Analysis of Foods for Starch and Vitamin C

Preparation of indicator starch solution: Place a portion of a peeled potato cut into small pieces (about 50 mL) into a 250 mL beaker. Add 100 mL of water and boil for about 10 min. Some of the potato starch will dissolve in the boiling water, and the resulting solution can be used as an indicator for the titrations.

Calibration of the iodine solution: Place a 250 mg vitamin C tablet into a 250 mL beaker and add 100 mL of water. Crush the tablet with a stirring rod and stir well to insure that all of the vitamin C is dissolved. Using your graduated cylinder, measure out 2.0 mL of the vitamin C solution, pour it into a 125 mL flask, add 10 drops of the potato starch solution and 18 mL of deionized water. Fill a 10 mL graduated cylinder with your iodine solution, and record the exact volume. Draw some of the iodine solution into a dropper, and add it dropwise to the 20 mL of vitamin C solution. Stir the solution after each addition of iodine. Continue the addition until the blue-black color appears and persists. Refill the dropper from the graduated cylinder as required. When the blue-black color persists, return any iodine solution which remains in the dropper back to the graduated cylinder. Record the final volume of iodine solution in the graduated cylinder. The difference between the initial and final volumes of solution in the cylinder is the volume of iodine solution which was required to titrate the vitamin C solution.