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

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Introduction

Experiment 3 Determination of the Percentage of Oxalate in Iron(II) Oxalate by Redox Titration 28-9-2005 Objective To determine the percentage by mass of oxalate in a sample of iron(II) oxalate Introduction Acidified potassium permanganate is a strong oxidizing agent. It oxidizes substances and reduces itself. MnO4- + 8H+ + 5e- -------------> Mn2+ + 4H2O (Purple) (Colourless) Both iron (II) and oxalate ions can be oxidized. When they come in contact with an oxidizing agent, the following reactions occur: 1. C2O42- ------------------> 2CO2 + 2e- 2. Fe2+ ------------------> Fe3+ + e- (Pale green) (Yellow) So, when potassium permanganate is added to acidified iron (II) oxalate, two redox reactions occur. 1. 5C2O42- + 2MnO4- + 16H+ + -------------> 2Mn2+ + 8H2O +10CO2 2. 5Fe2+ + MnO4- + 8H+-------------> Mn2+ + 4H2O + 5 Fe3+ Notice the color changes of the solution. At the beginning, the color of solution inside the conical flask should be pale green due to the iron (II) ions present. Then after a certain amount of potassium permanganate is added, it should change to yellow because iron (III) ions are formed. Finally, the color of the solution should change from yellow to purple after 1 excess drop of permanganate is added. ...read more.

Middle

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. ...read more.

Conclusion

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. ...read more.

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