Redox Titration: Permanganate against Oxalate (Ethanedioate)

Titrate a 25cm3 portion of the diluted mixture against the given standard Sodium Hydroxide (0.05M). Add 3 drops of Phenolphthalein indicator. At the end point, the solution will change from colorless to pink. Repeat the titration for 3 more times to find out the average volume.

(C) Redox titration

Titrate another 25cm3 portion of the diluted mixture against the given standard potassium permanganate KMnO4 (0.02M). For each titration, you should add about 20cm3 2M sulphuric acid and heat the mixture to above 343K before adding the potassium permanganate. Further heating is necessary during the titration as the mixture is cooled down. Repeat the titration at least twice.

Data:

(A) Morality of standard sodium hydroxide solution: 0.05M

Titration volume (cm3)

Trial

2

3

Final burette reading

4.00

7.50

0.95

4.50

Initial burette reading

035

4.00

7.50

0.95

Volume of NaOH used

3.65

3.50

3.45

3.55

(B) Morality of standard potassium permanganate solution: 0.02M

Titration volume (cm3)

Trial

2

3

Final burette reading

8.70

1.95

5.10

8.25

Initial burette reading

5.10

8.70

1.95

5.10

Volume of NaOH used

3.60

3.25

3.15

3.15

Calculation:

(1) H2C2O4 + 2 NaOH ~~~~>2 H2O + Na2C2O4

No. Of mole of NaOH = 0.05 * (3.5/1000)

=0.000175

Molar ratio of H2C2O4 : NaOH = 1 : 2

No. Of mole of H2C2O4 = 0.000175*0.5

=0.0000875

Morality of H2C2O4 = [0.0000875/25/1000]*10

=0.035

Therefore the concentration of oxalic acid is 0.035M

(2) 16H+ + 5 C2O4 + 2 MnO4 ~~~> 8 H2O + 2 Mn + 10 CO2

No. Of mole of MnO4 = 0.02*3.18/1000

=0.0000637

Mole ratio of C2O4 : MnO4 = 5 : 2

No. Of mole of C2O4 = 0.0000637*5*0.5

=0.000159

Morality of C2O4 = [0.000159/25/1000]*10

=0.0636

Therefore the concentration of the oxalate salt in the original mixture is 0.0636M

Discussion:

We can also use the methyl orange, as the indicator because it also has a clear colour change during the titration .It will change to the yellow when it is in the acid condition. When it turns to alkali, it will turn to colourless. During the titration, if the temperature is below 70 C the reaction will very slow. It is because the optimum temperature of reaction is 70 C, when it reachs70 C the reaction become very effective.

We cannot determine the concentration by weighting and dissolving in water because there are some impurities occurs. During the process, the solid is not 100% dissolve in the water and may polluted.