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Determine the Amount of Vitamin C in Various Foods by Using Titration Method

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Author: Janice VanCleave

Vitamin C is required for good health. This vitamin is not produced by your body and must be obtained from foods or vitamin tablets.

In this project, you will use the titration method to determine the amount of vitamin C in various foods and vitamin tablets. You will also determine whether fruit drinks and fruit juices have comparable amounts of vitamin C. Vitamin C's antioxidant properties will be studied, and its part in an oxidation-reduction reaction will be examined. You will also look at the sources and uses of different vitamins.

Getting Started

Purpose: To determine the amount of iodine needed to react with a standard solution (carefully measured quantity) of vitamin C.

CAUTION: Be careful not to allow the vitamin C to touch your skin, in case you have any sensitivity to the chemical.

Materials

  • 100-mg ascorbic acid (vitamin C) tablet
  • wax paper
  • hammer
  • 1/2 cup (125 ml) of distilled water
  • 1-quart (l-liter) jar
  • spoon
  • 4 baby-food jars
  • marking pen
  • masking tape
  • 1-teaspoon (5-ml) measuring spoon
  • starch solution (see Appendix 5)
  • sheet of white paper
  • eyedropper
  • tincture of iodine

Procedure

CAUTION: Keep the iodine out of reach of small children. It is poisonous and is for external use only.

  1. Prepare a standard vitamin C solution by:
    • crushing the vitamin C tablet (place it between two sheets of wax paper and hit it gently with a hammer).
    • pouring 1/2 cup (125 ml) of distilled water into the quart (liter) jar.
    • adding the crushed vitamin C powder to the water in the jar.
    • stirring until the powder dissolves.
  2. Pour equal portions of the standard vitamin C solution into four baby-food jars.
  3. With the marking pen, write "A." "B," "C," and "D" on pieces of masking tape and tape one label to each jar.
  4. Add 1 teaspoon (5 ml) of the starch solution to each jar.
  5. Place jar A on the sheet of white paper.
  6. Fill the eyedropper with tincture of iodine. Slowly add the iodine in the eyedropper to jar A. counting each drop added (see Figure 7.1). Swirl the jar after each addition of five drops. Continue to add the iodine until the jar's contents remain a blue-black color.
  7. Record the number of drops required to turn the jar's contents this blue-black color.
  8. Repeat the procedure using jars B, C, and D.
  9. Add the results for the four jars and divide by four to compute the average number of drops of iodine needed to react with the 25 mg of vitamin C in each jar. Note: This number will be used to calculate the concentration of vitamin C in other substances.

Results

The vitamin C-starch solution is unaffected by the initial drops of iodine, but adding more iodine results in a blue-black solution. Note: The number of drops of iodine needed to react with the 25 mg of vitamin C in the solution will vary with the size of the eyedropper.

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 iodine (I2 and ethyl alcohol (C2H5OH). The combination of elemental iodine and vitamin C chemically changes vitamin C (ascorbic acid) to a compound ineffective as a vitamin called dehydroascorbic acid. The elemental iodine is changed to a charged particle called iodide (I).

Vitamin C Content Analysis of Food by Titration

Elemental iodine reacts with starch to produce a blue-black color, but mixing the charged particle iodide with starch does not produce a color change. When starch, vitamin C, and elemental iodine are mixed, the iodine is more attracted to the vitamin C molecules. The starch stays in the solution unchanged until all of the vitamin C has combined with the elemental iodine. When the last molecule of vitamin C reacts with the iodine, then the starch molecules combine with any remaining iodine, producing a starch-and-iodine, blue-black-colored complex molecule.

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