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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.
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 KMnO4 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:- C2H42- → 2CO2 + 2e–
Overall Ionic reaction:- 2MnO4– + 16H+ + 5C2H42- → 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.
- Oxalic acid
- Potassium permanganate solution
- 1.0M sulfuric acid
- Chemical balance
- Burette stand
- Conical flask
- Measuring flask
- Weighing bottle
- White tile
- Wire gauze
- In burette – KMnO4 solution
- In Conical flask – 10ml of oxalic acid + Sulfuric acid
- Indicator – Self indicator (KMnO4)
- End Point – Appearance of permanent pale pink colour.
(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
- Weigh an empty watch glass using a chemical balance.
- Weigh 6.3g of oxalic acid accurately in the watch glass.
- With the help of a funnel transfer the oxalic acid into the measuring flask.
- Now wash the funnel with distilled water without removing the funnel from the flask.
- Make the solution up to the marked point with distilled water and make sure the oxalic acid fully dissolved.
- This solution is 0.1N standard solution of oxalic acid.
(b) Titration of potassium permanganate solution against standard oxalic acid solution:
- Rinse the burette with the potassium permanganate solution and fill the burette with potassium permanganate solution.
- Fix the burette in the burette stand and place the white tile below the burette in order to find the end point correctly.
- Pipette out 10ml of 0.1N standard oxalic acid solution in a conical flask.
- Add a test tube full of sulfuric acid in order to prevent oxidation of manganese to form manganese dioxide.
- Heat the mixture upto 60oC before titrating with potassium permanganate.
- Note down the initial reading in the burette before starting the titration.
- The hot solution is titrated against potassium permanganate solution and simultaneously swirl the solution in the flask gently.
- Initially the purple colour of KMnO4 is discharged with oxalic acid. The appearance of permanent pink colour reveals the end point.
- Repeat the titration until concordant values are obtained.
- 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.
|S.No||Volume of oxalic acid in ml||Burette Reading||Volume(V) of KMnO4 used
V = (y-x)ml
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
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.
KMnO4 = Oxalic acid
5M2V2 = 2M1V1
M2 = (2M1V1/5M2V2)
The strength of KMnO4 is calculated by using the molarity.
Results and Discussion:
- Molarity of KMnO4 is ______
- The Strength of KMnO4 is _____M.
- Clean all the apparatus with distilled water before starting the experiment and then rise with the solution to be taken in them.
- Rinse the pipette and burette before use.
- Potassium permanganate is dark in colour, so always read the upper meniscus.
- Use dilute sulfuric acid for acidifying the potassium permanganate.
- Take accurate readings once it reaches the end point and don’t go with average readings.
- Use antiparallex card or autoparallex card while taking the burette readings.
- Do not use rubber cork burette as it is can be attacked by KMnO4.
- The strength of the unknown solution should be taken upto two decimal places only.
How to convert M/10 oxalic acid solution into N/10 oxalic acid solution?
To make the conversion add an equal volume of water so that the solution converted to N/10.
What is the formula and basicity of oxalic acid?
The formula for oxalic acid is (COOH)2.2H2O. The basicity of oxalic acid is 2 means it is a dibasic acid.
In this titration of KMnO4 vs oxalic acid, what is the indicator used?
Potassium permanganate itself is purple in colour and acts as a self indicator.
What is meant by endpoint?
The endpoint is also called equivalence point or stoichiometric point means the conclusion of the chemical reaction. It is the point where no more titrant is required and the reaction is complete.
What is meant by redox titration?
In redox titrations, both oxidation and reduction reactions take place simultaneously. During titration, one will get oxidised at the same time the other reactant will get reduced also called a redox reaction.
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