International A LevelChemistryEdexcelRevision Notes6. Practical Skills in Chemistry II6.2 Advanced Inorganic & Organic Chemistry Core Practicals6.2.2 Redox Titration -Thiosulfate & Iodine

Redox Titration -Thiosulfate & Iodine (Edexcel International A Level Chemistry)

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Core Practical 13b: Thiosulfate & Iodine Titration

Iodine-Thiosulfate Titrations

  • A redox reaction occurs between iodine and thiosulfate ions:

2S2O32– (aq) + I2 (aq) → 2I–(aq) + S4O62– (aq)

  • The light brown/yellow colour of the iodine turns paler as it is converted to colourless iodide ions
  • When the solution is a straw colour, starch is added to clarify the end point
  • The solution turns blue/black until all the iodine reacts, at which point the colour disappears.
  • This titration can be used to determine the concentration of an oxidising agent, which oxidises iodide ions to iodine molecules
  • The amount of iodine is determined from titration against a known quantity of sodium thiosulfate solution

Worked example

Analysis of household bleach

Chlorate(I) ions, ClO-, are the active ingredient in many household bleaches.

10.0 cm3 of bleach was made up to 250.0 cm3. 25.0 cm3 of this solution had 10.0 cm3 of 1.0 mol dm-3 potassium iodide and then acidified with 1.0 mol dm-3 hydrochloric acid.

ClO- (aq) + 2I- (aq) + 2H+ (aq) → Cl- (aq) + I2 (aq) + H2O (l) 

This was titrated with 0.05 mol dm-3 sodium thiosulfate solution giving an average titre of 25.20 cm3.

2S2O32- (aq) + I2 (aq) → 2I- (aq) + S4O62- (aq)

What is the concentration of chlorate(I) ions in the bleach?

   Answer:

  • One mole of ClO- (aq) produces one mole of I2 (aq) which reacts with two moles of 2S2O32- (aq)
    • Therefore, 1 : 2 ratio of ClO- (aq) : S2O32- (aq)
  • Number of moles of S2O32- (aq) equals space fraction numerator 0.05 space cross times space 25.20 over denominator 1000 end fraction equals space1.26 x 10-3 moles 
  • Number of moles of I2 (aq) and ClO- (aq) in 25.0 cm3 equals space fraction numerator 1.26 space cross times space 10 to the power of negative 3 end exponent over denominator 2 end fraction space equals6.30 x 10-4 moles 
  • Number of moles of ClO- (aq) in 250.0 cm3 = 6.30 x 10-4 x 10 = 6.30 x 10-3 moles 
  • The 250.0 cm3 was prepared from 10.0 cm3 bleach
    • 10 cm3 bleach = 6.30 x 10-3 moles of ClO- ions
    • 1.0 dm3 bleach = 0.630 moles of ClO- ions
    • Therefore, the concentration of ClO- ions in the bleach is 0.630 mol dm-3 

Examiner Tip

General sequence for redox titration calculations

  1. Write down the half equations for the oxidant and reductant
  2. Deduce the overall equation
  3. Calculate the number of moles of manganate(VII) or dichromate(VI) used
  4. Calculate the ratio of moles of oxidant to moles of reductant from the overall redox equation
  5. Calculate the number of moles in the sample solution of the reductant
  6. Calculate the number of moles in the original solution of reductant
  7. Determine either the concentration of the original solution or the percentage of reductant in a known quantity of sample

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