Why Is KMnO4 Used in the Titration of Oxalic Acid?
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 Oxalic Acid Titration with KMnO4: Procedure, Calculations & Tips
1. What is the fundamental principle behind the titration of oxalic acid with KMnO₄?
This titration is based on a redox reaction. Oxalic acid (C₂H₂O₄) acts as a reducing agent and is oxidised to carbon dioxide (CO₂), while potassium permanganate (KMnO₄), a strong oxidising agent, is reduced to manganese(II) sulphate (MnSO₄) in an acidic medium. The volume of KMnO₄ required to completely oxidise a known volume of oxalic acid is used to determine the concentration of either solution.
2. What is the balanced chemical equation for the reaction between potassium permanganate and oxalic acid?
In an acidic medium provided by sulphuric acid, the overall balanced chemical equation for the reaction is:
2KMnO₄ + 3H₂SO₄ + 5H₂C₂O₄ → K₂SO₄ + 2MnSO₄ + 8H₂O + 10CO₂.
This stoichiometric relationship is essential for all calculations related to the titration.
3. Why is potassium permanganate (KMnO₄) considered a self-indicator in this experiment?
Potassium permanganate acts as its own indicator due to its intense purple colour from the permanganate ion (MnO₄⁻). During the titration, as it reacts, it is reduced to the manganese(II) ion (Mn²⁺), which is colourless. Once all the oxalic acid has been consumed, the very next drop of excess KMnO₄ is not decolourised and imparts a persistent light pink colour to the solution, signalling the endpoint without needing a separate indicator.
4. What is the endpoint in the titration of oxalic acid with KMnO₄, and how is it precisely detected?
The endpoint is the point at which the reaction between the titrant (KMnO₄) and the analyte (oxalic acid) is just complete. It is detected visually when the addition of a single drop of KMnO₄ changes the solution in the conical flask from colourless to a permanent faint pink. This pink colour should persist for at least 30 seconds to confirm that the reaction is complete.
5. Why must dilute sulphuric acid (H₂SO₄) be added to the oxalic acid solution before starting the titration?
Dilute sulphuric acid is crucial because it provides the acidic medium (H⁺ ions) required for the complete reduction of the permanganate ion (MnO₄⁻) to the colourless manganese(II) ion (Mn²⁺). The reaction, MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O, will not proceed correctly in a neutral or alkaline medium, where manganese can form brown precipitates of MnO₂, leading to inaccurate results.
6. Why is the oxalic acid solution heated to about 60-70°C before titrating?
The reaction between oxalic acid and potassium permanganate is extremely slow at room temperature. Heating the solution to 60-70°C acts as a catalyst, increasing the kinetic energy of the reacting particles and thus raising the reaction rate to a reasonable level. If the solution is not heated, the endpoint will be reached very slowly and inaccurately. Furthermore, the Mn²⁺ ions produced during the reaction act as an auto-catalyst, speeding up the reaction once it begins.
7. What would happen if we used hydrochloric acid (HCl) or nitric acid (HNO₃) instead of sulphuric acid?
Using other acids would lead to incorrect results and side reactions:
- Hydrochloric acid (HCl): KMnO₄ is a powerful oxidising agent and would oxidise the chloride ions (Cl⁻) from HCl into chlorine gas (Cl₂). This secondary reaction would consume extra KMnO₄, leading to an artificially high and inaccurate volume reading.
- Nitric acid (HNO₃): Nitric acid is itself an oxidising agent. It would compete with KMnO₄ to oxidise the oxalic acid, leading to a lower volume of KMnO₄ being used and thus an incorrect result.
8. Explain the difference between a primary and a secondary standard in the context of this titration.
The key difference lies in their purity and stability:
- A primary standard is a substance of very high purity, stable, and non-hygroscopic, used to prepare a solution of an exactly known concentration. In this experiment, hydrated oxalic acid (H₂C₂O₄·2H₂O) is the primary standard.
- A secondary standard is a substance whose solution concentration is not stable over time and must be determined by titrating it against a primary standard. Potassium permanganate (KMnO₄) is a secondary standard because it is hard to obtain in 100% pure form and can decompose in the presence of sunlight or impurities.
9. How are the molarity and strength of the KMnO₄ solution calculated from the titration results?
The molarity is found using the titration formula derived from the balanced equation's stoichiometry (n₁ = 2 for KMnO₄, n₂ = 5 for oxalic acid):
(M₁V₁ / n₁) = (M₂V₂ / n₂)
Where M and V represent Molarity and Volume respectively. Rearranging for the molarity of KMnO₄ (M₁) gives: M₁ = (M₂ × V₂ × n₁) / (V₁ × n₂). The strength, expressed in grams per litre (g/L), is then calculated by multiplying the resulting molarity (M₁) by the molar mass of KMnO₄ (158.034 g/mol).
10. What is meant by auto-catalysis in the KMnO₄ and oxalic acid titration?
Auto-catalysis is a unique phenomenon where one of the reaction products itself acts as a catalyst for that same reaction. In this titration, the reaction is initially slow. However, the reduction of MnO₄⁻ produces manganese(II) ions (Mn²⁺). These Mn²⁺ ions catalyse the subsequent reaction between permanganate and oxalate ions, causing the reaction rate to increase significantly after it has started. This is why the first few drops of KMnO₄ take a noticeably longer time to decolourise compared to later drops.
