The standardisation of potassium manganate(VII).

Potassium manganate(VII) solution cannot be accurately prepared by weighing the salt, for several reasons:

• it is not commercially obtainable in a high state of purity;
• it readily oxidises traces of organic materials (e.g. dust) which find their way into the solution;
• the potential for the MnO₄^-/MnO₂ couple is such that it slowly oxidises solvent water - this is why potassium manganate(VII) solution gradually deposit manganese(IV) oxide on storage.

The solution must therefore be standardised against a primary standard. Such a compound has the following characteristics:

• it can be obtained commercially in high purity;
• it is stable in solution;
• it does not absorb water or carbon dioxide from the atmosphere nor vaporise whilst being weighed;
• it can be dried readily in an oven or by stronger heating without decomposition.

A suitable primary standard for the standardisation of potassium manganate(VII) solution is sodium ethanedioate, Na₂C₂O₄. The reaction is

2MnO₄^- + 5C₂O₄^2- + 16H⁺ à 2Mn²⁺ + 10CO₂ + 8H₂O

Method:

1 Weigh accurately about 2.0 g of sodium ethanedioate (this substance is poisonous);

2 Transfer this solid quantitatively to a 250 cm³ graduated flask, add about 75 cm³ of bench sulphuric acid, and shake until the solid has dissolved. Make to the mark with bench sulphuric acid, and mix well.

3 Pipette 25.0 cm³ of the sodium ethanedioate solution to a 250 cm³ conical flask, add about 50 cm3 of bench sulphuric acid, and heat to about 80^oC.

4 Titrate the mixture with potassium manganate(VII) solution until the mixture is just permanently pink. (The solution must be kept above 60^oC. If a brown precipitate starts to form, the solution is either too cool or the titration is being performed too quickly.)

5 Repeat to obtain three consistent titres.

6 Calculate the concentration of the potassium manganate(VII) solution.

Results:

Mass of sodium ethanedioate/g:

Mean titre/cm³:

Hence calculate the concentraton of the potassium manganate(VII) solution.

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