Magic Bullets: Chemistry vs. Cancer

    Go to teacher's guide

    You Become What You Eat:
    What's in There?
    Vitamin C in Fruit Juice

    Menu

    Safety
    Advance Preparation
    Calculations
    Answers to Analysis and Conclusions
    Notes
    For more information, at other Web sites...
    Relevant National Science Education Standards
    Relevant New Jersey State Science Education Standards

    Introduction

    This lab activity is included for several reasons. First, the titration represents a standard activity in chemistry. You might want to use this titration and the application to nutrients in place of another type of titration. Second, the process illustratesm how nutritionists can detect and quantify microamounts of nutrients in a mixture.

    This is an application of the methods by which chemists analyze complex mixtures in many areas. The detection and quantification of phytochemicals in foods fits this pattern. And, third, vitamin C is an antioxidant as are many phytochemicals. There is no suggestion that vitamin C is an anticancer agent, Linus Pauling's opinions aside.

    If your classes have done microscale labs prior to this activity, they will know the type of equipment involved. If this is your first venture into microscale with a class, you might want to show Beral pipettes and microplates as you assign the lab so students can visualize what they will do.

    Safety

    General Safety Guidelines

    The following MSDS sheets are available on-line from Cornell University:

      Potassium iodate
      Potassium iodide

    Advance Preparation

    1. Prepare the vitamin C solution. Note that vitamin C is stable when dry, but in solution it deteriorates as it is exposed to oxygen. Prepare this solution as close to the time of use as possible. Dissolve 1 g of L-ascorbic acid in approximately 500 ml of distilled water. Swirl to dissolve the solid. Dilute to make 1 liter. The concentration of this solution will be about 0.006 M. You will not need the concentration for the lab. It is given as 1 mg vitamin C per 1 ml of solution.
    2. Prepare the iodine solution. Dissolve 10.00 g KI in 250 ml 0.01 KIO3 solution (2.14 g KIO3 in 1.0 L). Transfer to a 1 L flask and add 60 ml of 3M sulfuric acid. Add enough distilled water to make 1 L of solution.
    3. Prepare starch solution. To make a 1% solution, add 10 g soluble starch to distilled water to make a smooth paste. Pour the starch paste into 1 L boiling water with constant stirring. Cool. This solution deteriorates relatively quickly. An easier way to make the solution is to spray regular spray starch into distilled water.
    4. Supply each lab table with:

      1. Stoppered bottle of each of the three solutions along with a labeled Beral pipette for each solution.
      2. Microplate for each group.
      3. Beral® pipets with blank labels, enough for each type of juice you assign.
      4. Toothpicks.

    5. If you ask students to bring in their own juices, decide how you will dispense them. Caution students not to drink any of the juices brought into the lab.
    6. You may wish to demonstrate for the class the color change at the end point of the titration. It can be done using an overhead projector and clear microplate. Show the color as you approach the end point to help student judge this.

    Calculations

    You may have to guide your students through the calculations, especially the conversion factor math, even though it is shown in the student data section. An example of student data:

        Number of drops vitamin C to equal 1 ml=28 drops

        Number of drops of iodine solution=23 drops

        Conversion Factor Calculation (see Step 7 in student procedure)

        Mg vitamin C/drop iodine=a×b×c×d

        This ratio is then multiplied by the number of drops of iodine used to titrate each juice sample. There is sufficient space on the data and calculation sections for five juices.

    Answers to Analysis and Conclusions

    1. What conditions in this lab allow you to make direct comparisons of vitamin C content from your calculations?

        Answer: Each time a titration is done in this lab, 25 drops of juice is used. Since the volume of each juice is the same, we can use the calculated milligram results comparatively.

    2. Rank the juices you analyzed from most vitamin C to least.

        Answer: Results will vary from class to class, depending on the juices selected.

    3. If label information is available for the juices you used, how does information from the labels compare with your results?

        Answer: Results will vary from class to class, depending on the juices selected. To make these comparisons, students may have to do a little more calculating. The results of the calculations in this lab are actually mg vitamin C per 25 drops juice. Encourage students to read juice labels looking for units on the label. As long as students remember to look at their "number of drops per 1 ml" data, they can convert.

    4. Is the vitamin C content of any one juice much greater or much less than the rest?

        Answer: Results will vary.

    Notes

    A piece of white paper under the microplate may help students see the color change.

    For more information, at other Web sites...

      Natural Food-Fruit Vitamin C Content — from The Natural Food Hub.

    Relevant National Science Education Standards

      Science as Inquiry — This lab is based on inquiry skills.

    Relevant New Jersey State Science Education Standards

      5.1 The lab requires problem-solving and inquiry skills.
      5.3 Mathematical calculations are important in reaching conclusions in this lab.
      5.6 The interactions between vitamin C and other compounds are important in this lab.

     

    Back to:

    You Become What You Eat

    Teacher's Guide Directory | Student Version Directory | Pharmaceutical Achievers Home

    Copyright ©2001 The Chemical Heritage Foundation