Redox Titration between MnO4- and Fe2+

Описание: This video demonstrates the redox titration between potassium permanganate (KMnO4) solution and iron (ii) (Fe2+) solution.

MnO4- + 5Fe2+ + 8H+ → 5Fe3+ + Mn2+ + 4H2O

This titration is 'self indicating': it does not require an indicator because there is a clear colour change from deep purple to (almost) colourless. A white tile is often used to help identify the endpoint.

The potassium permanganate is deep purple in colour and this makes the bottom of the meniscus almost impossible to see. It is common in this experiment to read from the top of the meniscus instead.

This is a common titration because potassium permanganate, although often used in titrations, is relatively unstable (disproportionation/thermal decomposition, 2 KMnO4 → K2MnO4 + MnO2 + O2, becomes feasible above 230 0C, but can still occur at lower temperatures). It is not a 'primary standard'. This is because preparation of the solution will always cause formation of solid manganese dioxide (MnO2). Solutions of potassium permanganate must be filtered before being used, to remove this. It is then necessary to find the exact concentration using the titration shown in this video. This is called 'standardising the potassium permanganate solution'. Iron (ii) ammonium sulfate is a primary standard; in solution it is often mixed with sulfuric acid to make it even more stable.

Interestingly, an even more accurate alternative is the use of arsenious acid, instead of iron(ii). This is too dangerous to be used in a school.