dehydroascorbic acid + 2 I-Vitamin C, or ascorbic acid, is an essential vitamin that is necessary for people to remain healthy. A deficiency in vitamin C leads to a disease called scurvy. Many natural foods, one of the most well known being citrus fruits, contain high levels of vitamin C. Today many juice producers supplement their products with extra vitamins in addition to the natural content. You can find the vitamin C content on the labels of most commercially available juices. How close is their reported number to the actual amount of vitamin C in the juice? In this lab, you will find out. You will do so by titrating the sample with an iodine solution, using starch as an indicator.
Using a starch indicator, the endpoint is reached when all of the vitamin C has reacted and you add a little more iodine (excess). The excess iodine binds to the starch indicator, producing a deep blue color.
In this experiment you will be titrating the vitamin C in a sample of fruit juice. The titration involves the reaction shown below:
Ascorbic acid + I2
dehydroascorbic acid + 2 I-
The results of a ten year study concerning Vitamin C were just released and can be found on the web at http://www.ultimatecitrus.com/pressrel.html and http://www.ultimatecitrus.com/vitaminc.html.
Titrating with iodine is called iodimetry and the sample is found to be at the equivalence point when the solution turns a blue-black color and persists for 30 seconds. This color change is due to the adsorption of the excess unreacted iodine to the starch indicator. At this point, all of the ascorbic acid has been converted to dehydroascorbic acid.
The first thing you will need to do is to make your starch indicator solution. Do this by dissolving about 1.5 g of starch in about 5 mL of deionized water. Add this slurry to 250 mL of boiling water and continue to boil for 5 minutes. Let the solution cool and let the particles settle out. The clear supernatant is your indicator.
You will be given an iodine solution that is approximately 0.01M. Next, you will need to standardize iodine solution in order to determine its exact molarity. To do this, titrate the iodine solution against an ascorbic acid solution of known concentration (try something close to 0.010g ascorbic acid in about 75 mL distilled water); don't forget to add your indicator! You will need to use about 15 drops of the starch indicator in each titration. Do the titration in triplicate and then average the three molarities; you can consider your result the known molarity of the iodine solution.
Use the standardized iodine solution to determine the vitamin C concentrations in commercial juices. Remember that reproducibility is very important, so you should do each experiment multiple times!
We will provide 3 juices to be tested. Choose a fourth specimen (either a juice, or you can also choose a fruit or vegetable) and bring it with you to lab to test. Make sure to get nutritional information about the juice or fruit sample you bring. If you choose a fruit or vegetable, consider how you will process it for testing. You may test more than four specimens if you would like.
Use your book or the web sites indicated to look up the structure of Vitamin C. You should include this structure in your formal report. Determine the molecular weight of vitamin C for use in your calculations.
Does the amount of vitamin C in orange juice vary? Explain. What kind of juice has the highest amount of vitamin C levels? As far as you can tell is the manufacturer telling the truth on the label about how much vitamin C is in the fruit juice? How does the color of the juice itself affect your titration? How did you account for this? Do your results verify what is on the label for each juice? How much of each juice would need to be consumed to equal the RDA for the average adult?