Kids, let's make your own acid/base pH indicator by doing a little cooking - just by boiling red cabbage. The juice is used to test the pH of different liquids.
Please note: All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.
Tear up 1/2 head of red cabbage (or use a grater with supervision), place the pieces in a pot and add just enough water to cover them. Boil (with supervision) for 20-30 min until the liquid turns a dark purple color. Let cool and pour through a strainer into a large jar. You can even save the cabbage for later - add a little vinegar and it's a relish for hotdogs, etc. The collected liquid should be blue/dark purple. Make some test acid and base solutions in cups: white vinegar, clear soda, diluted lemon juice (acids) and some detergent in water or some baking soda in water (bases), and pure water (neutral). Add a few drops of the cabbage juice to each solution and note any color changes.
The juice should turn pink in acidic solutions, green in basic solutions, and not change in neutral solutions. Try some other solutions of interest. You can also try making your own pH paper: soak a coffee filter in the juice and let it dry, then cut into test strips. Or pre-cut the filter into any interesting shape. Dip your papers into the test solutions are record what happens. What might happen if you dip a paper first into a base and then into the vinegar? You can even try a little creative art by using a cotton swab dipped in vinegar to draw on your pH paper. Can you then make your art disappear? Try all of these things and have fun!
Further comment: Red cabbage works so well because of the highly colored anthocyanin dye it contains. Other plants that contain these molecules are beets, cranberries, and blueberries.
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Kathleen Carrado Gregar, PhD, Argonne National Labs
[email protected]
April 1999
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References: www.madsci.org, the Mad Scientist Network based at Washington University Medical School in St. Louis, and WonderScience, February 1988.
