Vitamin C Redox Reaction Cleans Up Iodine Stains on Hand

I learned this demo from a visitor to campus a couple years ago. This demo takes only a couple minutes. To put this together, you need to go to the local drugstore/grocery store and pick up the following:

-iodine solution (tincture of iodine, used as a disinfectant)

-spray starch

-vitamin C tablets

Step 1 - Smear some of the iodine solution on the palm of your hand. DO NOT let the iodine solution dry out (otherwise it will be hard to remove).

Step 2 - While the iodine solution is still wet, spray some starch onto the palm of your hand. The iodine spot turns blue-black as the starch molecules wrap around the iodine molecules. Again, DO NOT let this mess on your hand dry out.

Step 3 - While the iodine/starch solution is still wet, take a vitamin C tablet and rub it across the stain (I make a smiley face first). The dark stain disappears though sometimes there might still be some yellow tint left. The vitamin C tablet acts as an antioxidant, reducing the iodine to iodide ions and breaking apart the starch/iodine complex. The tablet itself stays fairly white, this it not simply rubbing the stain off, it is a chemical reaction.

CAUTIONS:

Wash your hands with soap and water as soon as you get the chance after the demo (the starch on your hand will still be sticky/slimy).

Iodine stains all sorts of things, so do not spill it.

DO NOT eat the vitamin C tablet that you used in the demo, though you should be able to use the same tablet multiple times.

You should try this demo by yourself first in case I missed any subtle details.

VARIATION: A counterfeit money marking pen, which apparently contains iodine, can be used instead of tincture of iodine. The iodine in the pen does not react with the cloth fibers in real money, but it does react with the cellulose fibers in counterfeit money. The pen can be used to draw light tinted patterns on skin, but the markings dry out fairly quickly so the starch must be applied quickly to turn them dark. These starch/iodine marks can also be erased with a vitamin C tablet.

ABOVE: (LEFT) Step 1 (MIDDLE) Step 3 and (RIGHT) Variation: Erasing counterfeit money pen marks.