I am assuming by rate of vitamin C you are talking about testing for the vitamin C levels in various fruit juices. Vitamin C is the common name for the chemical L-ascorbic acid, which is an important chemical for human body chemistry. There is a very simple test that can be done to test for vitamin C. Vitamin C is an antioxidant that can be used to reduce other chemicals. When a solution of iodine (I2) is added to vitamin C, the iodine is reduced to the iodide anion (I-). I2 is a deep purple colored liquid but I- is colorless. So take a cup of water and add a couple of drops of iodine. The solution will turn purple. Add a fruit juice to this solution one drop at a time until the mixture becomes colorless. The fewer drops used means that the fruit juice has a large amount of vitamin C. The more drops used means that the fruit juice has a smaller amount of vitamin C. By comparing the number of drops of the different juices used in the experiment you can rank them in terms of their vitamin C content.