Expert: Dr. Henry Boyter, Jr. - 4/12/2004

Question
Dear Sir:

I have recently concluded data collection on a research project on the retention of Ascorbic Acid (vit. c) as related to container type.   In other words i bought orange juice in four different containers and monitored the mg vit. c/ml juice over a period of a week.   The four container types I chose were tin, polyethylene, wax carton, and a conglomeration of mixed unknown plastics (recycle number 7).   

Here is the method used to determine vit c. content:

Titration was used in order to determine the amount of ascorbic acid (Vit.C) in given samples of orange juices (Kutscher, 1988).   In titration, a certain known solution is used to find information about an unknown solution.   For this procedure the known substance being reacted is Iodine.   Approximately 20 mL of orange juice and 2.5 mL of starch were reacted with Iodine.   Starch was used to show the end of the reaction known as the endpoint.   The endpoint of this reaction is when the solution turns a grayish-brown color.   Since there is a one to one mole ratio between ascorbic acid and iodine, one can hold true that at the endpoint of the reaction there are equal amounts of Iodine and Ascorbic Acid present (molar amounts).   The volume of Iodine and orange juice used were recorded.   Using the mLs of Iodine reacted and its given molarity (.00557 M), the number of moles of Iodine was determined.   Next, since there is a one to one mole ratio, we assume that there is the same number of moles of vitamin C present as there was Iodine.   With the moles of Vitamin C and its molar mass (176.1 mg/mol) the grams of Vitamin C present were determined.   Finally, the grams were converted to milligrams and this value was divided by the volume of juice reacted (mL).   This unit analysis process yields mg Vit. C/ mL juice.

    For polyethylene, wax carton, and the number 7 plastic results were as expected.   All three sloped down very slowly in vit. c content.   I did expect tin to fare better then all others, which it did.   The problem is my data indicates that the juice within the tin container "grew" vitamin c.   In other words more Iodine was used as time went on.   The only possible explanation i can come up with for this is that perhaps something else was being generated in the juice due to the contact with the tin over time. Perhaps this hypothetical something caused more iodine to be used over time, ultimately throwing off the whole purpose of the titration.

Note: all juices were stored in refrigerator

If you could please email back any suggestions or ideas as to why more iodine was used over time in the tin containers only it would be appreciated.   This project accounts for half of my final marking period grade.

Also three cans of tin were tested independantly.   All three cans used more iodine as time went on.

Your tjme is greatly appreciated.
Note: My teacher has given permission and actually encourages me to seek information from scientists.   Of course you will be cited as a source for any information provided.   

Sincerely
Nathan Graf
Honors Chem I Student
Cumberland Valley High School

Answer
Another thought: did you standardize the iodine each time?  Your solution may lose potency over time.  If it does, it will throw off your calculation.






A couple of things to think about.

1. I don't know what a tin container really is, since cans and whatnot are not really tin anymore.  Really low grade steel of some sort.

2. What happens if some of the metal in the can dissolves in the orange juice over time?  Would the iodine go up or down?  If it would go up, it shouldn't go up much since the can should not decompose to any extent.