[23.3] iodine / thiosulfate redox titrations

Question 1

what are the oxidation, reduction, and overall reactions in iodine / thiosulfate titrations?

Answer 1

• oxidation - 2S₂O₃²⁻ (aq) —> S₄O₆²⁻ (aq) + 2e⁻
• reduction - I₂ (aq) + 2e⁻ —> 2I⁻ (aq)
• overall - 2S₂O₃²⁻ (aq) + I₂ (aq) —> 2I⁻ (aq) + S₄O₆²⁻ (aq)

Question 2

what can iodine / thiosulfate titrations be used to determine?

Answer 2

• ClO⁻ content in household bleach
• Cu²⁺ content in copper (II) compounds
• Cu content in copper alloys

Question 3

describe the stages in an iodine-thiosulfate titration

Answer 3

1. add standard solution of Na₂S₂O₃ to the burette
2. prepare a solution of oxidising agent to be analysed
3. add an excess of KI. the oxidising agent reacts with iodide ions to produce iodine, which turns the solution a yellow-brown colour
4. titrate this solution with Na₂S₂O₃ (aq)
5. during the titration, iodine is reduced back to iodide ions. yellow-brown colour fades to a pale straw colour
6. end point approaches when iodine colour fades to become a pale straw colour
7. add a small amount of starch indicator, which has a blue-black colour
8. as more sodium thiosulfate is added, blue-black colour fades
9. at end point, blue-black colour disappears - all iodine has been reduced to I⁻

Question 4

what can iodine-thiosulfate titrations be used to analyse?

Answer 4

• oxidising agents eg. chlorate (I) ions, ClO⁻ (aq), copper (II) ions, Cu²⁺ (aq)
• need to be capable of oxising I⁻ ions to I₂