Vitamin C test & titration

[nancy_kemper]

I am doing a science fair project called "Which orange juice has more vitamin C, fresh squeezed, carton, or frozen concentrate?" I found the idea on Science Buddies and ordered all the supplies to conduct the experiment. I cannot seem to get past the initial titration process with vitamin C tablets. I never could get a color change. I switched the measurements and used 20 mL of starch and 10 drops of vitamin C an then added my iodine an i got dramatic color changes, until we mixed it then it went back to clear its original color. It's supposed to stay that blue-black color the whole time was my understanding. What do i need to do to get this titration process to work right?

[deleted-71417]

Hi,

It is fairly obvious you have not added enough iodine to fully oxidize the vitamin C. You are correct that at the end point of the titration the solution should turn blue-violet and stay that way. You might start by adding just one drop of the vitamin C to your starch solution and titrating that. From the results of titrating that you can judge how many drops of vitamin C to add to require a reasonable amount of iodine solution to reach an end point.

Good luck on the experiment!

Best regards,

Barrett L. Tomlinson