Principle Reaction Equation Procedure and Calculation of Oxalic Acid Titration
Aim – To determine the concentration/molarity/strength of potassium permanganate (KMnO4) solution by titrating it against a 0.1 M standard solution of oxalic acid (COOH-COOH).
Theory - KMnO4 is an oxidizing agent which works in acidic medium more strongly than alkaline medium. So, for quantitative analysis potassium permanganate is generally used in acidic medium only. Its oxidizing action can be represented by following reaction in an acidic medium –
Reaction – MnO4- + 8H+ + 5e- 🡪 Mn2+ + 4H2O
We use sulphuric acid in this titration with KMnO4. The solution which contain MnO4- ion in it is purple in colour. While the solution containing Mn+2 ions is colourless. Thus, potassium permanganate when reacts with a reducing agent it works as 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 of indicator as potassium permanganate will act as self-indicator. Thus, oxalic acid can be titrated against potassium permanganate which includes following reactions –
Reduction half reaction: 2KMnO4 + 3H2SO4 🡪 K2SO4 + 2MnSO4 + 3H2O + 5[O]
Oxidation half reaction: H2C2O4 + [O] 60℃→ 2CO2 + H2O x 5
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2MnO4- + 5C2O42- +16H+ 🡪 2Mn2+ + 10CO2 + 8H2O
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Here in the above reaction, 2MnO4- reduced to Mn2+ while C2O42- oxidized to CO2 .
Initially when we add potassium permanganate into conical flask containing oxalic acid, it gets discharged and the solution remain colourless. After complete consumption of oxalic acid ions, the end point is indicated by a pink colour due to excess of unreacted potassium permanganate (pink in colour). Potassium permanganate reacts with sulphuric acid and forms manganous sulphate which works as a catalyst for reduction of MnO4- . That’s why 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 conical flask.
Procedure – 1. Preparation of 0.1M standard solution of oxalic acid –
Equivalent weight of oxalic acid = molecular weight / number of electrons lost by one molecule
= 126/2 = 63
Equivalent weight of oxalic acid is 63.
Normality = molecular weight/Equivalent mass of solute x molarity
= 126/63 x 0.1
= 0.2 N
Strength = Normality Equivalent weight
= 0.2 x 63
= 12.6g/l
For the preparation of 1 litre of N/20 oxalic acid solution amount of oxalic acid is required is 12.6g. Following steps should be followed -
1. Weigh 12.6 g of oxalic acid using a weighing machine.
2. Now take the weighed oxalic acid in a measuring flask and add distilled water to make the volume 1litre.
3. This solution is 0.2 N standard solution of oxalic acid.
2. Potassium permanganate solution titration with oxalic acid solution –
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.
Remove the air bubbles from the nozzle of the burette.
Take 10 ml of oxalic acid solution in a clean conical flask. Add 5ml of 1.0M sulphuric acid in it.
Heat the solution upto 50-60℃ before titrating it with potassium permanganate solution.
To increase visibility of the colour, keep the white tile below the conical flask.
Note the initial reading of the burette containing potassium permanganate.
Now start adding potassium permanganate through the burette in small quantity into hot oxalic acid solution.
Keep swirling the contents of the flask slowly.
Initially purple (pink) colour of potassium permanganate is discharged on reaction with oxalic acid.
When permanent pink colour appears, it indicates the end point.
Note down the upper meniscus of the burette containing potassium permanganate.
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 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 Potassium permanganate
2M1V1 = 5M2V2
M2 = 2 M1V15V2
For calculating 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 -
Always rinse the burette and take out the bubbles at nozzle of the burette.
Always rinse the burette and other flasks with distilled water before using.
KMnO4 is a dark colour solution so always use upper meniscus reading.
Use sulphuric acid for acidifying the potassium permanganate. Don’t use HCl or nitric acid.
Maintain the temperature of oxalic acid solution at 50-60’C.
Do not use rubber cork.
The strength of the solution should be taken till three decimals.
Never use a burette with a broken nozzle.
While noting reading at the end point, no drop should be hanging at the nozzle of the burette.
End point should be detected carefully and precisely.
Pipette should be used carefully.
FAQs on Titration of Oxalic Acid with Potassium Permanganate
1. What is titration of oxalic acid?
The titration of oxalic acid is a redox titration in which oxalic acid is quantitatively reacted with a standard solution of potassium permanganate to determine its concentration. In this process:
- Oxalic acid (H2C2O4) acts as a reducing agent.
- KMnO4 acts as an oxidizing agent in acidic medium.
- The reaction is carried out in the presence of dilute H2SO4.
2. What is the balanced chemical equation for titration of oxalic acid with KMnO4?
The balanced redox equation for the titration of oxalic acid with potassium permanganate in acidic medium is 2KMnO4(aq) + 5H2C2O4(aq) + 3H2SO4(aq) → K2SO4(aq) + 2MnSO4(aq) + 10CO2(g) + 8H2O(l). In this reaction:
- MnO4- is reduced to Mn2+.
- Oxalic acid is oxidized to CO2.
- The medium must be acidic for proper reduction of permanganate.
3. Why is sulphuric acid used in the titration of oxalic acid?
Dilute sulphuric acid (H2SO4) is used to provide the acidic medium required for the reduction of permanganate ions to Mn2+. In acidic medium:
- MnO4- is reduced to Mn2+ (colorless).
- The reaction proceeds smoothly and completely.
- Undesired side reactions are minimized.
4. Why is the titration of oxalic acid heated?
The titration of oxalic acid is heated to about 60–70°C to increase the rate of the redox reaction between oxalic acid and permanganate ions. This is necessary because:
- The reaction is slow at room temperature.
- Heating helps overcome activation energy.
- Once started, Mn2+ ions formed act as a catalyst (autocatalysis).
5. What is the indicator used in oxalic acid titration?
No external indicator is required because KMnO4 acts as a self-indicator in oxalic acid titration. During the titration:
- The purple color of MnO4- disappears as it is reduced.
- At the end point, a faint permanent pink color appears.
- This light pink color persists for about 30 seconds.
6. What is the end point in the titration of oxalic acid with KMnO4?
The end point is the appearance of a faint permanent pink color due to a slight excess of KMnO4. At this stage:
- All the oxalic acid has been oxidized to CO2.
- A small amount of unreacted MnO4- remains.
- The pink color persists for about 30 seconds.
7. How do you calculate the strength of oxalic acid by titration?
The strength of oxalic acid is calculated using the formula N1V1 = N2V2 based on the equivalence principle of titration. Steps involved:
- Note the normality (N1) and volume (V1) of KMnO4.
- Measure the volume (V2) of oxalic acid used.
- Calculate unknown normality (N2) of oxalic acid.
8. What type of reaction is the titration of oxalic acid?
The titration of oxalic acid is a redox (oxidation–reduction) reaction where electrons are transferred between reactants. In this process:
- Oxalic acid is oxidized to CO2.
- MnO4- is reduced to Mn2+.
- The reaction occurs in acidic medium.
9. What is the equivalent weight of oxalic acid in KMnO4 titration?
The equivalent weight of oxalic acid (H2C2O4·2H2O) in KMnO4 titration is 63 g/equiv. This is calculated as:
- Molar mass = 126 g/mol.
- n-factor = 2 (each molecule loses 2 electrons).
- Equivalent weight = 126 ÷ 2 = 63 g/equiv.
10. Why is KMnO4 considered a self-indicator in oxalic acid titration?
KMnO4 is considered a self-indicator because its intense purple color changes during the reaction and signals the end point without any added indicator. Specifically:
- MnO4- is purple in solution.
- It becomes nearly colorless as Mn2+ after reduction.
- A slight excess produces a faint pink color at the end point.
