1. Testing Table Salt for KI Content By: Stephanie Njegovan
2. 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.
3. 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!!
4. Regular vs. Iodized Weigh out 20.0 g of regular salt. Weigh out 20.0 g of iodized salt.
5. 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+
6. 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
7. 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-
8. 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).