Natural pH indicators include red cabbage juice, turmeric, beets, horse chestnut leaves, onions and various berries and flowers. These indicators show the presence of acids or bases by changing color with pH differences.
Red cabbage contains red pigment, known as anthocyanin, that reacts very visibly to changes in pH. Acids turn anthocyanin red, a neutral pH turns it purple and a basic substance turns it green. It is possible to approximate a pH value by observing the precise color of a substance's reaction with red cabbage juice. Similar color changes occur in red onions.
Grapes, cherries and blackberries all appear reddish in the presence of acids, but change to shades of blue and violet upon exposure to a base. Beets react in a similar way, becoming more purple in basic solutions. This pattern repeats in the petals of many flowers, such as delphiniums, geraniums, morning glories and roses. Blueberries remain blue at a pH of 3 or higher, but turn red in the presence of strong acids.
Curcumin, the yellow pigment of turmeric, becomes red with bases stronger than an 8.6 pH, such as baking soda or ammonia. Soaking horse chestnut leaves in alcohol yields the pigment esculin, which turns blue at a pH of 2 or higher and fluoresces under a black light.