You may have heard that citrus juices are acids or that ammonia is a base. The terms acid and base refer to the concentration of hydrogen ions (pH) in the substance; acids have a high concentration of hydrogen ions, while bases have a low concentration of hydrogen ions. But how can you test whether a substance is an acid or base? In this experiment, you will make your own pH indicator from red cabbage and use it to measure some liquids to see whether they are acids or bases.
What You Need to Do the Experiment
You will need:
- A head of red cabbage
- A stove
- A pot
- Distilled water
- A funnel or coffee filter basket
- Coffee filters
- A set of eyedropper bottles (e.g., clean infant medicine bottles)
- A medicine cup with teaspoon or milliliter markings
- Several small paper cups
- Some clear test substances (e.g., lemon juice, clear sports drink, lemon-lime soda, window cleaner, vinegar).
Performing the Experiment
To prepare the indicator, shred some red cabbage leaves, place them in small volume of water (1/2 cup) in the pot, bring the pot to a boil on the stove, and boil for 5-10 minutes (Note - You should wear some form of eye protection and use pot holders when handling boiling solutions). Pour the water with the boiled leaves through the coffee filter to remove the pieces of cabbage, collect the filtered juice in a clean glass, and let it cool; it should appear purple. You can then pour the juice into the eyedropper bottles for use and storage. This filtered juice is your pH indicator. Note that the concentration of the indicator solution depends upon the amount of cabbage and the volume of water used; the more cabbage in the smallest possible volume of water will yield a high concentration of indicator molecules.
To use your pH indicator, pour about 2 teaspoons (10 ml) of each test substance into a separate Dixie cup. To each cup, add approximately 1/2 teaspoon (2 ml) of cabbage juice indicator. Now watch for the color changes! If the substance is an acid (i.e. lemon juice), the indicator should turn bright pink. If the substance is neutral (i.e. Gatorade), then the indicator should remain reddish-purple. Finally, if the substance is a base (i.e. window cleaner), then the indicator should turn green.
What Actually Happened
By boiling the red cabbage leaves, you extracted a class of pigment molecules called anthocyanins into solution. Anthocyanin molecules will change their color depending upon the pH of their environment and can indicate the pH of a solution). This experiment will tell you whether a substance is an acid or base, but not the exact value of pH; the pH scale ranges from acid (0-6), through neutral (7) to base (8-14). If you want to calibrate your cabbage juice pH indicator, you will have to test your substances with another quantitative indicator (e.g. litmus paper) and compare those results to the colors of the cabbage juice pH indicator in those solutions; litmus paper can be obtained from several scientific suppliers (i.e. Fisher Scientific, Carolina Biological, Edmund Scientific) or from your local swimming pool store.
To learn more, read "Where does the color come from in purple cabbage?"