The objective of this experiment is to use a redox reaction titration to accurately determine the amount of vitamin C in a sample of lemon juice, orange juice, or grapefruit juice.
Chemistry of Vitamin C
The chemical name for vitamin C is L-ascorbic acid. Its molecular formula is C6H8O6; its molar mass is 176.12 g/mole.
Ascorbic acid is found throughout the plant and animal kingdoms, occurring in citrus fruits, hip berries (such as rose hips), fresh tea leaves, tomatoes, broccoli, other fruits and vegetables, paprika, and the adrenal cortex of oxen. It can be obtained from any of these sources but was originally isolated from and identified in oxen. It was the first vitamin to be prepared in pure form. Ascorbic acid is a white solid that has a sharp, sour taste and dissolves in water. The pure compound is stable to air oxidation when dry, but when impure (as it is in many natural forms) it is readily oxidized when exposed to air and light. Vitamin C is a fairly strong reducing agent and decolorizes many dyes. Its aqueous solutions are rapidly oxidized by air; this reaction is accelerated in basic solution and in the presence of iron and copper ions. The vitamin-C content of juices can decrease rapidly with time once the juice is exposed to air. (Much of the information reported here about vitamin C was obtained from the Merck Index, Susan Budavari, Ed., Merck and Co., Inc., Rahway, NJ, 1989, which is a good source for physical, chemical, and physiological properties of chemicals, drugs, and biological agents.)
Vitamin C is essential to humans. It is involved in the synthesis of collagen, which constitutes about one-third of the total protein in the human body. A deficiency of vitamin C results in a disease called scurvy, which is characterized by weakness, swollen joints, bleeding gums and loose teeth, and delayed healing of wounds. Scurvy was common in sailors, who had no fresh fruits or vegetables for long periods. In 1735 James Lind found that scurvy could be avoided if sailors were provided citrus fruit. To this day British sailors are called “limeys” because they were provided limes to prevent scurvy. A quantity of 60 mg vitamin C per day is enough to prevent the disease, and this is the recommended daily dietary allowance (RDA).
Vitamin C is also involved in iron metabolism, and many believe that very large doses are effective in preventing or curing the common cold. In 1970 Linus Pauling, a two-time Nobel Prize winner, published “Vitamin C and the Common Cold”, which stated that doses of 1-2 g per day (18 times the RDA) would prevent colds, and 4010 g/day would cure an existing cold. However, some recent studies do not support this hypothesis. At large doses vitamin C causes problems such as diarrhea and the induction of kidney stones.
Titrations can be completed on both Acid-base reaction and oxidation-reduction reaction systems. Both types of reactions occur rapidly in aqueous solution, the balanced equations for such reactions can be determined, and there exist techniques (such as changes in color of indicators or the color of the reactants themselves) for determining when the reactants have been mixed in stoichiometric ratios. Since vitamin C is a weak acid and also a good reducing agent, either type of reaction might be used. This experiment makes use of an oxidation-reduction reaction in which elemental iodine oxidizes ascorbic acid. Iodine is chosen because it is a weak oxidizing agent so it will not oxidize substances other than the ascorbic acid in the sample of fruit juice.
As a strong reducing agent, ascorbic acid will reduce I2 to I- very easily. We will use this reaction in conjunction with a starch indicator to determine the number of moles of Vitamin C present. A number of reactions occur during a single titration. The solution to be titrated will consist of KI, acetic acid, starch solution, and...