How to Make a Red Cabbage pH Indicator

Three different colored jars against a white background
Three jars containing red cabbage juice, turned red by adding lemon (acid), green by adding soap (alkali), and blue with nothing added. Clive Streeter / Getty Images

Make your own pH indicator solution. Red cabbage juice contains a natural pH indicator that changes colors according to the acidity of the solution. Red cabbage juice indicators are easy to make, exhibit a wide range of colors, and can be used to make your own pH paper strips.

Cabbage pH Indicator Basics

Red cabbage contains a pigment molecule called flavin (an anthocyanin). This water-soluble pigment is also found in apple skins, plums, poppies, cornflowers, and grapes. Very acidic solutions will turn anthocyanin into a red color. Neutral solutions result in a purplish color. Basic solutions appear in greenish-yellow. Therefore, you can determine the pH of a solution based on the color that it turns the anthocyanin pigments in red cabbage juice.

The color of the juice changes in response to changes in its hydrogen ion concentration; pH is the -log[H+]. Acids will donate hydrogen ions in an aqueous solution and have a low pH (pH 7).

Materials You Will Need

  • Red cabbage
  • Blender or knife
  • Boiling water
  • Filter paper (coffee filters work well)
  • One large glass beaker or another glass container
  • Six 250 mL beakers or other small glass containers
  • Household ammonia (NH3)
  • 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)
  • Antacids (calcium carbonate, calcium hydroxide, magnesium hydroxide)
  • Seltzer water (carbonic acid, H2CO3)
  • Muriatic acid or masonry's cleaner (hydrochloric acid, HCl)
  • Lye (potassium hydroxide, KOH or sodium hydroxide, NaOH)

Procedure

  1. Chop the cabbage into small pieces until you have about 2 cups of chopped cabbage. Place the cabbage in a large beaker or other glass container and add boiling water to cover the cabbage. Allow at least 10 minutes for the color to leach out of the cabbage. Alternatively, you can place about 2 cups of cabbage in a blender, cover it with boiling water, and blend it.
  2. Filter out the plant material to obtain a red-purple-bluish colored liquid. This liquid is at about pH 7. The exact color you get depends on the pH of the water.
  3. Pour about 50–100 mL of your red cabbage indicator into each 250 mL beaker.
  4. Add various household solutions to your indicator until it changes color. Use separate containers for each household solution—you don't want to mix chemicals that don't go well together.

Red Cabbage pH Indicator Colors

pH 2 4 6 8 10 12
Color Red Purple Violet Blue Blue-Green Greenish Yellow

Tips and Safety

This demo uses acids and bases, so use safety goggles and gloves, particularly when handling strong acids (HCl) and strong bases (NaOH or KOH). Chemicals used in this demo may be safely washed down the drain with water.

You can conduct a neutralization experiment using a cabbage juice indicator. First, add an acidic solution such as vinegar or lemon, then juice until a reddish color is obtained. Add baking soda or antacids to return the pH to a neutral 7.

You can make your own pH paper strips using a red cabbage indicator. Take filter paper (or coffee filter) and soak it in a concentrated red cabbage juice solution. After a few hours, remove the paper and allow it to dry (hang it by a clothespin or string). Cut the filter into strips and use them to test the pH of various solutions. To test a sample, place a drop of liquid on the test strip. Don't dip the strip in the liquid because you'll get cabbage juice in it. An example of a basic solution is laundry soap. Examples of common acids include lemon juice and vinegar.