Iodine-Sodium Thiosulfate Titrations

Iodine-Sodium Thiosulfate Titrations


  • A iodine-sodium thiosulfate titration is a redox reaction used to determine the amount of iodine produced in a reaction.
  • It involves the reactant iodine (I2) being converted to iodide ions (I-) and the thiosulfate ions (S2O3^2-) are oxidised to tetrathionate ions (S4O6^2-).


  • The basic method revolves around adding a known volume of sodium thiosulfate to a solution containing iodine until all the iodine is reacted and the solution turns a light straw colour.
  • To aid in identifying this end point of the reaction, a starch indicator is added which turns the solution into a blue-black colour.
  • The volume of sodium thiosulfate required to reach this point, or titrate the reaction to completion, is noted.
  • With the volume and the concentration of the sodium thiosulfate known, the amount of iodine that was in the original solution can be calculated using stoichiometry.


  • The balanced equation for this reaction is: 2S2O3^2- (aq) + I2(aq) -> S4O6^2- (aq) + 2I^- (aq)
  • So the ratio of sodium thiosulfate to iodine is 2:1. This is used, along with iodine’s molar mass, in the stoichiometric calculations to find the amount of iodine.
  • Moles of thiosulfate can be calculated using the equation: n = CV, where n is moles, C is concentration in mol/L, and V is volume in L.
  • By knowing the moles of thiosulfate, you can then find the moles of iodine by multiplying the moles of thiosulfate by 0.5 (due to the 2:1 ratio).

Common Errors

  • Errors can often occur in the detection of the end point. The colour change from blue-black to colourless is sharp but can be missed. Thus, titrations should ideally be repeated to minimize this source of experimental error.
  • The sodium thiosulfate solution is susceptible to decomposition upon prolonged exposure to air. Thus, it should always be freshly prepared and stored properly to maintain accurate concentrations.
  • Not fully rinsing equipment (e.g. pipettes, burettes) before use; left over chemicals can negatively impact experiment results.


  • Iodine-sodium thiosulfate titration plays a significant role in the industrial production of the halogen iodine, and the accurate measurement of iodine concentrations.
  • It is essential in wastewater treatment as it can detect the presence of certain disinfectant agents such as chlorine.

Remember it is important to understand the process and the principle behind this redox reaction since A Level Chemistry questions can often involve applications, or ask to explain the underlying reasons.