Science Buddies: "Ask an Expert"

donnahardy2 wrote:Hi Puja,

If you mix your iodine solution with the starch solution, it should turn blue immediately. If you are measuring vitamin C, the vitamin will oxidize the iodine and the appearance of the blue color will not occur until there is an excess of the iodine solution. So if your standard contains chemically equivalent more vitamin C compared to iodine, then the blue color won’t appear.

Please let me know if you see the blue color when you mix just starch and iodine. If you don’t, then let me know exactly how you prepared the starch and what source of iodine you are using. You need to isolate the problem.

Donna Hardy

donnahardy2 wrote:Hi Puja,

Did you mix the iodine solution with the starch without adding any vitamin C? You should see an immediate dark blue color. This should be a good way to check your reagents. Try this first when you get a chance to start working on your project again.

It's possible that there were other antioxidants in the multivitamin pill. I would recommend using vitamin C tablets that don't contain any other ingredients.

Donna Hardy

Make a fresh vitamin C standard solution (1 mg/mL). Do this on each day that you make vitamin C measurements from oranges.

donnahardy2 wrote:Hi,

Yes, staryl13 is correct. I was suggesting that you check your reagents to make sure they are working. If you add your iodine solution to the starch you should see the dark blue color. If you don’t see this, then try putting some of the iodine solution on a piece of freshly cut potato. This should help isolate the problem to the multivitamin, the iodine, or the starch solution. We think the most likely cause is the multivitamin, but this basic troubleshooting will help you identify the source of the problem so you can correct it and continue with your project.

Here is a website that describes the reaction between iodine solution and starch:

http://www.elmhurst.edu/~chm/vchembook/ ... odine.html

Vitamin C donates an electron to, or reduces iodine to iodide and disrupts the triodide complex and prevents the color from forming:

http://chemistry.about.com/od/demonstra ... ration.htm

This is an example of a reduction/oxidation, or redox reaction.

Thanks, what i dont get is why do you have to titrate vitamin c standered soultion, your measuring vitamin c from oranges, so where do measuring vitamin c from tablest come from?

http://www.shodor.org/unchem/advanced/redox/

Please let us know if you need more explanation of the chemistry. Understanding the chemistry of the reaction is very important for this project.

Donna Hardy