Titration of Oxalic Acid with KMnO4

Table of Contents

AimTheoryMaterials RequiredApparatus SetupProcedureObservationCalculationsResults and DiscussionPrecautionsViva Questions

The titration of potassium permanganate (KMnO₄) against oxalic acid (C₂H₂O₄) is an example of redox titration. In close proximity to the endpoint, the action of the indicator is analogous to the other types of visual colour titrations in oxidation-reduction (redox) titrations.

Aim:

To determine the strength of potassium permanganate by titrating it against the standard solution of 0.1M oxalic acid.

Theory:

Potassium permanganate is a strong oxidising agent and in the presence of sulfuric acid it acts as a powerful oxidising agent. In acidic medium the oxidising ability of KMnO₄ is represented by the following equation.

In acidic solution,

MnO₄^– + 8H⁺ + 5e^– → Mn²⁺ + 4H₂O

Solution containing MnO₄^– ions are purple in colour and the solution containing Mn²⁺ ions are colourless and hence permanganate solution is decolourised 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 oxidised to carbon dioxide by KMnO_4, which itself gets reduced to MnSO₄. Oxalic acid reacts with potassium permanganate in the following way.

The chemical reaction at room temperature is given below.

Reduction Half reaction:- 2KMnO₄ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 3H₂O + 5[O]

Oxidation Half reaction:- 5(COOH)₂ + 5[O] → 5H₂O + 10CO₂↑

The overall reaction takes place in the process is

Overall reaction:- 2KMnO₄ + 3H₂SO₄ + 5(COOH)₂ → K₂SO₄ + 2MnSO₄ + 8H₂O + 10CO₂↑

The ionic equation involved in the process is given below.

Reduction Half reaction:- MnO₄^– + 8H⁺ + 5e^– → Mn²⁺ + 4H₂O

Oxidation Half reaction:- C₂H₄^2- → 2CO₂ + 2e^–

Overall Ionic reaction:- 2MnO₄^– + 16H⁺ + 5C₂H₄^2- → 2Mn²⁺ + 10CO₂ + 8H₂O

This titration cannot be carried out in the presence of acids like nitric acid or hydrochloric acid because itself is an oxidising agent. So hydrochloric acid chemically reacts with KMnO₄ solution forming chlorine which is also an oxidising agent.

Materials Required:

1. Oxalic acid
2. Potassium permanganate solution
3. 1.0M sulfuric acid
4. Chemical balance
5. Burette
6. Burette stand
7. Pipette
8. Conical flask
9. Funnel
10. Measuring flask
11. Weighing bottle
12. White tile
13. Burnet
14. Wire gauze

Apparatus Setup:

1. In burette – KMnO₄ solution
2. In Conical flask – 10ml of oxalic acid + Sulfuric acid
3. Indicator – Self indicator (KMnO₄)
4. End Point – Appearance of permanent pale pink colour.

Titration of Oxalic Acid with KMnO4

Procedure:

(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 follows.

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

For the preparation of 1 litre of N/10 oxalic acid solution amount of oxalic acid required = 6.3 g

1. Weigh an empty watch glass using a chemical balance.
2. Weigh 6.3g of oxalic acid accurately in the watch glass.
3. With the help of a funnel transfer the oxalic acid into the measuring flask.
4. Now wash the funnel with distilled water without removing the funnel from the flask.
5. Make the solution up to the marked point with distilled water and make sure the oxalic acid is fully dissolved.
6. This solution is 0.1N 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 test tube full of sulfuric acid in order to prevent oxidation of manganese to form manganese dioxide.
5. Heat the mixture upto 60^oC 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 colour of KMnO₄ is discharged with oxalic acid. The appearance of permanent pink colour 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 given below in order to calculate the molarity of KMnO₄ given.

Observation:

Calculations:

To calculate the strength of given KMnO₄ in terms of molarity the following formula is used

a₁M₁V₁ = a₂M₂V₂

Where a₁ and a₂ are stoichiometric coefficient of oxalic acid and KMnO₄ in a balanced chemical equation.

a₁ = 2

a₂ = 5

Where

M₂ and M₁ are molarities of potassium permanganate and oxalic acid solutions used in the titration.

V₂ and V₁ are the volume of potassium permanganate and oxalic acid solutions used in the titration.

Therefore,

KMnO₄ = Oxalic acid

5M₂V₂ = 2M₁V₁

M₂ = (2M₁V₁/5M₂V₂)

The strength of KMnO₄ is calculated by using the molarity.

Strength = Molarity x Molar mass

Results and Discussion:

1. Molarity of KMnO₄ is ______
2. The Strength of KMnO₄ is _____M.

Precautions:

1. Clean all the apparatus with distilled water before starting the experiment and then rise with the solution to be taken in them.
2. Rinse the pipette and burette before use.
3. Potassium permanganate is dark in colour, so always read the upper meniscus.
4. Use dilute sulfuric acid for acidifying the potassium permanganate.
5. Take accurate readings once it reaches the end point and don’t go with average readings.
6. Use antiparallex card or autoparallex card while taking the burette readings.
7. Do not use rubber cork burette as it is can be attacked by KMnO₄.
8. The strength of the unknown solution should be taken upto two decimal places only.

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