DCPIP (2,6-dichlorophenolindophenol) is an indicator solution that turns from blue to colorless in the presence of Vitamin C, which is an antioxidant that can donate electrons. By titrating a known volume of fruit juice with a standard solution of DCPIP, and measuring the volume of DCPIP solution required to decolorize the fruit juice, one can calculate the amount of Vitamin C in the fruit juice.