Acid-Base indicators provide a quick, visual reference. They show (indicate) by color whether a material is acidic (less than pH 7) or basic (greater than pH 7). An acid-base indicator does not tell you how acidic or basic a solution is, only which. This limits the ways a single indicator can be used, but they are quite helpful in certain situations. There are both chemical indicators and natural indicators, though in truth the natural indicators are simply plants which contain an indicating chemical. The color of the chemical changes because it can gain (acid) or lose (base) a hydrogen atom, and doing so alters the way in which the molecule absorbs light.
Litmus is one of the best known indicators. It is a natural indicator – a dye derived from lichens. Students often encounter it at a young age when they are first introduced to acids and bases. Gardeners are fond of it when checking soil pH. Most commonly, it is bonded to paper strips (litmus paper) and used in that form. Litmus, of course, is blue in bases, red in acids.
Red cabbage provides another natural indicator. In acid, the red color disappears, returning when placed in basic conditions. This indicator is popular in classroom activities, as the juice can be extracted by crushing the leaves for use, without any concern over toxicity, nor a need for an expensive chemical supplier.
Hydrangea, when in bloom, is a good, natural indicator of soil pH. It blooms pink in acidic soils, purple-blue in basic soils. While it thrives in either, having a hydrangea nearby can be helpful when caring for other, more pH-sensitive plants. For instance, if your hydrangea is blue when your nearby blueberry bush likes acidic soil, you would know to adjust your soil pH. (A little vinegar might do the trick.)
When it comes to chemical acid-base indicators, phenolphthalein is probably the best known, and the hardest to say. [The “phth” sound is an f-th which is best achieved by sliding directly from the “f” sound to the “th” in the same exhalation. To do this, start on your “f” and then slide your tongue forward between your teeth to shift it to a “th”.] Phenolphthalein behaves just like red cabbage juice, turning a lovely pink/magenta in bases, clear in acids. Unlike cabbage juice, you don’t want to consume phenolphthalein. (It works as a laxative in people.)
Phenol Red is another chemical indicator. It is slightly more dramatic, having color under both acidic (yellow) and basic (red) conditions.
Many other chemical indicators are available, on paper test strips, as crystals, and in solution. Most of them are not strictly acid-base indicators, however. Each has a specific pH range at which it transitions from one color to another. If the color change occurs around a pH of 5, as in the case of Bromocresol Green (yellow at lower pH, blue at higher pH, and green near pH 5), the indicator will show blue for both basic and weakly acidic conditions. Indicators such as these are not as useful for a quick determination of whether something is acidic or basic. They are excellent, however, for monitoring chemical reactions with pH changes, as in titrations. A whole library of indicators exists so that a chemist can select the indicator most appropriate to the reaction of interest.
Since these other indicators are beyond the scope of this article, here are a pair of good resources which list indicators, the pH range they are useful for, and the corresponding colors.