SAY pH WITH FLOWERS, FRUITS, AND VEGETABLES

PURPOSE

DESCRIPTION

TIME REQUIRED

MATERIALS

Chemicals

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)*

acetone

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.

Equipment

test tubes*

ziplock bags

stirring rods

10-mL graduated cylinders

medicine droppers

spot plate*

HAZARDS

MODIFICATIONS/SUBSTITUTIONS

1. 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.
2. NaOH is available as lye from a grocery store.
3. 2-Propanol is available as rubbing alcohol.
4. Acetone is available as unscented, plain nail polish remover.
5. Beakers or plastic cups may be used in place of test tubes.
6. Plastic coffee stirrers may be used in place of stirring rods.
7. Tissue culture plates, white styrofoam egg cartons, or ice cube trays for making small ice cubes may be used in place of spot plates.

PROCEDURE

Extracting the indicator

1. 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.
2. 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.
3. Remove petals, skins, etc. from the extract by decanting or filtering.

1. Label 13 test tubes from 1 to 13.
2. Place 9.0 mL of distilled or deionized water in all test tubes except #1 and #13.
3. Prepare solutions in the acid range in the following manner:
   1. Place 10.0 mL of 0.1 M HCl in test tube #1. (pH = 1)
   2. Transfer 1.0 mL of 0.1 M acid from test tube #1 to test tube #2 and mix thoroughly. (pH = 2)
   3. Transfer 1.0 mL of acid solution from test tube $2 to test tube #3 and mix thoroughly. (pH = 3)
   4. 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.
4. Add 10.0 mL distilled or deionized water to test tube #7. (pH = 7)
5. Prepare solutions of base in the following manner:
   1. Place 10.0 mL of 0.1 M NaOH in test tube #13. (pH = 13)
   2. Transfer 1.0 mL of 0.1 M NaOH from test tube #13 to test tube #12 and mix thoroughly. (pH = 12)
   3. 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.
6. 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.
7. Add a drop or two of the flower indicator to each well. Observe the pH at which the indicator changes color.
8. 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.

DISPOSAL

TIPS

1. 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.
2. 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?

REFERENCES

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 lpt@www.woodrow.org
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