9. TO STANDARDISE THE GIVEN KMnO4 SOLUTION AGAINST THE STANDARD OXALIC ACID SOLUTION (PRIMARY STANDARD).

THEORY:

It is a redox titration between acidified KMnO₄ and oxalic acid.

Formula unit equation

2KMnO₄+ 3H₂SO₄ + 5H₂C₂O₄ ————> K₂SO₄ + 2MnSO₄ + 8H₂O + 10C0₂

Ionic equation

[MnO₄ ^– + 8H⁺+ 5e ——–> Mn⁺⁺ + 4H₂O] × 2

[C₂O₄ ^—–  2e  ————-> 2CO₂] × 5

Adding, 2MnO₄ ^— +16H⁺ + 5C₂0₄ ^— ————–>2Mn⁺⁺ +8H₂O + 10CO₂

APPARATUS:

1) Burette 

2) Pipette 

3) Conical flask

4) Burette stand

5) Clamps

CHEMICALS:

No external indicator

PROCEDURE:

i) Wash the whole apparatus with water.

ii) Rinse the burette with the given KMnO4, and fill it up to the zero mark (read the upper meniscus in this case)

iii) Rinse the pipette with the standard oxalic acid solution, and pipette out 10 ml of the solution into a clean conical flask.

iv) Add 1 test tube full of bench H₂SO₄ and heat to about 60°- 70°C.

v) While hot, titrate it by adding KMnO₄ solution from the burette in small amounts, swirling the flask all the while.

N.B. Do not add KMnO4 rapidly in the beginning.

vi) The pink color of KMnO₄ will go on disappearing. Continue adding KMnO4 solution till with a last single drop, a light pink color persists for a few seconds. This is the end point of titration. Read the upper meniscus.

vii)  Repeat the titration till you get two or three concurrent readings.

RESULTS:

Solution taken in the burette = KMnO₄ solution

Oxalic acid taken for each titration = 10 ml

Indicator = KMnO₄ self indicator

End point = From colorless to light pink.

Normality of oxalic acid = N / 10 (f = 1.048)

BURETTE READINGS:

No. of obs	Initial reading	Final reading	Vol. of KMnO4 (ml)	Mean
1			11.0 —->	(Approx)
2			10.8
3			10.6	10.6
4			10.6

CALCULATIONS :

KMnO₄ solution Oxalic acid

V₁ = 10.6 ml V₂ = 10 ml

N₁ = ? N₂ = N / 10 (f = 1.048)

By applying the normality equation

V₁N₁ = V₂N₂

N₁ = 10x Nx 1.048 / 10.6 x 10

     = N / 10 (f = 0.9887).