Iron(II) titrations

This is an example of a redox titration.

Acidified potassium manganate(VII) is a popular standard solution for use in redox titrations. It is an oxidizing agent and can be used to oxidize a number of other substances. A common titration is the oxidation of iron(II) to iron(III) ions:

MnO₄^-(aq) + 5Fe²⁺(aq) + 8H⁺(aq) ⇒ Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O(l)

MnO₄^- º 5Fe²⁺

The solution needs to be well acidified to provide the H⁺ ions shown in the equation. Without sufficient acid alternative reactions take place and the link between the MnO₄^- and Fe²⁺ will be lost.

		This titrant is self-indicating. This means that a separate indicator is not needed. The manganate(VII) ion is bright purple, but it is reduced to the virtually colourless manganese(II) ion (it's actually very pale pink). When the iron(II) is finally used up, the manganate(VII) is no longer reduced, so the purple colour remains. As the titration should have been done to the nearest drop, there should be no more than a drop of purple manganate(VII) in the titration mixture. Consequently, it does not look purple, but appears pink. The end-point is the first permanent pink colour. If you put your mouse over the first picture you will see the actual end-point - it's not easy to see! If you are not sure whether you have reached an end-point, make a note of the burette reading and add another drop of titrant, This has been done and shown in the second picture - this has definitely gone!

The method can be adapted to measure the concentration of iron(III) ions. The iron(III) solution is first reacted with zinc metal and acid. This reduces the iron(III) to iron(II). This can be seen in the following video (note the final formation of a green precipitate of iron(II) hydroxide):

Video - the reduction of iron(III) by zinc metal

Excess zinc is now filtered off and the iron(II) solution is titrated with potassium manganate(VII) as before. Care needs to be taken with iron(II) solutions as they are oxidized to iron(III) by the air.

The calculations are very similar to the acid-base titration calculations. Try the following two exercises:

Exercise 5 and exercise 6 are iron(II)-manganate(VII) titration calculations.