Redox Titrations
- In a titration, the concentration of a solution is determined by titrating with a solution of known concentration.
- In redox titrations, an oxidizing agent is titrated against a reducing agent
- Electrons are transferred from one species to the other
- Indicators are sometimes used to show the endpoint of the titration
- However, most transition metal ions naturally change colour when changing oxidation state
- There are two common redox titrations you should know about manganate(VII) titrations and iodine-thiosulfate titrations
Manganate(VII) Titrations
- A redox reaction occurs between acidified manganate (VII) ions and iron (II) ions:
MnO4– (aq) + 8H+ (aq) + 5Fe2+ (aq) → Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l)
- This reaction needs no indicator as the manganate (VII) is a strong purple colour which disappears at the end point, so the titration is self-indicating
- This reaction is often used for the analysis of iron for example in iron tablets (health supplement)
Iodine-Thiosulfate Titrations
- A redox reaction occurs between iodine and thiosulfate ions:
2S2O32– (aq) + I2 (aq) → 2I–(aq) + S4O62– (aq)
- The light brown/yellow colour of the iodine turns paler as it is converted to colourless iodide ions
- When the solution is a straw colour, starch is added to clarify the end point
- The solution turns blue/black until all the iodine reacts, at which point the colour disappears.
- This titration can be used to determine the concentration of an oxidizing agent, which oxidizes iodide ions to iodine molecules
- The amount of iodine is determined from titration against a known quantity of sodium thiosulfate solution
- This reaction can be used for the analysis of chlorine in bleach