Sources: B. Z. Shakhashiri, 1989, Chemical Demonstrations: A Handbook for Teachers of Chemistry, vol. 3, pp 162-166.

Description and Concept: Red cabbage juice will change to a variety of colors when added to solutions of various pH. Red cabbage juice is a pH indicator.

Materials:

red cabbage

blender

hot or boiling water

filter paper (coffee filters work well)

1 liter beaker (or jar)

Six 250 mL beakers (or jars)

household ammonia (NH₃(aq))

baking soda (sodium bicarbonate, NaHCO₃)

washing soda (sodium carbonate, Na₂CO₃)

lemon juice (citric acid, C₆H₈O₇)

vinegar (acetic acid, CH₃COOH)

cream of tartar (Potassium bitartrate, KHC₄H₄O₆)

Other household items that may be used include:

antacids (calcium carbonate, calcium hydroxide, magnesium hydroxide), seltzer water (carbonic acid, H₂CO₃)

muriatic acid or masonry's cleaner (hydrochloric acid, HCl),

or lye (potassium hydroxide, KOH or sodium hydroxide, NaOH).

Safety:

Hydrochloric acid and sodium hydroxide are strong irritants. You should wear safety goggles and disposable gloves for this demo, especially if you choose to use HCl and NaOH.

Procedure:

Chop cabbage into small pieces. Put cabbage in 5 liter beaker and then cover with boiling water, let stand for 10 minutes or more (the more you wait the more potent your indicator).

OR

Put about 2 cups of cabbage in a blender, cover with boiling water, and blend. Filter out the plant material to get the red-purple-bluish colored liquid. This liquid is at about pH 7. Pour about 50 - 100 mLs of your red cabbage indicator into each 250 mL beaker. Add your various household solutions to your indicator until you see a color change.

For example: Ammonia will turn your indicator green, NaOH -yellow, vinegar -red, and HCl - a deep red.

You could do a neutralization experiment with the indicator. First add an acidic solution such as vinegar or lemon juice to get a reddish color, then add baking soda or antacids to bring the pH back towards a neutral 7.

Clean-Up: All of the chemicals used in this demo can be washed down the drain with water.

Background:

Red cabbage contains pigments called anthocyanins, which belong to a class of compounds called flavonoids. This pigment is also found in red poppies, blue cornflowers, grapes, and apple skin. Depending on the pH of the fruit or plant the pigment will give a different color. This pigment can be used to indicate pH of solutions.

pH is the -log[H₃O⁺]. Acids increase the concentration of hydronium ions ([H₃O⁺]) in a solution and therefore have a low pH. A base will increase the concentration of hydroxide ions ([OH^-]) (decreasing the [H₃O⁺]) in a solution and will have a high pH.