Message-Id: <v01540b06b3380b4dc46b@[192.174.2.182]>
Date: Mon, 12 Apr 1999 12:44:28 -0800
To: pinhole@exploratorium.edu
From: karenk@exploratorium.edu (Karen Kalumuck)
Subject: Vitamin C assay
Dear Pinholers:
In response to Yibi's question about Vitamin C -- I have a
well-developed activity on titrating vitamin C; the indicator is tincture
of iodine (medicinal iodine bought in any pharmacy). You create a
"standard" of vitamin C and add 1% starch solution; as you titrate this
solution with iodine, the vitamin C preferentially binds to the iodine.
When the vitamin C is all bound up with the iodine, the iodine will react
with the starch producing the characteristic blue-black color of this
reaction. Recording the volume of iodine needed to reach this endpoint,
you then repeat the titration with your "unknowns", incuding the addition
of starch. Proportions are used to calculate the amount of vitamin C in
the unknowns. Because colored solutions mask the endpoint, a 1:10 dilution
of things like orange juice is typically used for testing.
This activity will be part of the Explortorium Human Body
Explorations book currently being classroom tested, and due to be published
about one year from now. If anyone wants me to send them an electronic
copy of this activity, contact me directly (karenk@exploratorium.edu) and
I'll be glad to email it.
----Karen
Karen E. Kalumuck, Ph.D.
Biologist
Exploratorium Teacher Institute
3601 Lyon St.
San Francisco, CA 94123
415-561-0313
karenk@exploratorium.edu