Analysis of Vitamin C I2 + Starch

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Analysis of Vitamin C I2 + starch

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Vitamin C (ascorbic acid) was one of the first vitamins which established a relationship between a disease and its prevention by proper diet. Scurvy has been known for ages. Prevention of scurvy was prescribed by eating fresh vegetables and fruits. The active ingredient in fruits and vegetables that helps to prevent scurvy is ascorbic acid which primates, including humans, cannot synthesize. Ascorbic acid is an important biological antioxidant (reducing agent). It helps to keep the iron in the enzyme prolyl hydroxylase in the reduced form and thereby it helps to maintain the activity of the enzyme. Prolyl oxidase, on the other hand, is essential for the synthesis of normal collagen. In scurvy, abnormal collagen is synthesized that causes skin lesions and broken blood vessels. The minimum daily requirement of vitamin C to prevent scurvy is 30 mg. Some people, including Linus Pauling, advocate mega doses of vitamin C (250-10,000 mg/day) to prevent cancer, the common cold, etc. In this experiment we will determine the vitamin C content of certain foods by titrating the solution with a water soluble form of iodine (I2). Vitamin C is oxidized by I2 (as I3– ) according to the following chemical reaction: CH 2OH O O OH

Iodine-starch complex (black-blue color)

It is worthwhile to note that although vitamin C is very stable when dry, it is readily oxidized by oxygen when in solution. Therefore, a solution of vitamin C should not be exposed to air for an extended period of time. The amount of vitamin C can be calculated by using the following conversion factor: 1 ml of I2 (0.01 M) = 1.76 mg of vitamin C

(1 mL I2 ) ⎜

⎛ 0.01mol ⎞ ⎛ 1 mol Vit C ⎞ ⎛ 176 g ⎞ g Vit C ⎟⎜ ⎟⎜ ⎟ = 1.76 1000 mL I 2 ⎠ ⎝ 1 mol I 2 ⎠ ⎝ mol Vit C ⎠ mL I 2 ⎝

PROCEDURE: 1. Pour about 60 ml of a fruit drink that you wish to analyze into a clean, dry 100-ml beaker. Record the kind of drink on the report sheet. Alternative sources of vitamin C can be used, such as fresh oranges, lemons, lemon juice, and red cabbage.

2. If the fruit drink is cloudy or contains suspended particles, it can be made clear by the following procedure: Filter the solution through a glass funnel, bedded with a small piece of glass wool. Collect the filtrate in a 50 ml Erlenmeyer flask. 3. Using a 10 ml volumetric pipette transfer 10 ml of the fruit drink into a 125 ml Erlenmeyer flask. Then add 20 ml of distilled water, five drops of 3 M HCl (as a catalyst), ten drops of 5% starch solution to the flask, and a magnetic stir bar. 4. Clamp a clean, dry 50 ml burette onto a burette stand. Rinse the burette twice with 5 ml portions of the iodine solution (0.01 M). Let the rinsings run through the tip of the burette and discard them. Fill the burette slightly above the zero mark with a standard iodine solution. Record the molarity of standard iodine solution. Air bubbles should be removed by turning the stopcock several times to force the air bubbles out of the tip. Record the initial reading of standard iodine solution to the nearest 0.01 ml. 5. Place the flask that contains the vitamin C sample on a stir plate under the burette. Add the iodine solution drop wise to it while stirring the solution, until the indicator just changes to dark blue. This color should persist for at least 20 seconds. Record the final burette reading.

HO

OH

Vitamin C (Ascorbic Acid) (Reduced)

CH 2OH O O OH + 2e- + 2H+

6. Now add ten additional milliliters (10 ml volumetric pipette) of vitamin C drink to your flask. Repeat the titration. Record the volume of Iodine solution. 7. Repeat step 6 a second time by adding 10 ml more of the vitamin C drink for a total of 30 ml.

O

O

Dehydroascorbic Acid (Oxidized)

Vit-C + I2

Dehydroascorbic Acid + 2 HI


Adapted from the Pierce chemistry lab manual

As vitamin C is oxidized by iodine, I2 becomes reduced to I . When the end point is reached (no vitamin C is left), the excess...
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