MadSci Network: Chemistry

Subject: Orange juice/Iodine titration results seem backwards

Date: Sun Feb 7 07:18:46 2010
Posted by Katie
Grade level: 4-6 School: Traut Elementary
City: Fort Collins State/Province: CO Country: USA
Area of science: Chemistry
ID: 1265552326.Ch

I wanted to test for Vitamin C in different orange juices.  I used 20 ml of 
OJ, 10 drops of corn starch solution, and titrated with iodine solution.  I 
expected the amount of iodine to decrease over time (due to less vitamin C), 
but over time the amount of iodine needed to turn solution blue increased!  
Based on research, it seems that the OJ is gaining Vit. C over time, but I 
know this is wrong.  Why does it take more iodine soln. to turn the OJ blue?  
I am using the same iodine and starch solutions to test the OJ over time.  
Also, the frozen concentrate OJ takes more iodine solution to react (turn 
blue) and this is opposite what I expected from my research (it should have 
more vit. C than fresh).  Thanks!

Re: Orange juice/Iodine titration results seem backwards

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