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Students have asked recently: How does titrating with potassium permanganate test for hydrogen peroxide? Why does hydrogen peroxide turn KMnO4 clear? 2. How does catalase break down hydrogen peroxide?
Question Date: 2002-09-26
Answer 1:

Potassium permanganat is used as an oxidizing agent to determine the hydrogen peroxide amount in a solution. Hydrogen peroxide reduces the permanganate to a colorless product. This is a great example for a redox reaction.

2 KMnO4 + 3 H2SO4 + 5H2O2 = 2 MnSO4 + K2SO4 + 5O2 + 8H2O

or less confusing

5 H2O2 + 2MnO4- + 6H+ = 5O2 + 2Mn2+ + 8H2O

Manganese can exist in all oxidation states from +2 to +7. Manganese II forms salts with all common anions. They are mostly light pink in color. Manganese VII is found in the intense purple colored permanganate ions MnO4-. As long as you have hydrogen peroxide in your solution, the permanganate will always be reduced to a clear solution. But as soon as the hydrogen peroxide is gone the intense purple color of the permanganate will be visible.

Note: It is important that the hydrogen peroxide solution is acidic. The hydrogen ions take part in the reaction as a reactant. The protons are needed to form water. It is also important to use sulfuric acid and not hydrochloric acid because the permanganate is such a powerful oxidant, it would oxidize chloride ions to form chlorine.

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