IB DP Chemistry: SL复习笔记9.1.7 Redox Titrations

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) + I(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