What is the test for calculating the exact percentage of Vitamin C contained in a Vitamin suppliment?

I am required for my A-level assignment to analyse a vitamin suppliment. I know about the titration method involvin DCPIP but cannot understand how it is used to calculate the exact percentage. please help!!

What is the test for calculating the exact percentage of Vitamin C contained in a Vitamin suppliment?
i believe the common test is read the label
Reply:An Iodine solution is the best way to test for the presence of starch. The distinctive blue-black color which results from mixing starch and iodine is a simple and effective test.





Having produced a solution of this blue indicator, some researchers suggest its use as an indicator for the presence of Vitamin C. By adding drops of the solution in question to the blue indicator, the presence of Vitamin C is confirmed by the bleaching out of the blue color.





The definitive test for Vitamin C is done with an organic indicator solution of 2,3-dichlorophenol indophenol solution which is available from chemical supply houses such as Flinn and Science Kit and Boreal Laboratories. Put about one to one and half inches of indicator in each test tube and add drops of the various juices to be tested until a color change is noted. Strength of Vitamin C can be observed by the number of drops taken to effect a change.





My experience with my students indicated that the DCPIP (dichlorophenol indophenol) did not change color in an acid medium. The bottle we received from Science Kit and Boreal Labs noted that the indicator was effective only in a basic medium of pH 12.0 - 14.0. This is very alkaline! We therefore neutralized the acid by adding a weak base (1% Sodium Hydroxide) until the color change from blue to green was observed. It should also be noted that most juices have a distinct color of their own which may alter the final effect. To compensate for this, we spun the various samples in a centrifuge to separate the colored particles from the mostly clear liquid. You might also use coffee filters.


Once you have effectively learned how to make the indicator change color, you might want to make a standard indicator with a known quantity of Vitamin C by buying Ascorbic Acid tablets and dissolving them in a known quantity of water. Real laboratories used to compare the color of the unknown with a known by using a device called a "colorimeter." It is basically a light meter focused on the test tube to measure how much light is transmitted by a known sample and how much is transmitted by the unknown. Much more sophisticated equipment is now available, however, the old colorimeter is easier to understand and can be used for a variety of color tests in which you create a standard and an unknown.


Unfortunately, most Vitamin C analysis as done in General Chemistry or wet


lab


involves some kind of titration. There are many variants of this:


iodine/starch indicators with NaOH, phenolphthalein indicator with NaOH, etc.


However, all these analyses have to assume that the only acid present in the


sample is Vitamin C and this is not often true.


Instrumental analysis for Vitamin C usually involve some kind of


chromatography. The Vitamin C content is first separated before its


percentage is calculated.





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