SAY pH WITH FLOWERS, FRUITS, AND VEGETABLES
This experiment explores the extraction of natural indicators from common flowers, fruits, and vegetables and the pH at which these natural indicators change color.
Many flowers, fruits, and vegetables contain chemical substances that change color in solutions of different pH's. In this experiment, some of these indicators will be extracted. The pH's at which the indicators change color will be investigated.
Two lab periods.
- 0.1 M HCl solution (add 8.3 mL of concentrated HCl solution to enough deionized or distilled water to make 1.0 L of solution)*
- 0.1 M NaOH solution (dissolve 4.0 g NaOH in enough deionized or distilled water to make 1.0 L of solution)*
- 2-propanol (isopropyl alcohol)*
- deionized or distilled water
- fruits - strawberries, blueberries, black berries, raspberries, grapes, plums, etc.
- vegetables - red cabbage, beets, red radishes, red onions, etc.
- flowers - purple dahlias, purple hollyhocks, red geraniums, blue iris, hydrangeas, etc.
- test tubes*
- ziplock bags
- stirring rods
- 10-mL graduated cylinders
- medicine droppers
- spot plate*
Caution should be used when working with solutions of hydrochloric acid and sodium hydroxide. Both can be irritating to the skin. Goggles must be worn throughout the experiment. If it is necessary to heat the extract in order to concentrate the indicator, do not heat those containing the flammable solvents, 2-propanol or acetone, over an open flame. Use a water bath or hot plate.
Have each lab group bring in at least one flower, one vegetable, and one fruit for investigation (dark colors are preferred).
- HCl solution is available from a hardware as muriatic acid, 28% HCl. To make a 0.1 M HCl solution add 17 mL of this solution to enough distilled or deionized water to make 1.0 L-of solution.
- NaOH is available as lye from a grocery store.
- 2-Propanol is available as rubbing alcohol.
- Acetone is available as unscented, plain nail polish remover.
- Beakers or plastic cups may be used in place of test tubes.
- Plastic coffee stirrers may be used in place of stirring rods.
- Tissue culture plates, white styrofoam egg cartons, or ice cube trays for making small ice cubes may be used in place of spot plates.
Extracting the indicator
Testing the pH range of the indicator
- Place a small sample of each fruit, vegetable, or flower to be tested in a different ziplock bag. For flowers, only a few petals are normally needed. It is important to keep stems, leaves, etc. out of the bag so that chlorophyll is not extracted along with the flower pigments. For fruits and vegetables, finely chopped pieces should be used. Use only that portion of the fruit or vegetable that is most pigmented.
- Add about 10 mL of solvent to each bag, close the bag, and mash the contents in the closed bag to make the extract. Leave combinations that seem most productive overnight to enhance the extraction. Suggested solvents include water, 2-propanol, 50-50 water/2-propanol mixture, or acetone. One of these should easily extract the color. If the extracts are very dilute, the color can be concentrated by heating the opened bag in a warm water bath or by pouring the extract in a beaker and evaporating on a hot plate.
- Remove petals, skins, etc. from the extract by decanting or filtering.
Testing the pH of other liquids
- Label 13 test tubes from 1 to 13.
- Place 9.0 mL of distilled or deionized water in all test tubes except #1 and #13.
- Prepare solutions in the acid range in the following manner:
- Place 10.0 mL of 0.1 M HCl in test tube #1. (pH = 1)
- Transfer 1.0 mL of 0.1 M acid from test tube #1 to test tube #2 and mix thoroughly. (pH = 2)
- Transfer 1.0 mL of acid solution from test tube $2 to test tube #3 and mix thoroughly. (pH = 3)
- Continue making the serial dilutions by transferring 1.0 mL of the most recently diluted acid solution to the next test tube until six acid solutions of pH 1 to 6 have been prepared. Be sure to mix each thoroughly before the transfer.
- Add 10.0 mL distilled or deionized water to test tube #7. (pH = 7)
- Prepare solutions of base in the following manner:
- Place 10.0 mL of 0.1 M NaOH in test tube #13. (pH = 13)
- Transfer 1.0 mL of 0.1 M NaOH from test tube #13 to test tube #12 and mix thoroughly. (pH = 12)
- Continue making serial dilutions of the base going from pH 12 down to pH 8 by transferring 1.0 mL of the most recently diluted basic solution to the next test tube and mixing thoroughly each time.
- Label the wells of a spot plate from 1 to 13. Transfer a few drops of each of the solutions prepared in steps 3, 4, and 5 to the corresponding well in the spot plate.
- Add a drop or two of the flower indicator to each well. Observe the pH at which the indicator changes color.
- Repeat steps 6 and 7 with the fruit extract and then the vegetable extract.
Once the pH ranges of the indicators have been determined, they can be used in acid-base titrations or to test the pH of household chemicals.
All of the solutions produced should be flushed down the drain with plenty of water.
- If large amounts of red cabbage indicator are to be used, it may be more efficient to boil 1/4 of a cabbage in a beaker or pot for a few minutes to provide the extract for an entire class.
- A resourceful student could expand on this experiment to develop very successful science projects such as: What particular combinations of natural indicators would result in an effective universal indicator? Can the individual components of an extract be effectively isolated through the use of chromatography? How do indicators from different colored flowers of the same species differ?
Phanstiel, O., J. Chem. Ed. 62, 322 (1985).
- Describes how this type of experiment can be used to involve students in experimental design.
Summerlin, L.R., Chemistry of Common Substances, Silver Burdett, New York, 1986.
- This work describes a similar experiment.
Submitted by Larry Kresse, Angie Matamoros, Patti Ruff, Sam Sakurada, and Lillie Tucker-Akin
Woodrow Wilson Leadership Program in Chemistry
The Woodrow Wilson National Fellowship Foundation
CN 5281, Princeton NJ 08543-5281