9.1.7 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:

2S₂O₃^2– (aq) + I₂ (aq) → 2I^–(aq) + S₄O₆^2– (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