TO STANDARDIZE THE GIVEN POTASSIUM PERMANGANATE SOLUTION BY USING N/10 STANDARD OXALIC ACID SOLUTION

Experiment. No. 4

OBJECTIVE:

TO STANDARDIZE THE GIVEN POTASSIUM PERMANGANATE SOLUTION BY USING N/10 STANDARD OXALIC ACID SOLUTION AND EXPRESS THE CONCENTRATION IN TERMS OF MOLARITY, GRAM PER LITRE AND PERCENTAGE STRENGTH

APPARATUS REQUIRED

• conical flask
• pipette
• burette
• beaker
• glass tube

CHEMICALS REQUIRED

• Potassium permanganate
• oxalic acid
• sulphuric acid

THEORY

Standardization means the determination of the concentration of an unknown solution with the help of the primary standard solution by finding the endpoint of the reaction. The reaction between the acidified KMnO₄ solution and the oxalic acid solution is oxidation-reduction and is said to be a redox reaction. KMnO₄ acts as an oxidizing agent and Oxalic acid acts as a reducing agent. The titration between oxidizing and reducing agents is called redox titration. The concentration of unknown KMnO₄ solution is determined by titrating it against a certain volume of oxalic acid. At the endpoint, the KMnO₄ solution persists in its pink colour. Therefore, potassium permanganate itself acts as self indicator.

Formula unit equation

KMnO₄ + 3H₂SO₄ + 5H₂C₂O₄ → K₂SO₄ + 2MnSO₄ + 8H₂O + 10CO₂

Ionic equation

In this reaction, H₂SO₄ acts as an acidifying agent. Here Magnesium is reduced from +7 to +2 oxidation state by gaining 5 electrons. So, the equivalent weight of KMnO₄ is calculated by dividing molecular weight by 5.

Eq. wt. = 158/5 = 31.6

PROCEDURE

A clean burette was taken and clamped in a stand in a vertical position. Then the burette was rinsed with KMnO₄ solution and filled with it. The pipette was rinsed with standard oxalic acid solution and pipetted out 10 ml of the solution in a clean conical flask. Then, 1 test full of H₂SO₄ solution was added and heated at about 60-70°C. While hot, it was titrated by adding KMnO₄ solution from the burette in a small amount swilling the flask all the while. The pink colour of KMnO4 will go on disappearing. The addition of KMnO4 solution was continued till a last single drop until a light pink colour persisted for a few seconds. This was the endpoint of the titration. The titration was repeated till till concurrent reading was obtained.

OBSERVATION TABLE

CALCULATION

From Normality equation,

V_{1}N_{1} = V_{2}N_{2}\\
10.1 \times N_{1} =10 \times (1/10)\\ N_{2} = 0.099 N

Normality = 0.099 N

Molarity = Normality/n = 0.099/5 = 0.0198 M

Gram per litre = Normality x eq. wt. = 0.099 x 31.6 = 3.1284 gm/lit

Percentage strength = (gm/lit)/10 = 3.1284/10 = 0.31284 %(w/v)

RESULT

Hence, the given KMnO₄ solution was standardised and it’s concentration was found to be 0.099N

CONCLUSION

Redox titration can be carried out without using an indicator.

PRECAUTIONS

1. The burette was not used with a rubber cock.
2. The reading was taken coinciding with the upper meniscus of the burette.
3. Titration must be carried out in a hot solution.
4. A sufficient quantity of dilute sulphuric acid was added to the oxalic acid solution in the titrating flask before titration.
5. Care should be taken while sucking oxalic acid so that no solution comes into the mouth as it is poisonous.
6. A small quantity of KMnO₄ was added at a time and shaken till it was decolourized before adding the next lot.
7. At the end of the experiment, the whole apparatus was washed with water. Burette and beaker, which contained KMnO4 solution usually, get a brown stain. Clean these with FeSO₄ solution and wash them again with water.