Determination of the Percentage of Oxalate in Iron(II) Oxalate by Redox Titration

Procedure

1. Approximately 2 grams of iron (II) oxalate was weighed out with a weighing bottle.
2. The oxalate powder was poured into a beaker. Dilute sulphuric acid was added.
3. The beaker was heated and the solution inside was stirred until the solution turns very pale yellow..
4. The solution in the beaker was poured into a volumetric flask and made up to 250 cm3 with distilled water.
5. Potassium permanganate was poured into a burette.
6. 25 cm3 of the oxalate solution was transferred to a conical flask. About 10 cm3 of sulphuric acid was added into the conical flask.
7. The conical flask was heated until the solution inside reaches 60oC.
8. A few drops of potassium permanganate were added to the conical flask. The flask was shaken until a yellow colour appeared.
9. Potassium permanganate was further added until the solution turned pink. The reading was taken.
10. Steps 5 to 9 was repeated 3 more times.

Data and Calculation

        Weight of bottle with FeC2O4.2H2O = 10.093g

        Weight of bottle after FeC2O4.2H2O was poured into beaker = 8.311g

        Mass of FeC2O4.2H2O used = 1.782g

Volume of KMnO4 used in titration

        Average volume of KMnO4 used = (27.3 + 27.4 + 27.4)/3 = 27.367 cm3

        No. of moles of KMnO4 used = 27.367/1000 X 0.02 = 5.473 X10-4 moles

        For every 3 moles of MnO4- used, 5 moles of FeC2O4 is reacted.

        So, no. of moles of FeC2O4 .2H2O in every 25cm3= 5.473 X 10-4 /3 X 5 =

9.122 X10-4 moles

        There are also 9.122 X 10-4 moles of oxalate in every 25cm3. Then there would be 9.122 X 10-3 moles of oxalate in 250cm3 of the solution.

        Weight of C2O42- in the sample = 9.122 X 10-3 X (12 X 2 + 16 X 4) = 0.803g

        Percentage by mass of C2O42- in the sample (found by experiment)

 = 0.803 / 1.782 X 100% = 45.04%

        Molar mass of FeC2O4 .2H2O as written on its bottle = 179.9 g/mole

        Percentage by mass of C2O42- in the sample (theoretically)

                                = (24+16 X 4)/ 179.9 X 100%= 48.92%

Conclusion

Percentage by mass of C2O42- in the sample, as found by experiment, is 45.04%. Percentage by mass of C2O42- in theory is 48.92%.

Discussion

1. There was one procedure that I mentioned in the procedures section. It was that the iron (II) oxalate needed to be heated to 60oC before it can be titrated. Why is this step necessary? It was because the redox reaction between oxalate ions and permanganate ions were too slow. Why was the redox reaction between the two ions slow? This is due to 2 reasons. First, both ions are anions. They both carry the same negative charge. So, when they come close together, they would repel each other. The second reason is because of the structure of the 2 ions. The reactions between the two ions involve a lot of bond breaking and bond formation. So, it would take a long time for reaction.

Then to speed up the reaction of the oxalate and permanganate ions, the solution needs to be heated before it is titrated. But doing so needs a lot of time. How can we shorten the time required for heating the solution up to 60oC? There are 2 ways. First, we can heat the conical flask with a stronger flame. We can adjust the flame intensity by enlarging the air hole. Second, we can heat 1 conical flask and titrate another one which has been heated at the same time. These methods can be used but I only used the first one. Why? If I use the second method, it requires a lot of skill. If the conical flask being heated boils while I was titrating the other one, it would be very dangerous. So, I only used a stronger flame.

Answers to Study Question

1. This is because the permanganate ions itself can act as an indicator. Before it is reacted, the permanganate ions are purple in colour. When it is dropped into the conical flask and reduced, it turns to manganese ions and changes to colorless. So, the solution in the conical flask retains its color before titration. But after all the reducing agents in the conical flask has reacted, adding 1 more drop of permanganate ion will change the whole color of the solution pink.
2. If brown precipitates were formed during titration, I need to add more sulphuric acid and reheat the solution. Adding sulphuric acid can dissolve the manganese (IV) oxide. The temperature is lowered in this step. So, the solution needs to be reheated again.
3. The redox reaction takes a relatively long time to complete. So, to allow the redox reaction to go to completion before permanganate ions are further added, it has to be shaken well and for a period of time. This would allow the colour to change before any permanganate ions are added.
4. As mentioned above, the heating of the solution is to speed up the reaction. If heating is insufficient, it would take a long time for the purple color of the permanganate which is just added to change colour. So, a persistent colour change of the solution would not indicate the end point.
5. To make sure that exactly 25cm3 of oxalate solution is obtained, pipetting of it must be done with great care. Otherwise, if more or less of the solution is obtained, the titrations would become inaccurate.
6. It takes time to let the oxalate ions, iron (II) ions and permanganate ions to react with each other. So, the permanganate ions may not be decolourized directly. Also, at the start, the permanganate ions experience more repulsion from the oxalate ions. So, it may need more time for the permanganate ions to react with the oxalate ions.
7. Add an excess of potassium iodide and sulpuric acid into 25cm3 of potassium iodate. Pour this solution into a conical flask. Then, pour sodium thiosulphate into a burette. Titrate the solution in the conical flask with the thiosulphate. Just as the brown colour of iodine fades, add a drop of starch indicator. Find out the volume of thiosulphate required to decolourize the blue-black color of the starch solution. Then calculate the number of moles of thiosulphate ions used in the titration, and the molarity would be found.