Redox Titration of Hydrogen Peroxide Background In this experiment you will determine the concentration of H2O2 in a dilute hydrogen peroxide solution by titrating the solution against a standardized potassium permanganate (KMNO4) solution. The hydrogen peroxide (H2O2) solution that is commonly available in drug stores is a 3%(m/m) solution. The industrial solutions is 30%(m/m). At this higher concentration the solution must be handled with rubber gloves. In time hydrogen peroxide decomposes to form water and oxygen gas. This is why all hydrogen peroxide solutions are stored under conditions that protect them from light and contamination that may catalyze decomposition. Its efficiency as a germicide in wounds depends upon the release of oxygen gas, which is catalyzed by an enzyme in the blood. In H2O2, the oxygen gas is assigned an oxidation state of -1 and can be oxidized to the zero state (O2) or reduced to the -2 state; as in H2O. Therefore, hydrogen peroxide is both an oxidizing agent and a reducing agent. Actually it is a very powerful oxidizing agent and a rather poor reducing agent. It is considered a reducing agent only in the prsence of a very strong oxidizing agent; usually in an acidic solution. In this experiment, H2O2(aq) plays the role of a reducing agent where as the other reactant, potassium permanganate, acts as a strong oxidizing agent. The net ionic equation for the redox reaction is shown below: 5 H2O2(aq) + 2 MnO2 (aq) + 6 H+(aq) +502 + 2 Mn2+ + 8 H2O(1) -1 Potassium permanganate, KMnO4, is widely used as an oxidizing agent in volumetric analysis. In acid solution, the MnO ion undergoes reduction to Mn2+. Since the Mno ion is violet and the Mn2+ ion is nearly colorless, the endpoint in titrations is the appearance of excess KMnO4 solution which appears as the first permanent pink color in the solution. Complete these problems in your laboratory notebook before you begin the lab work on the Redox Titration of H2O2. 1. Wtite balanced net ionic equations for the reaction between H2O2(aq) and MnO2 (aq), which occurs in an acid solution. (a) the oxidation half-rection (b) the reduction half-reaction 2. If 10.00 mL of an H2O2(aq) solution requires 22.55 mL of a 0.0153 M KMnO4 to reach the endpoint of the titration, what is the molarity of the H2O2 solution? 3. A hydrogen peroxide solution is labeled as 9.97 M and has a density of 1.13 g/mL. Calculate its concentration in terms of mass percent of H2O2; %(m/m)H2O2. 4. If 1.39 g of KMnO4(s) is dissolved to prepare a 500.0 mL solution, what is the molar concentration of the MnO anion in this solution? 5. In this experiment, 10.00 mL of the unknown solution to be analyzed is diluted to 100.0 mL. Then 10.00 mL of the dilute solution is titrated with a standardized permanganate solution. Why not skip all this dilution business and just pipet 1.00 mL of the original unknown solution and titrate it directly? Assume that the uncertainty in each pipet reading is = 0.02 mL. 27 Experimental Procedure Safety: Safety: Wear safety goggles throughout the experiment. Observe the usual precautions in handling the dilute (3M) sulfuric acid solution. (Wash skin promptly with water upon contact. Then neutralize any spill with solid sodium bicarbonate before cleaning up). The permanganate solution in the dilute concentration used presents little hazard except for possible staining of skin or clothing. Wash it off promptly with water. 1. Obtain a 50 mL buret, a 10-mL volumetric pipet, and a 100-mL volumetric flask. Rinse the glassware and ensure that they are in fact clean (i.e., water drains without forming beads). 2. Take a clean and dry 250 mL beaker to the reagent area and obtain about 100 mL of the standardized potassium permanganate solution; KMnO4(aq). Do not waste the KMnO4 solution. You can always come back for more, but you cannot return any leftover excess to the reagent bottle. Be sure to record the Molariy (M) of the KMnO4 solution in your laboratory notebook. Cover the 250 mL beaker with a watch glass. 3. Fill the 50 mL buret with the permanganate solution, being careful not to spill. One difference in this titration from other titrations you have performed is that, because the permanganate is so intensely colored, you cannot see the bottom of the meniscus in the buret. What is visible is the top level of the solutyion inthe buret. However, since the volume used in data analysis is calculated from the difference of two volume readings, no error is introduced. 4. Obtain your hydrogen peroxide unknown from your instructor. Do not forget to record the unknown ID code of hydrogen peroxide in your laboratory notebook. Dilution of the hydrogen peroxide unknown: Even though the unknown issued to you is less than 3%(m/m) H2O2, it is still too concentrated for volumetric analysis. Any reasonable sized sample, say 10.0 mL or more would require an excessive amount of the permanganate solution; more than the 50 mL buret can handle. (As a general rule, a volume greater than 50 mL, which is one full buret-is excessive.) 5. Using a pipet bulb, carefully pipet 10.00 mL of your unknown solution (labeled OS) into a 100 mL volumetric flask (See Figure 1 on Page 30). Using a beaker add deionized water until the level is close to, but still below the calibration mark on the flask. Finally, bring the deionized water level to the calibrated mark by adding water dropwise from a medicine dropper. Mix thoroughly by inverting the stoppered flask 20 times. Aliquot Analysis: The term aliquot refers to a sample taken for analysis (the sample is typically a fraction of the solution being analyzed). Therefore when you remove a fraction of the diluted solution for analysis, that fraction is called an aliquot. Also, when performing calculations as well as in reporting results, it is important to distinguish between the original solution (labeled as OS) and the diluted solution (labeled as DS). 28 6. Pipet a 10.00 mL aliquot of the DS solution into a 125-mL Erlenmeyer flask; this is the titration flask. For preparation of titration sample see Figure 2 on Page 30 7. Add about 25 mL of deionized water and 10 drops of 3M H2SO4 acid to the titration flask. Set the 125 mL erlenmeyer flask on a piece of white paper during titration to improve visibility of the end point. 8. Make a reading of the buret volume and record this as the "initial buret reading" (Note: burets should be read to + 0.01 mL). Start adding the permanganate solution to the the H2O2 sample in the 125 mL erlenmeyer flask; swirl the flask as you do. The purple permanganate solution will be readily decolorized upon contact with the H2O2 solution. As you continue to add permanganate solution the purple color will persist a bit longer; this means that the moles of H2O2 in solution is decreasing. Take this as a signal to slow down the rate at which permaganate solution is added. When all the hydrogen peroxide is oxidized, the next drop of the permanganate solution will color the entire solution in the 125 mL erlenmeyer flask pink. This is the endpoint of the titration. (It is possible to refine the endpoint to a half drop; simply touch a drop of permanganate hanging from the buret tip with the inside wall of the erlenmeyer flask. Then wash the permanganate drop into the flask with a stream of DI water from your wash bottle). The proper endpoint color is "very light pink”. Make a reading of the buret volume (+ 0.01 mL) and record it as the "final buret reading". The volume of permanganate used is the the final buret reading minus the initial buret reading. 9. Analyze 3 or 4 10 mL aliquots of the DS H2O2 solution. That is, perform 3 or 4 trials. The volume of permanganate used for each trial should be very close, but don't be biased. It is fine to let previous experimental trials tell you when to slow down, but make each endpoint determination independent of the others. 10. Dispose of titrated samples in the designated waste container located in the fume hood. Any excess potassium permanganate solution in the same waste container as the titrated samples. Cleaning the Buret: Rinse the buret barrel and tip thoroughly with tap water. On standing in glassware, permanganate solutions are known to leave a brown stain of MnO2. Such stains can be removed by treatment with dilute hydrochloric acid followed by a rinsing with water. 29 DI H20 Pipette 10 ml Sample Mix well DS OS OS 100 mL Volumetric Flask Figure 1: Preparation of Diluted solution (DS) of H2O2 from the Original Solution (OS). 50 mL Buret Standard KMnO4 Pipette 10 mL Sample 25 mL DI H2O 10 Drops 3 M H2SO4 DS 125 mL Volumetric Flask Titration Sample Figure 2: Preparation of Titration Sample.
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