Determine the Amount of Vitamin C in Various Foods by Using Titration Method (page 2)
Try New Approaches
- Analyze a multiple-vitamin tablet to determine the amount of vitamin C in it. Repeat the experiment substituting a multiple-vitamin tablet for the vitamin C tablet. Note: Avoid using a multiple vitamin that contains vitamin E, which may affect the results.
- Determine the concentration of vitamin C in citrus fruit juices. Repeat the original experiment replacing the vitamin C solution with 1/2 cup (125 ml) of each fruit juice to be tested. If the juices contain pulp, strain them through cheesecloth before beginning the experiment. Again, use the equation comparing the drops of iodine needed to react with 25 mg of vitamin C to calculate the number of milligrams of the vitamin in each sample of fruit juice.
- Does temperature affect the concentration of vitamin C? Repeat the original experiment twice, first substituting 25 ml of boiled orange juice for the standard vitamin C solution, and then using 1/2 cup (125 ml) of frozen orange juice that has been thawed. Use the equation again to compute the amount of vitamin C in the test solution.
- Does storing orange juice in an open container affect the vitamin C content? Repeat the original experiment using 2 cups (500 ml) of orange juice prepared by diluting a small can of frozen orange juice according to the directions on the container. Pour 1 cup (250 ml) of orange juice into two separate jars. Place a lid on one container. Each day, test a 1/8 cup (31 ml) sample from each container.
The number of drops of iodine needed to react with the vitamin C mixture depends on the size of the drops, which is determined by the eyedropper. Thus, the same size eyedropper must be used throughout the experiment. Use the following equation and the known number of drops of iodine required to react with 25 mg of vitamin C to compute the amount of vitamin C in the test material (the multiple-vitamin tablet). See Appendix 6 for an example calculation.
Design Your Own Experiment
In the analysis experiments for vitamin C content, the vitamin C acts as a reducing agent (causes other chemicals to gain electrons). As a reducing agent, vitamin C is readily oxidized itself and therefore prevents other chemicals from being oxidized. A chemical that undergoes oxidation has gained oxygen and/or lost electrons. You will study more about the redox (oxidation-reduction) reaction under "Get the Facts."
Apples contain enzymes that speed surface browning where the surface is exposed to the oxygen in the air. Vitamin C prevents this "enzymatic browning" due to oxidation of the fruit To demonstrate vitamin C's antioxidation abilities, cut an apple in half (see Figure 7.2). Crush a vitamin C tablet and sprinkle it on one of the apple pieces. Allow the apple to sit uncovered and make observations every hour for three hours.
The antioxidant property of fruit juices and drinks can give an indication of their vitamin C content Dip cut pieces of apple into several testing solutions and allow each piece to sit uncovered. Describe the appearance of the apple pieces immediately and then every hour for three hours. Rate the vitamin C content of the testing solutions by degree of enzymatic browning. Record the results in a data table such as shown here.