 g of reagent grade potassium permanganate (KMnO4 ) in 1 L of purified water
2. Heat on a steam bath for two hrs.
3. Cover and allow to stand for 24 hrs.
4. Filter through a fine porosity sintered glass crucible, discarding the first 25 mL.
5. Store in a glass-stoppered, amber-colored bottle. Avoid exposure to direct sunlight; cover the neck of the bottle with a small beaker as a protection against dust.
6. Standardized against standard solution of oxalic acid
(b) Titration of potassium permanganate solution against standard oxalic acid solution:
1. Rinse the burette with the potassium permanganate solution and fill the burette with potassium permanganate solution.
2. Fix the burette in the burette stand and place the white tile below the burette in order to find the end point correctly.
3. Pipette out 10ml of 0.1N standard oxalic acid solution in a conical flask.
4. Add a 2.5 mL of sulphuric acid in order to prevent oxidation of manganese to form manganese dioxide.
5. Heat the mixture up to 60 degree C before titrating with potassium permanganate.
6. Note down the initial reading in the burette before starting the titration.
7. The hot solution is titrated against potassium permanganate solution and simultaneously swirl the solution in the flask gently.
8. Initially the purple color of KMnO4 is discharged with oxalic acid. The appearance of permanent pink color reveals the end point.
9. Repeat the titration until concordant values are obtained.
10. Note down the upper meniscus on the burette readings. Record the reading in the observation table in order to calculate the normality of KMnO4 solution.