IS IT REALLY 3% HYDROGEN PEROXIDE?

ADAPTED FROM CHEMISTRY IN MICROSCALE

PURPOSE: To determine the concentration of hydrogen peroxide in a commercial product by redox titration INTRODUCTION: Labels on commercial hydrogen peroxide solutions read 3% hydrogen peroxide. Is this accurate? The mass percent of hydrogen peroxide, H2O2 may be determined by titrating H2O2 with potassium permanganate in an acidic solution. In this reaction, H2O2 is oxidized to oxygen gas and the permanganate ion is reduced to Mn2+. Using the masses of the solutions used and the stoichiometry of the reaction, the percentage of H2O2 can be determined. MATERIALS: Balance, Well plate, 2 Beral pipettes, toothpick, 3% H2O2, KMnO4 (aq), 6M H2SO4 SAFETY: BE CAREFUL WITH POTASSIUM PERMANGANATE SOLUTION, WHICH IS A STRONG OXIDIZING REAGENT AND 6M SULFURIC ACID, WHICH IS BOTH A STRONG ACID AND AN OXIDIZING AGENT. REPORT ALL SPILLS. PRE-LAB QUESTIONS: 1. Give the oxidation states of all the elements in the following species: H2O2, KMnO4, H2SO4, and O2. 2. Use the half-reaction method to balance the following equation: H2O2 + MnO4- O2 + Mn2+. Label

each half-reaction as reduction or oxidation and label the oxidizing agent and reducing agent. 3. In this lab, which solution is the titrant? The analyte? 4. What is the definition of equivalence point? In this lab, how would you recognize the equivalence

point? PROCEDURE: PREPARE YOUR OWN TABLES FOR QUALITATIVE AND QUANTITATIVE OBSERVATION/DATA. 1. Fill a labeled polyethylene pipette with commercial hydrogen peroxide solution. Record observation. 2. Take precautions to keep the solution off your skin. Fill a second pipette (be sure to label it) with

KMnO4 solution. Record the concentration and observation of the solution. 3. Record the initial mass of both pipettes. 4. Add 15 drops of the H2O2 solution to one of the wells in your 12 well plate. Add 3 drops of 6M H2SO4. Caution! Keep sulfuric acid off your skin. Save the polyethylene pipette with the H2O2 solution to mass later. 5. Add the KMnO4 solution one drop at a time to the same well until a faint pink color or a brown precipitate persists. If needed, use a toothpick to stir the solution. 6. Determine and record the final mass of each pipette. 7. DO TWO MORE TRIALS. All used solutions can be poured down the drain. Wash the well plate with

water.

ANALYSIS/CALCULATION: Show calculations for the first trial. For the other trials, simply record the numerical values. Trial 1 Trial 2 Trial 3 1) Concentration of KMnO4 solution:______________ _______________ _____________ 2) Initial Mass of KMnO4 solution: _______________ _______________ _______________ 3) Final Mass of KMnO4 solution: _______________ _______________ _______________ 4) Mass of KMnO4 solution used: _______________ _______________ _______________ 5) Initial Mass of H2O2 solution: _______________ _______________ _______________ 6) Final Mass of H2O2 solution: _______________ _______________ _______________ 7) Mass of H2O2 solution: _______________ _______________ _______________ 8) Mass of KMnO4 reacted: _______________ _______________ _______________ 9) Moles of KMnO4 reacted: _______________ _______________ _______________ 10) Moles of H2O2 reacted: _______________ _______________ _______________ 11) Mass of H2O2 reacted: _______________ _______________ _______________ 12) % H2O2 : _______________ _______________ _______________ 13) Avg % H2O2 _______________ POST-LAB QUESTIONS: 1. Based on your answer determine whether the label on the commercial product is correct. Explain. 2. Commercial hydrogen peroxide contains small amounts of organic compounds, which are added to

stabilize the hydrogen peroxide. If these compounds react with the permanganate ion, how will this affect the % H2O2? Explain.

3. If the pipette were wet with water prior to filling it with potassium permanganate, how would that affect

the % H2O2? Explain. 4. If the pipette were wet with water prior to filling it with hydrogen peroxide, how would that affect the

% H2O2? Explain.