Orange juice titration

[doctorC]

We have been testing the titration of orange juice to see how much vitamin c are in three different juices. We have been testing with no visible results. The orange juice solution has not been turning grayish/brown after we've added our drops of iodine solution. We think it might be our soluble starch solution. This is what we've tried: 1.) 1/2 tsp. of pure cornstarch mixed with 1 Tbs. of distilled water. We then poured into a cup of boiling distilled water.....that didn't work. 2.) 3 1/2 tsp. of cornstarch with 1 Tbs. of distilled water and mixed to make a thick paste, then added to 1 cup of boiling water....like her chemistry teacher said. This made a thick goo which did not totally dissolve. This also didn't work, when we added the ten drops to our orange juice, and then did our titration process. We didn't use the lugol's solution as directed in the sciencebuddies project idea guide.....which orange juice has the most vitamin c. Instead we used, per teacher's advice, just a pure potassium iodine solution. We are wondering what we are doing wrong. Is it the soluble starch or the iodine solution? Our project is due on Friday....PLEASE HELP US!!!!!

[deleted-71417]

Hi,

The problem is not the starch solution, it is the fact that you used pure potassium iodide solution. You must have iodine (the free halogen, elemental iodine) in addition to iodide ions in order to form the form the purple starch iodine complex. This experiment is based on free iodine reacting with the vitamin C until all the vitamin C is destroyed - only then does free iodine persist and form the purple complex with starch the marks the endpoint of the titration. For more information see:

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

You need to use Lugols solution to do this experiment. If you cannot obtain Lugols solution in time to meet your deadline you can try adding some hydrogen peroxide to your potassium iodide solution. The hydrogen peroxide should oxidize some of the iodide to iodine, giving you a substitute for the reagent you need. For this reaction to work very well the solution must be fairly acidic, otherwise the main thing that will happen is that hydrogen peroxide will just decompose into molecular oxygen and water and little iodine will be produced. Here is a procedure that might work (but I can’t guarantee it).

http://www.ehow.com/how_6210956_convert ... cture.html

If at all possible obtain Lugols solution and use it for the experiment. The above workaround is intended strictly as a last ditch effort to avoid total project failure.

Good luck!

Best regards,

Barrett L Tomlinson