Lab: Oxidation – Reduction Titrations ( Permanganometry )

Potassium permanganate, KMnO4, is probably the most widely used of all volumetric oxidizing agents. It is a powerful oxidant and readily available at modest cost. The intense color of the permanganate ion, MnO4- , is sufficient to detect the end point in most titrations.

Depending upon reaction conditions permanganate ion is reduced to manganese in the 2+, 3+, 4+ or 6+ state.

In solutions that are 0.1 M or greater in mineral acid the common reduction product is manganese (II) ion

MnO4-+ 8H++ 5e-↔ Mn2++ 4H2O E0 = 1.51 V

This is the most widely used of the permanganate reactions.

In solutions that are weakly acidic (above pH 4) neutral, or weakly alkaline manganese dioxideis the most common reduction product

MnO4-+ 4H++ 3e-↔ MnO2(s) + 2H2O E0 = 1.70 V

Titration in which manganese dioxide is the product suffer from the disadvantage that the slightly soluble brown oxide obscures the end point; time must be allowed for the solid to settle before an excess of the permanganate can be detected. Some important volumetric analyses based on permanganate involve reduction to manganese ion according to the half reaction given below;

MnO4-+ e-↔ MnO42-   E0 = 0.56 V

This stoichiometry tends to predominate in solutions that are greater than 1 M in sodiumhydroxide. Alkaline oxidations with permanganate have proved to be most useful in the determination of organic compounds.


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