The purple color of red cabbage comes from a pigment called anthocyanin. Anthocyanins are pigments that are responsible for pink, red, magenta and violet colors in plants. The color of anthocyanin is affected by the pH. Anthocyanin turns red in acids, purple in neutral solutions, and yellow-green in strongly basic (alkaline) solutions. Weak acids or bases will cause anthocyanin to turn an intermediate color: pink for weak acids or blue-green for weak bases. An extract from red cabbage can be used as a pH indicator to tell the approximate pH of solutions.
How to Make Red Cabbage Juice pH Indicator
1. Chop up a red cabbage into small pieces and place it in a pot.
2. Add enough water to cover the cabbage and bring to a boil.
3. Turn off the heat and allow it to stand until cool.
4. Filter through a coffee filter and collect the juice. It will be purple in color, because water is neutral, approximately pH 7.
How to Use Cabbage Juice as a pH Indicator for Bases and Acids
This experiment can be done by students in groups, or as a demonstration by the teacher. If the students are doing this themselves, make sure they are wearing goggles and gloves because acids and bases can be harmful to skin and eyes. If doing it as a demonstration, it works well to place the cabbage juice indicator in clear plastic petri dishes sitting on an overhead projector. The different colors are readily visible on the screen. For the demonstration, set up several test tubes or petri dishes containing cabbage juice.
Have students observe and record in their lab notebooks or on a worksheet the color of the cabbage juice; a purple color indicates that the pH is close to 7, neutral.
Add a small amount of lemon juice (pH 2) to one dish and have students note and record the color. It should change from purple to red.
Add some lye solution to a different dish, and have students observe and record the color. Lye is a very strong base, with a pH of around 13, and the color should be green or yellowish green.
These three solutions will serve as reference points. You can then try adding various other solutions to petri dishes containing the cabbage juice indicator, and have students observe the color change and estimate the pH by comparing it to the reference colors. Some possible things to try include: cola, vinegar, baking soda, cream of tartar, ammonia, and laundry detergent. Students can predict what they think the pH of each substance will be before adding it to the cabbage juice.
Keep the reference dishes on the overhead projector so students can see them and make color comparisons more easily.