Ask A Scientist Chemistry Archive

name         Laura W.
status       student
age          11

Question -   If iodine can indicate the presence of vitamin C in a fruit juice,why is starch solution commonly used to combine with iodine to form a test solution for vitamin C testing?
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The chemistry works like this: Iodine reacts with vitamin C.  When this happens, both the iodine 
and vitamin C turn into different chemicals.  If a sample contains vitamin C, iodine added to the 
sample will react with it and no longer be iodine.  At the same time, the vitamin C that reacts 
with the iodine also is no longer vitamin C.  When the amount of iodine added to the sample is 
greater than the amount of vitamin C that was there, all the vitamin C will be destroyed and 
there will be some iodine left over.  So to
find out how much vitamin C is in a sample, you can add small amounts of iodine, until the 
iodine you add no longer disappears as soon as it is added.

The starch plays another role.  Starch and iodine can pair up to make a dark blue material.  In 
this combination, the iodine can still react with vitamin C.  When that happens, the 
starch-iodine pair does not exist any more (because the iodine is gone), so the dark blue color 
disappears as well.  So if you add the starch-iodine solution to the sample with vitamin C, the 
blue color of the sample will disappear until you have added enough iodine to react with all of 
the vitamin C.  When that happens, the blue color will persist.  It is much easier to see the 
blue color of the starch-iodine solution than to see the faint yellow color of a weak iodine 
solution.  So
the starch acts as an "indicator" for iodine.  It makes it easy to see if any iodine is in the 
sample.


Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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