submitted by Howard Alpert
As a result of extensive searching, the following list of tests for Vitamin C should allow middle and high school students to test for the presence and quantify of Vitamin C.
As you are aware, 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.