Science Buddies: "Ask an Expert"

I have student doing the power of purple project. She is using povidone-iodine 10% and it is not working. SHe also tried tincture of decolorized iodine 7% and that did not work either. Should the iodine turn the oil purple? What iodine should be used? The description in the experiment does not make it clear once the student was at the drug store. Can you advise? Thanks, Lisa

In my eighth grade science experiment we used iodine and it reacted very differently with different substances. What is she using it on?

Please exscuse my thoughtfulness, I reread your post and now it is clear to me now. Has she tried looking on the computer or researching it at the library?

lisa_troy,

For everyone's reference, the project description is here: https://www.sciencebuddies.org/science- ... p062.shtml

The chemistry of iodine is pretty complicated -- lots of possible oxidation states. Povidone iodine is a "stable" complex of iodine with a polymer, which suggests that the iodine wouldn't interact with other molecules in the same way. I'm not sure exactly what decolorized iodine is, but something must be added to solution to change the color, and that almost certainly changes the chemical state of iodine.

http://en.wikipedia.org/wiki/Iodine#Iodine_chemistry
http://en.wikipedia.org/wiki/Povidone-iodine
http://en.wikipedia.org/wiki/Tincture_of_iodine

The solution you want should be called "tincture of iodine" or "iodine tincture" at a concentration of 7%. The project description links directly to this example product on Amazon:

http://www.amazon.com/gp/redirect.html? ... ative=9325

and there are more recommendations for obtaining it in a previous discussion: https://www.sciencebuddies.org/mentorin ... =26&t=6424

Hope that helps,
Amanda

lisa_troy wrote:I have student doing the power of purple project. She is using povidone-iodine 10% and it is not working. SHe also tried tincture of decolorized iodine 7% and that did not work either. Should the iodine turn the oil purple? What iodine should be used? The description in the experiment does not make it clear once the student was at the drug store. Can you advise? Thanks, Lisa

Neither providone-iodine nor decolorized iodine will work for this experiment!

The iodine reagent must start out purple. If it doesn't start out purple, the experiment won't work.

Adding purple iodine to saturated fat (double bond) will cause a decolorization of an iodine molecule and breaking of a double bond.
The more purple iodine that can be added to a saturated fat without remaining purple, the more saturated the fat is (more double bonds).

My daughter chose the same project and I ordered the 7% tincture of iodine from Amazon.com as suggested by science buddies. But the solution is not purple. It is brown. We tried it on starchy solution (corn starch with water) and it does turn purple when came in contact with starch. Has anyone use it on the cooking oil? I was puzzled by the brown, instead of purple, color as it stated in the project.
Sara

I also believe the color is brown, not purple. We are trying the experiment again, testing different oils at the same time. The hope is that we'll be able to tell the color difference between the oils with the same number of drops added. Please let me know if you make any progress. It sounds like we are both having the same problem.