Vitamin C Content (page 2)

Why?

Titration is the process of combining a measured amount of a solution of known concentration with a measured amount of solution of unknown concentration. Tincture of iodine is a mixture of elemental (natural noncharged form of an element) iodine (I₂⁰) and ethyl alcohol (C₂H₅OH). A chemical reaction in which electrons are transferred between reactants is known as an oxidation-reduction reaction, also called a redox reaction. Originally the term oxidation meant the combination of a substance with oxygen and reduction meant the loss of oxygen from a substance. Today, scientists extend these definitions to include all transfers of electrons. So oxidation is a loss of electrons or gain of oxygen and reduction is the gain of electrons or loss of oxygen. In the chemical reaction between vitamin C and elemental iodine (I₂⁰), vitamin C is easily oxidized (loses electrons), forming dehydroascorbic acid and iodine is reduced (gains electrons) forming the anion (a negatively charged particle) called iodide (I^–1).

Elemental iodine reacts with starch to form a complex molecule that has a blue-black color. However, when the iodide anion (the reduced form) is present, the blue-black color is not seen. When starch, vitamin C, and elemental iodine are mixed, the iodine quickly reacts with the vitamin C. When all of the vitamin C has been oxidized by the iodine, additional drops of iodine react with the starch and the blue-black color is seen.

Try New Approaches

Analyze a multivitamin tablet to determine the amount of vitamin C it it. Repeat the experiment substituting a multivitamin tablet for the vitamin C tablet. Note: Avoid using a multiple vitamin that contains vitamin E, which may affect the results.

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 calculated 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 multivitamin tablet). For example, if it takes 50 drops of iodine to react with 25 mg of vitamin C, determine the number of milligrams of vitamin C in a multiple vitamin tablet if 20 drops of iodine reacted with the multiple vitamin.

Design Your Own Experiment

1. Commercial foods are generally heated when being prepared for packaging. Design an experiment to determine if heating affects the amount of vitamin C in foods. One way is to compare heated with unheated citrus juices. Prepare fresh orange juice by squeezing enough oranges to collect 1 cup of juice. Strain the juice through cheesecloth to separate out any pulp from the orange. Heat half of the juice to boiling, then let it cool to room temperature. Repeat the original experiment\ twice, first substituting ¹⁄₂ cup (125 ml) of fresh orange juice for the vitamin C solution, and then using ¹⁄₂ cup (125 ml) of the heated orange juice. Note: Both juices need to be at room temperature when you test them.
2. Vitamin C is readily oxidized by combining with oxygen. Design an experiment to determine if storing orange juice in an open container affects its vitamin C content. One way is to store orange juice in two containers, one open (exposed to air, which contains oxygen) and one closed. Keep the containers in a refrigerator so the juice does not spoil. Each week for four or more weeks repeat the original experiment testing the vitamin C content in the juice in both containers. To determine the total amount of juice needed in each container, first decide on the number of days the testing will be done. Then multiply the total number of days by 1 cup (250 ml), the amount of juice needed for each day's testing.

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