Solutions used in permanganometric titrations

Potassium permanganate solution

Solid permanganate is difficult to prepare in form pure enough to be used as standard substance, so it is not possible to prepare its solution of known concentration just by dissolving known mass of solid. Commonly used solutions are 0.02M (0.1 normal).

This recipe can be used only for calculation of the approximate mass of the solid, as solution requires further actions before it can be used. Once permanganate is dissolved it should be boiled for about an hour, to oxidize organic contaminants that can be present in the solution. After cooling down, solution has to be filtered through sintered glass funnel to remove precipitated MnO₂, which can catalyze permanganate decomposition. Filtered solution should be kept in dark glass bottle with grinded glass stopper. Solution is not very stable - it should be filtered every 2-3 months and it should be standardized after each filtration. It is best to keep the solution neutral, as both low and high pH speed up permanganate decomposition.

Reinhardt-Zimmerman solution

Reinhardt-Zimmermann solution is used for iron titration. It combines concentrated sulfuric acid (necessary to lower pH to the values at which permanganate is a strong oxidizer, besides acid is consumed during the reaction), phospohric acid (used to complex iron (III) - complex is colorless and doesn't interfere with the end point detection) and manganese (II) sulfate (added to lower redox potential of permanganate so that the side reaction of oxidization of chlorides to free chlorine is eliminated).

Tin (II) chloride solution

Tin (II) chloride solution (in hydrochloric acid) is used to reduce iron (III) to iron (II) before titration.

Mercury (II) chloride solution

Mercury (II) chloride solution is used to oxidize excess tin (II) after the iron reduction. Product of the reaction - reduced mercury (I) - precipitates from the solution and is not oxidized by the permanganate.