Potassium manganate(VII) solution cannot be accurately prepared by weighing the salt, for several reasons:
The solution must therefore be standardised against a primary standard. Such a compound has the following characteristics:
A suitable primary standard for the standardisation of potassium manganate(VII) solution is sodium ethanedioate, Na2C2O4. The reaction is
2MnO4- + 5C2O42- + 16H+ à 2Mn2+ + 10CO2 + 8H2O
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 80oC.
4 Titrate the mixture with potassium manganate(VII) solution until the mixture is just permanently pink. (The solution must be kept above 60oC. 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.
Mass of sodium ethanedioate/g:
Hence calculate the concentraton of the potassium manganate(VII) solution.
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