The Synthesis of Aspirin

The Synthesis of Aspirin

Experiment 13: Vitamin C Content of Beverages

Vitamins are a class of structurally-diverse chemicals which are an essential component of our diet. Each vitamin has its own particular function and properties. For example, some vitamins are soluble in fats and oils (including Vitamins A and E), while others will dissolve in water. Vitamin C, also known as asorbic acid, is among the latter. This vitamin has been celebrated for years for its ability to prevent colds and deter infection. Linus Pauling, the greatest chemist of the 20th century, devoted many of the final years of his life to studying and promoting it. As you probably already know, citrus fruits (and their juices) are the main source of Vitamin C for most people. In addition, many “alternative beverages,” including Tang and Kool-Aid contain Vitamin C. In today’s experiment, we will attempt to determine the number of milligrams of Vitamin C present in a typical 8-ounce serving of a fruit juice or beverage. We will also analyze a standard Vitamin C tablet. For reasons which will become clear as we continue, this experiment tends to work best on liquids with little or no color. You will observe the reaction of Vitamin C with iodine (I2) in the presence of starch. As you add iodine to the juice, you gradually transform all of the Vitamin C. When none of the vitamin is left, the iodine will interact with the starch, causing the solution to change color.

This will be the signal that the reaction is complete.
The Reaction:

Procedure
Part A—Vitamin C in a Commercial Vitamin C pill

  1. Obtain a Vitamin C pill. Note the number of milligrams of vitamin C each pill should have according to the container. Crush the pill into a fine powder with in a mortar. Carefully scrape all of this powder onto a piece of weighing paper.
  2. Transfer the powder to a 250 mL or 125 mL Erlenmeyer flask. Add about 50 mL of distilled water to the powder and swirl for about 2 minutes to dissolve as much of the powder as possible. Some of the material may not dissolve; this is probably a filler material used to hold the tablet itself together. Finally add about 1 mL of the 1% starch solution (the indicator).
  3. Set up a buret according to your instructor’s example. Add the iodine solution to the buret; the solution should fill the buret to a level between the 0 and 1 mL markings. Be careful: the iodine solution stains clothing and skin! You should use a glass funnel to help you in this process .Record the value of this “starting point” on the report sheet.
  4. Add iodine solution to the flask, carefully controlling the rate with the stopcock (the small handle on the buret). Swirl the flask gently as you add the solution. At first, you will see color changes which quickly fade. As you get closer to the endpoint of the titration, the color will seem to persist for a longer period of time. You should slow down the rate at which you are adding the iodine solution. The titration is complete when the solution turns a deep blue; it must remain blue for at least 30 seconds, during which time you should gently swirl the flask. Record the “ending point” reading on the buret on your report sheet.
  5. Thoroughly rinse the flask and the buret with water.

Part B—Vitamin C in Fruit Juice or Sports Drink

  1. Using a graduated cylinder, measure out exactly 50.0 mL of your sample. Pour this into the Erlenmeyer flask you used in Part A. Add 1 mL of 1% starch solution and swirl. Note: Depending on your juice, it may be necessary for you to add 2 mL of 0.1 M acetic acid. Only add it if the experiment does not seem to work and you are running it over again.
  2. The iodine solution you used to titrate the Vitamin C pill is too concentrated for use in this part of the experiment; you must first dilute (water down) the solution. Measure out exactly 10.0 mL of the iodine solution you used in Part A with a the buret, and transfer this to your 150 mL beaker. Add exactly 90.0 mL of water to this (measuring the volume with your 100 mL graduated cylinder) and stir with a glass stirring rod. We will call this the “diluted solution”; it is 10 times less concentrated than the solution you used in Part A. Pour the diluted solution into the buret as you did in step 3 of the previous part and record your starting point.
  3. Using the buret, add the iodine solution to the juice solution you prepared in step one. While controlling the flow from the buret with one hand, you should use the other to gently swirl the flask as you did in step 4 of Part A.
  1. When the solution changes color and does not return to the original color, you have reached the end point of the titration. You should swirl the solution for another thirty seconds. If the solution in the flask does not return to the original color, then you are done. Otherwise, it will be necessary to add a few more drops of the iodine solution from the buret. The color maybe different from that of Part B if your juice or sports drink has any color to it. However, the color change should be very distinct. Record the ending point of the titration.
  1. Repeat this procedure at least once (twice if possible). You should rely on the information from the first trial to predict how much iodine you will need to add. You may use the same Erlenmeyer-flask, but be sure to pour the contents down the drain and rinse it several times with distilled water first.
  2. Follow the procedures indicated in the report sheet to determine the number of milligrams of Vitamin C present in one 8 oz. serving of the drink.

How Much Vitamin C?Name:______

Report Sheet
Part A—Vitamin C Pill

1. Milligrams of Vitamin C in each pill (according to label) ______mg

2. Final Buret Reading(mL)
3. Starting Buret Reading (mL)
4. Total Volume of Iodine Solution
Added (#2 minus #3)

5. Each milliliter of the iodine solution should react with 14 mg of Vitamin C. Find the amount of Vitamin C in the pill by taking the value you got in #4 and multiplying it by 14. Record this value on the right.
______mg Vit. C/pill
Part B—Fruit Juice or Sports Drink
1. What juice or other product did you use?______
2. After diluting the iodine solution, figure out how many milligrams of vitamin C should react with 1 mL of the diluted solution. (Think: “How much did I dilute the solution by?”)
______mg Vit C/mL diluted Iodine solution

Trial One / Trial Two
3. Final Buret Reading(mL)
4. Initial Buret Reading (mL)
5. Total Volume of Iodine Solution
Added (#3 minus #4)
6. Milligrams of Vitamin C in 50. mL
of your juice (#5 divided by #2)

7. Average number of milligrams of Vitamin C
in 50 mL of your juice (Add up the values for both trials
in #6. Then, divide the result by 2)______mg

8. Milligrams of Vitamin C in one 8 fluid ounce
serving of this juice (Multiply the value in
#7 by 4.73)
______mg

9. Percent of USDA-recommended Vitamin C
requirement in 1 serving of juice (Divide the value
in step 8 by 60. Then, multiply by 100) ______%
According to your data, does this juice appear to be a good source of Vitamin C? Explain.
This method would not work well for purple grape juice, which also contains Vitamin C. Explain why.
Is it likely that the same procedure as the one we used today could be used to analyze all vitamins? Briefly explain your answer, using the material you have read about vitamins to assist you.

Preparation Instructions:

Prepare at least 1 liter of iodine solution for every 10 students in class. The iodine solution is prepared by dissolving 20. grams of iodine and 40. grams of potassium iodide in 1 liter of water. Stir the solution, then gravity filter to prevent burets from clogging up.

In addition students will need:
1% starch solution (at least 50 mL per 10 students)

1 Vitamin C pill per student

6 mortar and pestles for class use

Students might also need to add a few drops of 0.1 M acetic acid (unlikely)

Also, please purchase the following from the supermarket:
Any colorless (or clear and very slightly colored) sports drink which contains at
least 50% of the USDA Vitamin C requirement (30 mg) per 8 fl. ounce serving
White grape fruit juice (Welch’s works well)

Apple Juice which is fortified with Vitamin C (Tropicana works well, but check the label for Vitamin C)
There should be enough juice/sports drink available for each student to have 150 mL of a given liquid (they only need 150 mL of one of these, not 150 mL of each)

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