Acidified Potassium Permanganate Titration Lab Report

Abstract

This experiment explores the use of acidified potassium permanganate as a strong oxidizing agent in titration. The objective is to determine the percentage by mass of oxalate ions in a sample of iron (II) oxalate. Acidified potassium permanganate is added to acidified iron (II) oxalate, resulting in a series of redox reactions. The change in color of the solution during the titration process is used to determine the endpoint. Theoretical and experimental calculations are performed to determine the accuracy of the results.

Introduction

Acidified potassium permanganate is a potent oxidizing agent capable of oxidizing various substances while reducing itself in the process. It undergoes the following redox reaction:

Both iron (II) and oxalate ions can be oxidized when they come into contact with an oxidizing agent, leading to the following reactions:

When potassium permanganate is added to acidified iron (II) oxalate, two redox reactions occur:

The color changes of the solution during the titration are as follows: initially, the solution is pale green due to the presence of iron (II) ions. After the addition of potassium permanganate, it turns yellow due to the formation of iron (III) ions. Finally, the solution changes from yellow to purple after the addition of excess potassium permanganate, indicating the absence of reducing agents to react with the permanganate ions.

Materials and Methods

1. Approximately 2 grams of iron (II) oxalate was weighed out with a weighing bottle.
2. The oxalate powder was poured into a beaker, and dilute sulfuric acid was added.
3. The beaker was heated and the solution inside was stirred until it turned 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, and about 10 cm³ of sulfuric acid was added.
7. The conical flask was heated until the solution reached 60°C.
8. A few drops of potassium permanganate were added to the conical flask, and the flask was shaken until a yellow color appeared.
9. Potassium permanganate was further added until the solution turned pink, and the reading was taken.
10. Steps 5 to 9 were repeated three more times.

Results

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

Number of moles of KMnO₄ used = (27.367 / 1000) * 0.02 = 5.473 × 10^-4 moles

For every 3 moles of MnO₄^- used, 5 moles of FeC₂O₄.2H₂O is reacted.

No. of moles of FeC₂O₄.2H₂O in every 25 cm³ = 5.473 × 10^-4 / 3 * 5 = 9.122 × 10^-4 moles

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

Weight of C₂O₄^2- in the sample = 9.122 × 10^-3 * (12 * 2 + 16 * 4) = 0.803g

Percentage by mass of C₂O₄^2- in the sample (found by experiment) = (0.803 / 1.782) * 100% = 45.04%

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

Percentage by mass of C₂O₄^2- in the sample (theoretically) = (24 + 16 * 4) / 179.9 * 100% = 48.92%

Discussion

1. There was one procedure mentioned in the procedures section, which was heating the iron (II) oxalate to 60°C before titration. This step is necessary because the redox reaction between oxalate ions and permanganate ions is slow. The slow reaction can be attributed to two main factors: both ions are anions with the same negative charge, causing repulsion, and the reaction involves numerous bond breaking and formation steps.
2. To expedite the reaction between oxalate and permanganate ions, the solution needs to be heated. Two methods to shorten the heating time include using a stronger flame by adjusting the flame intensity and heating one conical flask while titrating another simultaneously. The first method was chosen in this experiment to ensure safety.

Answers to Study Questions

1. The purple color of permanganate ions can act as an indicator. Initially, the permanganate ions are purple, and when they are reduced during the titration, they become colorless. Therefore, the solution in the conical flask retains its color until all reducing agents are consumed. Adding one more drop of permanganate ions after the reaction with reducing agents will change the solution's color to pink, indicating the endpoint.
2. If brown precipitates were formed during titration, additional sulfuric acid should be added to dissolve the manganese (IV) oxide. The solution should then be reheated to facilitate dissolution.
3. The redox reaction takes a relatively long time to complete, necessitating thorough shaking of the solution for a significant duration to ensure that the color changes before additional permanganate ions are added.
4. Incomplete heating of the solution can slow down the reaction, requiring more time for the purple color of permanganate to change. Therefore, a persistent color change does not indicate the endpoint.
5. To ensure accuracy, precise pipetting of exactly 25 cm³ of oxalate solution is crucial. Any deviation from this volume can lead to inaccuracies in the titration results.
6. The slow reaction between oxalate and permanganate ions may require more time for the permanganate ions to react with the oxalate ions, particularly at the beginning of the titration.

Conclusion

The experiment determined that the percentage by mass of C₂O₄^2- ions in the sample of iron (II) oxalate was found to be 45.04%, while the theoretical percentage was calculated to be 48.92%. The variation between the experimental and theoretical values may be attributed to experimental error and incomplete reaction kinetics.

Recommendations

To improve the accuracy of the experiment, careful pipetting and more precise temperature control during heating should be emphasized. Additionally, consideration could be given to exploring alternative methods for accelerating the reaction between oxalate and permanganate ions to reduce the overall duration of the experiment.

References

• Ethical Principles in Designing Psychological Experiments | Experimental Psychology | JoVE
• University of Oregon Chemistry Demos: "Potassium Permanganate as an Oxidizing Agent"