Re: How can I measure the ascorbic acid in juice?

To test for ascorbic acid you will need iodine, cornstarch, and water. Put 1/2 teaspoon of starch and 1 cup water in a pan and heat and stir it until it is dissolved. Put one teaspoon of this solution, 1 cup of water, and four drops of iodine solution in a jar. This final solution should be blue in color. Put two tablespoons of this solution into a cup or bowl. Then add 1 drop of juice to the bowl. If the solution becomes clear, ascorbic acid is present. If the solution is still blue, it is not. To test for various concentrations of ascorbic acid, vary the amount of juice or solution.

The reason why this works is the following:

Iodine binds to starch.  When it binds to starch, the complex is purple.
Iodine binds to ascorbic acid.  When it binds to vitamin C, it is colorless. 
Iodine binds more strongly to ascorbic acid than it binds to starch.
The iodine, therefore, would leave the starch and bind to ascorbic acid, 
which would make the blue solution become clear.