Ksp and the Common Ion Effect

Question

I just completed a lab in which I titrated Calcium Iodate with Na₂S₂O₃. During the titration we were told to add a starch once the liquid had turned light yellow. When we did this it became dark and we just had to titrate a few drops more. I'm writing the lab write up for this lab and was wondering why we had to add the starch and why the color changed so dramatically.

 

-Adam

Answer 1

Google it!  Seriously - search "starch iodine" and you will be directed to a great variety of articles on the use of starch as an indicator for Iodine.  Articles such as

 

 http://chemwiki.ucdavis.edu/Core/Biological_Chemistry/Carbohydrates/Case_Studies/Starch_and_Iodine

 

do a nice job explaining a few things.