Experiment 14.1: Comparing Vitamin C in Fruit Juices

Student Information

Purpose

To use an indicator to determine the relative amount of vitamin C in fruit juices.

Materials

  • Tincture of iodine—1 ounce bottle (You can find this at any drug store.)
  • Lemon juice
  • Apple juice
  • Orange juice
  • Grapefruit juice or pineapple juice (or another juice of your choosing)
  • 1000 mg vitamin C pill
  • Medicine dropper
  • A 1 quart jar
  • Measuring cup with milliliter markings
  • Water
  • Six 8 ounce clear plastic cups

Background

Different fruits or fruit juices all contain vitamin C, but which is the best source? Vitamin C is an organic acid called ascorbic acid. Like other acids, vitamin C reacts with indicators to produce a color change. Tincture of iodine is a brownish-red colored indicator. When vitamin C is added, it becomes colorless. This reaction can be used to test for the presence of vitamin C and to give some idea of the amount that can be found in the fruit or fruit juices.

Note: Do not consume any of the fruit or fruit juices during this experiment! Tincture of iodine can stain and should NOT be ingested, so you may want to place plastic under your lab area.

Hypothesis

Predict which fruit juice contains the most vitamin C. Record your prediction:

Procedure

  1. Prepare an iodine solution by mixing a bottle of tincture of iodine with 500 mL of water in the quart jar.
  2. Pour 50 mL of the iodine solution into each of 5 clear plastic cups. Line them up on your work area.
  3. Using a spoon, crush a vitamin C tablet and then dissolve it in 100 mL of water in the other clear plastic cup. This will be your control.
  4. Using the medicine dropper add one drop of the vitamin C control solution to a cup of iodine. Swirl the cup and note the color change. If the iodine solution did not become clear, continue adding drops of the vitamin C solution one at a time (and swirling after each drop) until the iodine solution turns clear or almost clear. Make sure to count the drops and then record the total number of drops required to turn the iodine solution clear.
  5. Rinse out the medicine dropper thoroughly so that your results will not be contaminated. If you have more than one medicine dropper, use a different one for the next juice while the first one dries.
  6. Repeat steps 4 and 5 for each of the juices.
  7. Clean up and put everything away.

Data Collection

Vitamin C Control (1000mg tablet in 100mL water)

Drops of vitamin C control solution needed to clear the iodine solution:

Fruit Juice Testing Results

Juice Type Drops Needed to Clear Iodine Relative Vitamin C Content
Lemon juice --
Apple juice --
Orange juice --
Grapefruit/Pineapple juice --

Note: Fewer drops needed = higher vitamin C content

Conclusions

Make a bar graph of your results. Which fruit juice had the most vitamin C? Write a paragraph explaining how your results showed the juices with the most vitamin C. Make connections to the text.

Vitamin C Content Bar Chart

Chart will update automatically as you enter your data above