How Can I Test for Vitamin C?

Vitamin C Determination by Iodine Titration


Vitamin C (ascorbic acid) is an antioxidant that is essential for human nutrition. Vitamin C deficiency can lead to a disease called scurvy, which is characterized by abnormalities in the bones and teeth. Many fruits and vegetables contain vitamin C, but cooking destroys the vitamin, so raw citrus fruits and their juices are the main source of ascorbic acid for most people.





One way to determine the amount of vitamin C in food is to use a redox titration. The redox reaction is better than an acid-base titration since there are additional acids in a juice, but few of them interfere with the oxidation of ascorbic acid by iodine.





Iodine is relatively insoluble, but this can be improved by complexing the iodine with iodide to form triiodide:





I2 + I- %26lt;--%26gt; I3-





Triiodide oxidizes vitamin C to form dehydroascorbic acid:





C6H8O6 + I3- + H2O --%26gt; C6H6O6 + 3I- + 2H+





As long as vitamin C is present in the solution, the triiodide is converted to the iodide ion very quickly.





Howevever, when the all the vitamin C is oxidized, iodine and triiodide will be present, which react with starch to form a blue-black complex. The blue-black color is the endpoint of the titration.





This titration procedure is appropriate for testing the amount of vitamin C in vitamin C tablets, juices, and fresh, frozen, or packaged fruits and vegetables. The titration can be performed using just iodine solution and not iodate, but the iodate solution is more stable and gives a more accurate result..

How Can I Test for Vitamin C?
dilute the specimen(vitamin C tablets/powder) with a little water





now to dat add a little baking or cooking soda........





now this mixture will start bubbling!!!!


with evolution of carbon-di-oxide





dis simple kitchen experiment......... cann be used to test for vitamin C...................
Reply:Many textbooks suggest DCPIP as the reagent to test for vitamin C (ascorbic acid) but I have found


that the end point of a titration for this reaction is difficult to ascertain due to the lack of complete


decolourisation of the DCPIP.


Whilst participating in a 2001 Science week activity at the Plant Sciences department, Cambridge, I


came across an ascorbic acid assay using starch-iodine in place of DCPIP (the source of which was


the Thinkquest website). The ability of ascorbic acid to interfere with the starch-iodine reaction has


been noted elsewhere, Sharma et al (1990) and Samotus et al (1994)


I have used the following successfully;


Iodine 0.5g dissolved in 100ml of 1% potassium iodide solution (approx 0.02 moles dm


-3


Iodine in


0.06 moles dm


-3


KI)


0.1% Starch solution


1ml of starch is placed in a suitable receptacle and 1 drop of iodine solution added.


The ascorbic acid solution is then added drop wise until the blue-black colour of the starch iodine


complex disappear to leave a colourless solution.


The above amounts were completely decolourised by 150µl of 0.05% ascorbic acid and also by a few


drops of fresh orange juice.


The ascorbic acid reduces the iodine to iodide so it no longer forms the starch-iodine complex.


Reduction of iodine might also be the mechanism by which rinse aid inhibits the same reaction and


thus be related to the findings of Hadi-Talab and Levinson (2000)


I have used this reaction system as a practical assessment for Y12 Biology students using microscale


science equipment and a micropipette dispensing 10 µl drops; this titration enabled quantitative


comparisons to be made.
Reply:bite your teeth and see if it's resistance is tough