- 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
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?
- 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) 1.26 x 10-3 moles
- Number of moles of I2 (aq) and ClO- (aq) in 25.0 cm3 6.30 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
General sequence for redox titration calculations
- Write down the half equations for the oxidant and reductant
- Deduce the overall equation
- Calculate the number of moles of manganate(VII) or dichromate(VI) used
- Calculate the ratio of moles of oxidant to moles of reductant from the overall redox equation
- Calculate the number of moles in the sample solution of the reductant
- Calculate the number of moles in the original solution of reductant
- Determine either the concentration of the original solution or the percentage of reductant in a known quantity of sample