Red cabbage juice is a good indicator of pH because it contains flavin, a pigment molecule that changes colors according to the concentration of hydrogen ions in a solution. The juice is prepared by soaking chopped red cabbage in boiling water for at least 10 minutes and then filtering the solution to eliminate any plant material.
The prepared red cabbage juice is red, purple or blue, depending on the pH of the water used in its preparation. The addition of lemon juice, an acid, lowers the pH and causes the juice to become red. The addition of ammonia, a base, raises the pH and causes the juice to turn blue. The addition of stronger bases raises the pH higher and the solution becomes green or yellow, depending on the strength of the base.
Scientists use pH to measure the acidity of a material. A low pH indicates a high hydrogen ion concentration and a high pH indicates a low concentration. At a pH of 7, the concentration of hydrogen and hydroxyl ions are equal, resulting in a neutral solution. Students can soak filter paper in the juice solution, allow it to dry, and then cut it into strips to create a portable pH indicator that allows them to test pH outside the lab.