Titration of Oxalic Acid with KMnO4

Titration of potassium permanganate with oxalic acid is a type of redox titration which is one of the titration experiments included in the chemistry practical syllabus of Class 12 CBSE. Here in this article, we will be discussing the complete experiment in detail. 

Aim

To determine the concentration/molarity/strength of potassium permanganate (KMnO₄) solution by titrating it against a 0.1M standard solution of oxalic acid (COOH-COOH). 

Theory

KMnO₄ is an oxidizing agent which works in an acidic medium more strongly than an alkaline medium. So, for quantitative analysis potassium permanganate is generally used in an acidic medium only. Its oxidizing action can be represented by the following reaction in an acidic medium – 

Reaction

MnO₄-+8H++5e-Mn₂++4H₂O

We use sulphuric acid in this titration with KMnO4. The solution which contains MnO4- ion in it is purple in color. While the solution containing Mn⁺² ions is colorless. Thus, potassium permanganate when reacts with a reducing agent it works as a self-indicator also.

In the experiment, oxalic acid acts as a reducing agent and KMnO₄ is taken in an acidic medium of H₂SO₄. So, there is no need for an indicator as potassium permanganate will act as a self-indicator. Thus, oxalic acid can be titrated against potassium permanganate which includes the following reactions – 

Reduction Half-reaction

2KMnO₄+3H₂SO₄K₂SO₄+2MnSO₄+3H₂O+5 [O]

Oxidation Half-reaction

5(COOH)2+5[O]→5H2O+10CO2 ↑

Overall the reaction that takes place in the given process:

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

The ionic equation in the above process is:

Reduction Half reaction

$[{MnO_4}^{-}+8H^{+}+5e^{-} \to Mn^{2+}+4H_2O]\times 2$

Oxidation Half reaction

$[{C_2O_4}^{2-} \to 2CO_2 + 2e^{-}] \times 5$

Overall Ionic reaction

$2MnO^{4–} + 16H^{+} + 5C_2O_4^{2-} \to 2Mn^{2+} + 10CO_2 + 8H_2O$

Initially, when we add potassium permanganate into a conical flask containing oxalic acid, it gets discharged and the solution remains colourless. After complete consumption of oxalic acid ions, the endpoint is indicated by a pink colour due to excess of unreacted potassium permanganate (pink in colour). Potassium permanganate reacts with sulphuric acid and forms manganese sulphate which works as a catalyst for the reduction of MnO4-. That’s why the rate of reaction is slow in the beginning while faster as it proceeds further. 

Material Required

Measuring flask, burette, burette stand, pipette, conical flask, funnel, weighing bottle, glazed tile(white), burner, wire gauze, chemical balance, oxalic acid, potassium permanganate solution, 1.0 M sulphuric acid. 

Apparatus Setup

Potassium permanganate solution should be taken in the burette and oxalic acid solution should be taken in a conical flask.

Procedure – 1

Preparation of 0.1M Standard Solution of Oxalic Acid.

Equivalent weight of oxalic acid = \[\frac{\text{molecular weight}}{\text{number of electrons lost by one molecule}}\]

=\[\frac {126} {2}\]=63 

Equivalent weight of oxalic acid is 63. 

Normality = molecular weight Equivalent mass of solute molarity

=12663 0.1

=0.2N

Strength = Normality Equivalent weight

\[Strength = \frac{\text{Normality}}{\text{Equivalent weight}}\]

= \[\frac {0.2} {63}\]

=12.6g/l

For the preparation of 1 litre of \[\frac {N} {20}\] oxalic acid solution the amount of oxalic acid required is 12.6g. The following steps should be followed - 

1. Weigh 12.6g of oxalic acid using a weighing machine. 

2. Now take the weighted oxalic acid in a measuring flask and add distilled water to make the volume 1 litre.

3. This solution is a 0.2N standard solution of oxalic acid.

Procedure 2

Potassium Permanganate Solution Titration with an Oxalic Acid Solution.

1. Rinse and fill a clean burette with potassium permanganate solution. Burette must have a glass stop cock as rubber will get affected by permanganate ions.

2. Remove the air bubbles from the nozzle of the burette.

3. Take 10 ml of oxalic acid solution in a clean conical flask. Add 5ml of 1.0M sulphuric acid to it.

4. Heat the solution up to 50-60℃ before titrating it with potassium permanganate solution. 

5. To increase the visibility of the colour, keep the white tile below the conical flask.

6. Note the initial reading of the burette containing potassium permanganate.

7. Now start adding potassium permanganate through the burette in small quantities into a hot oxalic acid solution.

8. Keep swirling the contents of the flask slowly.

9. Initially purple (pink) colour of potassium permanganate is discharged on reaction with oxalic acid.

10. When a permanent pink colour appears, it indicates the end point.

11. Note down the upper meniscus of the burette containing potassium permanganate.

12. Repeat the titration till three times to obtain 3 concordant readings.

Observation Table

Calculations

The strength of potassium permanganate solution in terms of molarity can be calculated by the following formula – 

a₁m₁v₁=a₂m₂v₂

a₁ = number of electrons lost per formula unit of oxalic acid in a balanced equation of half cell reaction which is 2. 

a₂  = number of electrons gained per formula unit of potassium permanganate in the balanced equation of half cell reaction which is 5. 

M₁ = molarity of oxalic acid solution 

M₂ = molarity of potassium permanganate solution. 

V₁ = volume of oxalic acid solution 

V₂ = volume of potassium permanganate solution 

On putting the value of a₁ and a₂,

Oxalic acid and Potassium permanganate 

2m₁v₁=5m₂v₂

M₂= \[\frac {2M_{1}V_{1}} {5M_{2}V_{2}}\]

For calculating the strength of the solution,

Strength = Molarity Molar mass 

Result – 1. Molarity of potassium permanganate solution - ____

                2. Strength of potassium permanganate solution - ____  

Precautions

Following precautions should be taken while performing the experiment - 

1. Always rinse the burette and take out the bubbles at the nozzle of the burette. 

2. Always rinse the burette and other flasks with distilled water before using. 

3. KMnO₄  is a dark colour solution so always use upper meniscus reading. 

4. Use sulphuric acid for acidifying the potassium permanganate. Don’t use HCl or nitric acid. 

5. Maintain the temperature of the oxalic acid solution at 50-60’C.

6. Do not use rubber cork. 

7. The strength of the solution should be taken till three decimals. 

8. Never use a burette with a broken nozzle.

9. While noting reading at the endpoint, no drop should be hanging at the nozzle of the burette. 

10.  Endpoint should be detected carefully and precisely. 

11.  The pipette should be used carefully.