Exp 3: IS IT REALLY 3% HYDROGEN PEROXIDE?

Introduction:

Labels on commercial hydrogen peroxide solutions read 3 percent hydrogen peroxide. Is this accurate? And what about that bottle you've had in the bathroom cabinet for 6 months, or one year? The percentage of hydrogen peroxide, H2O2, may be determined by reacting it with potassium permanganate solution. In the reaction, H2O2 is oxidized to O2 and Mn7+ is reduced to Mn2+. The reaction is:

2MnO4-(aq) + 6H+(aq) + 5H2O2(aq) ---> 5O2(g) + 2Mn2+ (aq) + 8H2O(l)

From the masses of solutions used and the stoichiometry of the reaction, the percentage of H2O2 can be determined.

NOTE: This experiment involves two potentially dangerous chemicals. One is potassium permanganate, which is a strong oxidizing agent and will react quickly with skin and clothing. The other is the 6M sulfuric acid which is corrosive and caustic. Wash off a spill of either solution with large amounts of water. Goggles, of course, must be worn.

Procedure:

Fill a Beral pipet with commercial hydrogen peroxide and label. Then fill a second pipet with the potassium permanganate solution. Be sure to record the concentration of the solution, and label this pipet also. Then find the mass of each pipet to the nearest .01g.

Add 15 drops of the hydrogen peroxide solution to a small test tube. Add 3 drops of the 6M sulfuric acid. Save the Beral pipet with the peroxide solution to mass later.

Add potassium permanganate solution dropwise, with stirring, to the test tube until a faint pink color persists. A piece of white paper held behind the tube will help your detection of the pink. A brown ppt will sometimes form, especially if the permanganate sol'n is not stirred as it is added. Determine the mass of each pipet when finished.

Repeat two more times, using the same two pipets, refilling if needed and massing before and after each time.

Results and Discussion:

Report the percent of peroxide for each trial, as well as the average percent of peroxide with the average deviation.

Commercial hydrogen peroxide contains small amounts of organic compounds which are added to stabilize the hydrogen peroxide. If these compounds react with permanganage, how will this affect your results?

If you also tested a previously opened bottle of peroxide, report the percentage of peroxide in it. What could be the cause(s) of any decrease below 3%?

Gwen Sibert
Roanoke Valley Governor's School