Use redox titrations, including permanganate titrations, to determine the concentration of an analyte from balanced half-equations and stoichiometry.

What this dot point is asking

You must be able to set up and interpret a redox titration, write and combine half-equations, and carry the stoichiometry through to a concentration. Permanganate titrations are the classic SACE example.

How a redox titration works

A redox titration is just like an acid-base titration except the reaction is electron transfer rather than proton transfer. A burette delivers a solution of known concentration (the titrant or standard) into a measured volume of the analyte until the reaction is exactly complete (the equivalence point).

Permanganate as a self-indicating titrant

Potassium permanganate, $\text{KMnO}_4$, is the most common SACE titrant because it is self-indicating. In acidic solution the permanganate ion is reduced:

$\text{MnO}_4^-$ is intensely purple; $\text{Mn}^{2+}$ is almost colourless. While analyte remains, each added drop is decolourised. At the endpoint the first slight excess of $\text{MnO}_4^-$ turns the solution a faint, permanent pink - no separate indicator is needed.

Worked stoichiometry

A common analyte is iron(II), which is oxidised:

To combine the half-equations, balance the electrons. Multiply the iron half-equation by 5 so it matches the 5 electrons gained by permanganate, then add:

The mole ratio is therefore $1\,\text{MnO}_4^- : 5\,\text{Fe}^{2+}$.

Reliability of the result

As with any titration, use the average of concordant titres (within $0.10\ \text{mL}$ of each other) and discard the rough trial. Rinse the burette with the titrant and the pipette with the analyte so residual water does not dilute either solution.

Other redox titrants

• Iodine/thiosulfate titrations measure oxidising agents indirectly; starch is added near the endpoint to sharpen the blue-black to colourless change.
• Dichromate ($\text{Cr}_2\text{O}_7^{2-}$) is a strong oxidiser used with a separate redox indicator because its colour change is less distinct.