23.3. - iodine/thiosulfate redox titrations

• What is oxidised in iodine/thiosulfate titrations?


  Thiosulfate ions, S₂O₃²⁻(aq).
• What is reduced in iodine/thiosulfate titrations?


  Iodine, I₂.
• Iodine/thiosulfate titration: oxidation half equation


  2S₂O₃²⁻(aq) → S₄O₆²⁻(aq) + 2e⁻
• Iodine/thiosulfate titration: reduction half equation


  I₂(aq) + 2e⁻ →2I⁻(aq)
• Iodine/thiosulfate titration: overall equation


  2S₂O₃²⁻(aq) + I₂(aq) → 2I⁻(aq) + S₄O₆²⁻(aq)
• How can the concentration of aqueous iodine be determined?


  By titration with a standard solution of sodium thiosulfate.
• What can iodine/thiosulfate titrations be used to determine?

  - The ClO⁻ content in household bleach.

  - The Cu²⁺ content in copper (II) compounds.

  - The Cu content in copper alloys.
• Steps of analysis of oxidising agents


  1. Add a standard solution of Na₂S₂O₃ to the burette.

  
  

  2. Prepare a solution of the oxidising agent to be analysed. Using a pipette, add this solution to a conical flask along with an excess of potassium iodide. The oxidising agent reacts with iodide ions to produce iodine, which turns the solution a yellow-brown colour.

  
  

  3. Titrate this solution with the Na₂S₂O₃(aq). The yellow-brown colour of the iodine turns paler as it is converted to colourless iodide ions.

  
  

  4. When the solution is a straw colour, starch indicator is added to clarify the end point. The solution turns blue/black until all the iodine reacts, at which point the colour disappears.
• What does the analysis of oxidising agents using iodine/thiosulfate titrations determine?


  The concentration of an oxidising agent, which oxidises iodide ions to iodine molecules.
• How can we determine the copper content of copper (II) salts or alloys.


  Iodine/thiosulfate titrations.
• What happens when copper (II) salts are dissolved in water?


  Cu²⁺(aq) ions are produced.
• How can we produce Cu²⁺ ions from insoluble copper (II) compounds?


  By reacting them with acids.
• How can we produce Cu²⁺ ions from copper alloys, such as brass or bronze?


  The alloy is reacted and dissolved in concentrated nitric acid, followed by neutralisation.
• Analysis of copper


  Cu²⁺(aq) ions react with I⁻(aq) to form a solution of iodine and a white precipitate of copper(I) iodide, CuI(s). The mixture appears as a brown colour.

  
  

  The iodine in the brown mixture is then titrated with a standard solution of sodium thiosulfate.
• Percentage purity calculation


  Percentage purity = (mass of sample / mass of impure sample) x100