You will determine the amount of iodine formed by measuring the volume of a standard solution of thiosulfate required to react with the iodine.

The indicator for the titration is a starch solution. You will need 5 mL of the starch solution.

The deep blue color that forms as the starch is added is a complex between iodine, I2, and starch.

Next you need to add about 2 grams of potassium thiocyanate, KSCN, to the dark blue mixture. I2 tends to adsorb strongly on the solid CuI precipitate. The KSCN is added to displace the adsorbed I2 making it accessible to the starch collodial particles, hence yielding a sharper endpoint.


The iodine will be titrated with a standardized solution of sodium thiosulfate (Na2S2O3). Use a buret so you can accurately measure the volume of sodium thiosulfate because the volume is needed in the calculations. Record the volume of the thiosulfate at the start of the titration.
As the solution of sodium thiosulfate is added from the buret your solution will start to turn a gray to tan color.
This is what your sample should look like at the end-point of the titration. Be sure to record to volume of sodium thiosulfate needed to react with the iodine. The difference between the volume at the end of the titration and the level at the start is the volume of thiosulfate needed to react with the iodine in the solution.
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