• 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) +? 5Fe^2+?(aq)? → Mn^2+?(aq) +? 5Fe^3+?(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) + I_2?(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