Iodine/thiosulfate redox titrations

• in iodine/thiosulfate titrations, thiosulfate ions, S2O32- (aq) are oxidised and iodine, I2 is reduced
• The concentration of aqueous iodine can be determined by titration with a standard solution of sodium thiosulfate
• Analysis of oxidising agents:

  • iodine/thiosulfate titrations can be used to determine:
  • the ClO- content in household bleach
  • the Cu2+ content in copper (II) cmpounds
  • the Cu content in copper alloys
• Procedure


  1. Add a standard solution of Na2S2O3 to the burette

  2. Prepare a solution of the oxidising agent to be analysed - using a pipette, add this solution to a conical flask. then add an excess of potassium iodide
• The oxidising agent reacts with potassium iodide


  Produces iodine, which turns the solution a yellow-brown colour
• Titration


  1. Titrate the solution with the Na2S2O3 (aq)

  2. The iodine is reduced back to I- ions and the brown colour fades quite gradually, making it difficult to decide on an end point
• Starch indicator


  Used when the end point is being approached, the iodine colour has faded enough to become a pale straw colour
• using starch for the end point:

  • when the end point is being approached and the iodine has faded enough to become a place straw colour, add a small amount of starch indicator
  • a deep blue-black colour forms to assist the identification of the end point
  • as more sodium thiosulfate is added, the blue-black colour fades
  • at the end point, all the iodine will have just reacted and the blue-black colour disappears
• • starch is added - solution turns blue-black
  • blue-black colour fades near the end point
  • the end point - the blue-black colour has just disappeared - all the iodine has been reduced to I-