Experiment 1 The Potentiometric Titration Of Hydrogen Peroxide

EXPERIMENT 1 : The Potentiometric Titration of Hydrogen Peroxide

INTRODUCTION Titration is a volumetric technique used to determine the concentrations of solutions. A titration involves the addition of a titrant to an analyte. The titration is carried out until it reaches an endpoint or equivalence point which is the exact point at which the reaction between the two solutions is complete. A chemical indicator is often used to aid in the identification of the end point but in this case our titrant is intensely coloured so an indicator is unnecessary. In this experiment, we will be titrating hydrogen peroxide with the permanganate ion and observing the redox reaction that occurs. Once the titration is completed (purple colour remains for at least 60 seconds) we can use the volumes measured for each solution, as well as the concentration of the titrant, to determine the concentration of the analyte. We e are using ORP (Oxidation-Reduction Potential) Sensor to measure the potential of the reaction. The data will look like an acid-base titration curve. The volume of KMnO4 titrant used at the equivalence point will be used to determine the concentration of the H2O2solution. OBJECTIVE In this experiment, I am able to conduct the potentiometric titration of the reaction between commercially available hydrogen peroxide and potassium permanganate, measure the potential change of the reaction and also determine the concentration of the hydrogen peroxide solution. METHOD: Measuring Volume Using a Drop Counter 1. An acidified and diluted hydrogen peroxide, H2O2, is prepared as solution for the titration a) 100mL of 0.3% H2O2 from stock solution of 3% H2O2 is prepared. b) 10mL of the diluted H2O2 solution is measured out precisely into a 250mL beaker. 25mL of distilled water and 10 mL of 4.5 M sulfuric acid, H2SO4, solution is mixed carefully. CAUTION: H2SO4 is a strong acid and should be handled with care. 2. The beaker of hydrogen peroxide solution is placed on a magnetic stirrer and a stirring bar is added

3. The apparatus is assembled as shown in Figure 1:

Figure 1 a) b)

4.

5.

6. 7.

The Drop Counter has been calibrated prior experiment. ORP Sensor is insert through the large hole in the Drop Counter into the beaker. c) the positions of the Drop Counter and reagent reservoir is adjusted so they are both lined up with the center of the magnetic stirrer. Lab Quest is switched on. Drop counter and ORP Sensor is attached to DIG 1 and Channel 1, respectively. The reading for volume (mL) and potential (mV) at the screen can be observed. Rinsed and filled the reagent reservoir with 0.020M MnO4– solution. Open both valve so that the tip is filled with the MnO4– solution, then close the top valve. CAUTION: Handle the KMnO4 solution with care; it has been mixed with H2SO4, which can cause painful burns if it encounters the skin. Magnetic stirrer switched on. Make sure the stirring bar does not hit the ORP Sensor. The titration is ready to begin. At the equivalence point, a faint pink color of unreacted MnO4– solution is observed. a) Before adding MnO4– titrant, tap ► to start data collection. b) Carefully open the top valve so that the MnO4– solution drops once for every second. You can see the plot at the meter screen as well as the volume of MnO4– solution added at the bottom of the screen. c) Wait for the plot to make a huge jump. A linear increase of the plot is observed. MnO4– solution is allowed to continue dropping for another 2mL. Then stop the titration. d) Tap ■ to stop data collection. e) The titration curve is examined and estimate the volume of MnO4– solution used to reach the equivalence point of the titration. This value is recorded in data table for Trial 1.

9. The reaction mixture is disposed as directed. The ORP Sensor is rinsed with distilled water in preparation for the second titration. 10. For the second trial, the Cabinet icon is tapped next to word Run 1, the graph for Run 1 will be stored temporarily. The screen will show new graph with name Run 2. The titration is repeted for another two times with a new sample of H2O2 solution. Be careful by adding the MnO4– solution drop by drop in the region near the equivalence point, so that you can precisely identify the equivalence point of the reaction. 11. After all trials are completed, tap the word Run 3. You will be able to select previous run or select all runs. The graph of the selected run will be displayed. 12. Print/ save a copy of the titration data curve for the trial that you intend to use in your data analysis. DATA TABLE

RESULTS AND CALCULATIONS 1. Plot the potential (E) versus reagent volume (V) for each trial and indicate the equivalence point. Trial 1

Trial 2

Trial 3

2. Plot the derivative of the titration curve (∆E/∆V) for each trial and determine the equivalence point from the point of inflection

Trial 1

Trial 2

Trial 3

3. Write the oxidation and reduction half-reactions for the redox reaction taking place in this laboratory. Identify the oxidizing agent for this reaction. Justify your answer. Reduction : MnO4 – (aq) + 8H+ (aq) + 5e–  Mn2+(aq) + 4H2O(l) Oxidation : H2O2(aq)  O2+(g) + 2H+ (aq) + 2e– MnO4 – is the oxidizing agent because it accepts electrons to be reduced to Mn2+ H2O2 is the reducing agent because it releases electrons to be oxidized to H+

4. How many moles of electrons are transferred in the balanced redox reaction? Justify your answer. To balance charge : ( MnO4 – + 8H+ + 5e–  Mn2+ + 4H2O ) x 2 ( H2O2  O2+ + 2H+ + 2e– ) x 5 Balanced redox equation : Reduction : 2MnO4 – (aq) + 16H+ (aq) + 10e–  2Mn2+(aq) + 8H2O(l) Oxidation : 5H2O2(aq)  5O2+(g) + 10H+ (aq) + 10e– 10 moles of electrons are transferred to balance the reaction

5. State the value of equivalence point obtained from the derivative plot (∆E/∆V). Calculate the moles of MnO4– used to reach the equivalence point. Run 1 Equivalence point : 15.786 mL Moles of KMnO4 – used : (0.020) (15.786) 1000 = 3.157 x 10-4 moles Run 2 Equivalence point : 15.929 mL Moles of KMnO4 – used : (0.020) (15.929) 1000 = 3.186 x 10-4 moles Run 3 Equivalence point : 14.895 mL Moles of KMnO4 – used : (0.020) (14.895) 1000 = 2.979 x 10-4 moles 6. Use the number of MnO4– moles to calculate the moles of H2O2 in the sample of solution. 2 moles of MnO4– react with 5 moles of H2O2 Run 1 Moles of H2O2 : 3.157 x 10-4 x 5/2 = 7.893 x 10-4 moles Run 2 Moles of H2O2 : 3.186 x 10-4 x 5/2 = 7.965 x 10-4 moles Run 3 Moles of H2O2 : 2.979 x 10-4 x 5/2 = 7.448 x 10-4 moles

7. Calculate

the molar concentration of the H2O2 solution.

Run 1 M=

7.893 x 10-4 x 103 10 = 0.07893 mol dm-3

Run 2 M=

7.965 x 10-4 x 103 10 = 0.07965 mol dm-3

Run 3 M=

7.448 x 10-4 x 103 10 = 0.07448 mol dm-3

DISCUSSIONS This experiment results in graphs which is nearly similar to acid-base titration curve. On the first trial, the equivalence point is 14.5535mL and the value of its derivative is 15.786mL. There is a significant deviation of 1.233mL from the values of the first trial. On second trial the equivalence point is 15.071mL and the value of its derivative is 15.929mL which have deviation of 0.855mL. For third trial, there is significant deviation of 0.288mL from the values of the two equivalence points. These difference in values are caused by several errors. One source of error that could have occurred is that the buret was kept open and the permanganate freely dripped into the peroxide.This free drip affected the results as it added too much permanganate to the peroxide solution and caused it to turn very pink instead of the light, faded pink that was wanted. Next, the error that could have occurred is because of permanganate splashing and sticking on the wall of beaker during titration. This causes the measured potential are not accurate since the reaction did not react completely. The error also caused by the LabQuest which is not correctly calibrated which results in inaccurate reading. The advantages of using potentiometric titration over a direct potentiometric measurement is it requires no indicator to determine the equivalence point. Potentiometric titration is also far more accurate and precise than manual titration with high accuracy up to three digits in millilitres. This kind titration can measure cell potential for every drop of titrant.

CONCLUSION

In conclusion, by conducting redox titrations using a standardized permanganate solution we were able to measure the potential change of the reaction and also determine the concentration of the hydrogen peroxide solution. REFERENCES 1. Dianne, J. (2018,May 15). The Advantages of Potentiometric Titration. Retrieved from https://sciencing.com/advantages-potentiometric-titration-6532902.html 2. Bell, R. N., Analyt. Chem., (1947), 19, 97.