Titration – Redox


The purpose of this experiment is to balance the equation for the reaction between sodium thiosulphate and iodine.


You are to determine the ratio of a to b and so determine the stoichiometry of the reaction. You do this by taking a known amount of iodine and titrating it with standard sodium thiosulphate.

a I2 (aq) + b S2O32- (aq)  → products

The indicator you use in this titration is starch solution, which is deep blue in the presence of iodine; it is added near the end of the titration when the solution is straw-coloured. If you add starch too soon, you may get a blue-black precipitate which does not dissolve again easily even though there is an excess of thiosulphate. The end-point i n this titration is the point at which the addition of one drop of sodium thiosulphate causes the disappearance of the deep blue colour.


  • safety glasses
  • filter funnel
  • burette, 50 cm3
  • 2 beakers, 100 cm3
  • sodium thiosulphate solution, standardized (0.01 mol dm3) (LOW HAZARD)
  • pipette, 10 cm3
  • pipette filler
  • iodine solution, standardized (0.01 mol dm3) (LOW HAZARD)
  • 4 conical flasks, 250 cm3
  • starch indicator solution
  • white tile
  • wash-bottle of distilled water


  1. Using the funnel, rinse the burette and tip with the sodium thiosulphate solution. Fill it with the same solution. Don’t forget to fill the tip. Record the initial burette reading in the results table.
  2. Rinse the pipette with some of the iodine solution and carefully transfer 10.0 cm3 of the solution to one of the conical flasks.
  3. Titrate this solution until the colour of the iodine has almost gone (as indicated by a pale straw colour).
  4. Add 1-2 cm3 of starch solution and continue the titration, adding sodium thiosulphate dropwise until the end-point. Use the first flask for a trial run. Record the final burette reading.
  5. Repeat the titration three more times. Enter your results into a copy of your results table. These titrations should agree to within 0.10 cm3.

Results Table




  1. Use your results to determine the stoichiometric coefficients, a and b,

   in the equation:

a I2 (aq) + b S2O32- (aq)  → products

  1. All the iodine forms sodium iodide NaI. There is one other product – work

          out its formula.