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Iodine and Vitamin C Test
Name: Laura W.
Status: student
Age: 11
Location: N/A
Country: N/A
Date: 4/21/2003
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?
Replies:
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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Update: June 2012
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