Testing Table Salt for KI Content

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Testing Table Salt for KI
Content

• By Stephanie Njegovan

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Prepare 0.04M sodium thiosulfate solution
(250mL)

• Weigh out 2.48g of sodium thiosulfate
  pentahydrate
• Transfer to 250mL volumetric flask
• Dissolve in distilled water
• Dilute to mark and mix well.

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Dilute to 0.004M sodium thiosulfate solution
(100mL)

• Transfer 10.0mL of the 0.04M solution, just made,
  to a 100mL volumetric flask
• Dilute to 100mL line with distilled water and mix
  thoroughly
• This is the solution we will later be using for
  titration!!

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Regular vs. Iodized

• Weigh out 20.0 g of regular salt.

• Weigh out 20.0 g of iodized salt.

Transfer each salt into separate 600mL beakers,
making sure to label each accordingly.
Regular
Iodized
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Add water bromine H20

• Using a graduated cylinder, add 200mL of
  distilled water to each beaker to dissolve salt
• Add 2mL of already prepared bromine water to each
  beaker and let stand for a few minutes
• At this point, the solutions must be yellowish,
  indicating an excess of bromine
• Any iodide in the salt is oxidized to iodate ion
  
• I- 3Br2 3H20 ? IO3- 6Br- 6H

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Add formate buffer KI

• To each beaker, add 2mL of formate buffer to
  reduce the excess bromine.
  Br2 HCOOH ? 2Br- CO2 2H
• Add 3.0g KI to each beaker
• The iodized salt will be a darker yellow because
  the triiodide ion forms when the iodate ion is
  reduced by the iodide in acidic solution
  IO3- 6H 8I- ?
  3I3- 3H2O

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Add starch titrate!

• Add 2mL starch indicator to the iodized salt
  solution
• Finally, titrate each solution with 0.004M sodium
  thiosulfate solution, that was made in the first
  steps, to complete the experiment
• I2- 2S2O42- ? 3I- S4O62-

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Test your knowledge..

• What was the purpose of adding 2mL of formate
  buffer to the salt solution?
• The formate buffer reduces the excess bromine in
  the salt solution (from the bromine water).