How to Make Red Cabbage pH Paper

These pH paper test strips were made using coffee filters dipped in red cabbage juice.
These pH paper test strips were made using paper coffee filters that had been cut up into strips and dipped in red cabbage juice. The strips can be used to test the pH of common household chemicals. Anne Helmenstine

It's easy, safe, and fun to make your own pH paper test strips. This is a project that kids can do and that can be done from home, though calibrated test strips would work in a lab, too.

Key Takeaways: Red Cabbage pH Indicator

  • The pigment that gives red or purple cabbage its deep color is a natural pH indicator.
  • You can crush the cells of the cabbage to release the pigment and use it to make pH test strips. The test strips are made of coffee filters or paper towels.
  • Cabbage juice changes to red in the presence of an acid (pH less than 7), is blue at neutral pH (pH around 7), and is purple in the presence of a base (pH greater than 7).

Difficulty: Easy

Time Required: 15 minutes plus drying time

What You Need

Basically, all you need is a red cabbage (or purple cabbage, if that's what they are called where you live), some form of porous paper, and a means of cutting and heating the vegetable.

  • Red Cabbage
  • Filter Paper or Coffee Filters
  • Blender - optional
  • Microwave - optional
  • Dropper or Toothpicks - optional

The reason you want to cut up the cabbage (ideally blend it) is to break open the cells and release the anthocyanins that are the color-changing pigment molecules. Heat isn't strictly necessary, but it does make it easier to break up the cabbage. For the pH paper, the easiest porous paper to find is a paper coffee filter. If you have filter paper, you likely already have access to pH paper. However, filter paper has a smaller pore size than a coffee filter and is a superior choice. In a pinch, you can use a paper towel to make the pH paper.

Here's How

  1. Cut a red cabbage (or purple) into pieces such that it will fit into a blender. Chop the cabbage, adding the minimum amount of water needed to blend it (because you want the juice as concentrated as possible). If you don't have a blender, then use a vegetable grater or chop your cabbage using a knife.
  2. Microwave the cabbage until it's at the boiling point. You'll see the liquid boil or else steam rising from the cabbage. If you don't have a microwave, soak the cabbage in a small volume of boiling water or else heat the cabbage using another method.
  3. Allow the cabbage to cool (about 10 minutes).
  4. Filter the liquid from the cabbage through a filter paper or coffee filter. It should be deeply colored.
  5. Soak a filter paper or coffee filter in this liquid. Allow it to dry. Cut the dry colored paper into test strips.
  6. Use a dropper or toothpick to apply a little liquid to a test strip. The color range for acids and bases will depend on the particular plant. If you like, you can construct a chart of pH and colors using liquids with a known pH so that you can then test unknowns. Examples of acids include hydrochloric acid (HCl), vinegar, and lemon juice. Examples of bases include sodium or potassium hydroxide (NaOH or KOH) and baking soda solution. You can use cabbage pH paper to tell if something is an acid, a base, or neutral, but you can't get highly specific pH readings like you would using a pH meter. If the liquid you are testing is very deeply colored, you can dilute it with water without altering its pH value.
  7. Another way to use your pH paper is as a color-change paper. You can draw on pH paper using a toothpick or cotton swab that has been dipped in an acid or base.
Red cabbage juice used as a pH indicator
Red cabbage juice turns red in lemon juice (acid), blue in alkali (baking soda), and is blue in tap water (neutral pH).  Ian_Redding / Getty Images

Tips

  1. If you don't want colored fingers, soak only half of the filter paper with the cabbage juice, leaving the other side uncolored. You'll get less usable paper, but you will have a place to grab it.
  2. Many plants produce pigments that can be used as pH indicators. Try this project with some of the other common home and garden indicators. Most red or purple flowers and vegetables are pH indicators. Examples include beets, red roses, and purple pansies.
  3. If you spill the cabbage juice and stain a surface, you can get the stain out using ordinary household bleach.

Sources

  • Howstuffworks. "Where does the color come from in purple cabbage?" science.howstuffworks.com/life/botany/question439.htm
  • Stanford University. "Red Cabbage Lab: Acids and Bases." web.stanford.edu/~ajspakow/downloads/outreach/ph-student-9-30-09.pdf