Iodine/thiosulfate titrations

• Two common redox titrations:

  • potassium manganate(VII) (KMnO4 (aq) ) under acidic conditions

  • sodium thiosulfate (Na2S203 (aq) ) for determination of iodine (I2 (aq) )
• thiosulfate ions, S2O3 2- (aq) are oxidised
• iodine, I2 is reduced
• Oxidation half equation


  2 S2O3 2- (aq) → S4O6 2- (aq) + 2e
• Reduction half equation


  I2 (aq) + 2e -> 2I- (aq)
• Overall equation

  2 S2O3 2- (aq) + I2 (aq) -> 2I- (aq) + S4O6 2- (aq)
• The concentration of aqueous iodine can be determined by titration with a standard solution of sodium thiosulfate
• lodine/thiosulfate titrations can be used to determine:

  • the CIO- content in household bleach
  • the Cu2+ content in copper (Il) compounds
  • the Cu content in copper alloys
• What is step 1 of iodine/thiosulfate titration?


  Add a standard solution of sodium thiosulfate (Na2S2O3) to the burette
• What is step 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
• What happens in step 2?

  The oxidising agent reacts with iodide ions to produce iodine, which turns the solution a yellow-brown
• Step 3?

  • Titrate this solution with the sodium thiosulfate (aq)
  • During the titration, the iodine is reduced back to iodide ions and the brown colour fades quite gradually, making it difficult to decide on an end point
  • This problem is solved by using starch indicator
  • 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 colour has faded enough to become a pale straw colour, a small amount of starch indicator is added
  • A deep blue-black colour forms to assist with 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 colour change in iodine-thiosulfate solution

  1. when starch is added, the solution turns blue-black
  2. the blue-black colour fades near the end point
  3. the end point - the blue-black colour has just disappeared - all the iodine has been reduced to iodide
• Bleach is NaClO, sodium hypochlorite
• Bleach is a solution of sodium chlorate (I)
• The active ingredient in household bleach is chlorate ions ClO- commonly known as hypochlorite
• Examples of iodine-thiosulfate titrations for the analysis of two different oxidising agents:

  • chlorate (I) ions, ClO-(aq)
  • copper (Il) ions, Cu2+(aq)
• The same principles can be applied for the analysis of other oxidising agents, provided that they are capable of oxidising iodide ions to iodine
• Analysis of bleach example:

  • CIO- ions from the bleach are first reacted with I- and H+ ions to form I2:
  • CIO- (aq) + 2I- (aq) + 2H+(aq) → Cl- (aq) + I2(aq) + H2O (l)
  • In the titration, I2 reacts with S2O3 2- ions:
  • 2S2O3 2- (aq) + I2 (aq) → 2I- (aq) + S4O6 2- (aq)
• 2 main alloys of copper

  • brass
  • bronze
• Brass is made of copper and zinc
• Bronze is made of copper and tin
• Analysis of copper for copper (II) salts

  • Cu2+ (aq) ions are produced simply by dissolving the compound in water
  • Insoluble copper (II) compounds can be reacted with acids to form Cu2+ (aq) ions
• Analysis of copper for copper alloys

  • The alloy is reacted and dissolved in concentrated nitric acid
  • Followed by neutralisation to form Cu2+ (aq) ions