23.3 Iodine/thiosulfate redox titrations

• Iodine/thiosulfate titrations are used to analyse oxidising agents

  • oxidising agent is first reacted with excess iodide ions
  • the iodine generated is then titrated with thiosulfate ions which reduces iodide ions back to iodine
• Carrying out a titration

  • standard solution of sodium thiosulfate is added to the burette
  • solution of oxidising agent is pipetted into conical flask with an excess of aqueous potassium iodide, turning into a yellow-brown colour
  • sodium thiosulfate is added from the burette
  • at endpoint, colour fades to a pale straw colour. starch indicator is added forming a blue-black colour
  • at endpoint, colour changes to colourless