T&P- iodine-thiosulphate titration

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  • Created by: anabob
  • Created on: 27-01-18 17:09

background info

used to find out conc of a chemical that's strong enough to oxidise iodine to iodide ions. The iodine is treates with thiosulphate ions, with starch as an indicator.

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part 1

chlorate ions are strong enough to reduce iodine.

  • pour chlorate solution into a beaker
  • rinse a volumetric pipette (ensures pipette is clean and solution hasnt been diluted)
  • transfer a 25cm3 aliquot of solutin into a conical flask using a volumetric pipette and filler
  • add excess iodide ions by using a measuring cylinder to transfer 15cm3 of o.5M potassium iodide to the conical flask
  • add excess hydrogen ions by using a measuring cylinder to transfer 20cm3 of 1M sulfuric acid
  • contents will be brown because of amount of iodine made 
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part 2

titrate the iodine made with sodium thiosulfate to work out he number of moles of iodine in the first part

  • wash a burette with water, and then with a standard solution of sodium thiosulfte solution
  • fill burette with sodium thiosulfate
  • put conical flask on white tile
  • record initial burette reading to the nearest 0.05cm3
  • start rough titration, pale straw colour near end point .
  • at this point, add a few drops of starch, contents will be blue/black
  • end point is colourless
  • record the final burette reading and calculate rough titre by subtraction
  • wash conical flask with distilled water
  • carry out accurate titration, run sodium thiosulfate solution into conical flask until 1cm3 below rough titre
  • add dropwise
  • continue until results are concordant - within 0.1 cm3 of each other
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