What are some natural acid/base indicators?

A visual acid-base indicator is just a weak acid with differently colored acid and conjugate base forms. Flower and leaf pigments often fit this description. For example, take rose petals and crush them with alcohol; you have an acid/base indicator solution. Stew some red cabbage and pour off the juice; you have an acid/base indicator solution.

Many indicators can be extracted from plants; others (like phenolphthalein*) and methyl orange are synthetic. Here are a few 'natural' acid/base indicators. For a longer list of house and garden materials that can be used to make acid-base indicators, see a related question.

  • Alizarin is an orange dye present in the root of the madder plant; it was used to dye wool in ancient Egypt, Persia, and India. In an 0.5% alcohol solution, alizarin is yellow at pH 5.5 and red at pH 6.8. Several synthetic modifications of alizarin are also used as acid/base indicators.
  • Cochineal is an acid-base indicator made from the bodies of dried female cochineal insects, found in Mexico and Central America. You'll have to grind about 70,000 insects to make one pound of dry indicator. The powder is about 10% carminic acid, which is yellow in acidic solution, and deep violet in alkaline solution. Cochineal solutions are not used much as acid/base indicators these days.
  • Curcumin, or tumeric yellow, is a natural dye found in curry powder. It turns from yellow at pH 7.4 to red at pH 8.6.
  • Esculin is a fluorescent dye that can be extracted from the leaves and bark of the horse chestnut tree. You'll need to shine a black (ultraviolet) light on the indicator to get the full effect. Esculin changes from colorless at pH 1.5 to fluorescent blue at pH 2.
  • Anthocyanin is probably the most readily available acid/base indicator; it is the plant pigment that makes red cabbage purple, cornflowers blue, and poppies red. It changes color from red in acid solution to purplish to green in mildly alkaline solution to yellow in very alkaline solution. The color changes for red cabbage juice are shown to the right.
  • Litmus is a blue dye extracted from various species of lichens. Although these lichens grow in many parts of the world, almost all litmus is extracted and packaged in Holland. Litmus is red at pH 4.5 and blue around pH 8.3. While most litmus is used to make litmus papers, some is used as a coloring for beverages.
  • Logwood is a dye obtained from the heartwood of a tree that grows in Central America and the West Indies. The extract contains hematoxylin and hematein, which turn bright red in alkaline solution.

Author: Fred Senese senese@antoine.frostburg.edu

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