In theory, any substance that undergoes a reversible chemical change when pH changes can be used as an acid-base indicator. In practice, a sharp change in some easily detectable property of the substance is required. Usually, the property is color; but other properties such as odor can also change with pH.
Here's a partial list of house and garden materials that can be used as acid/base indicators. Notice that most of the materials are derived from plants. Almost any flower, fruit, or plant part that is red, blue, or purple contains
a class of chemical compounds called anthocyanins that change color with pH. The color of a flower or fruit depends on which anthocyanins are present, the pH of the pigment-bearing tissues, and the presence of other pigments, like yellow flavones. To learn more about why indicator molecules change color with pH, see Water to Wine.
Baking soda, NaHCO3,
fizzes when added to an acidic solution, but no change occurs in basic solution. The reaction is
HCO3-(aq) + H+(aq) = H2O() + CO2(g)
The reaction isn't easily reversible so baking soda is more of a "spot test" for acids than an indicator.
Beets change from red to purplish in very basic solution.
Blackberries, black currants, and black raspberries change from red in acids to dark blue or violet in basic solution.
Blue and red grapes contain several different pH-sensitive anthocyanins. For example, blue grapes are colored by a monoglucoside of malvinidin that changes from deep red in acidic solutions to violet in basic solution. Red wines naturally contain these same pigments.
Blueberries change from blue (around pH 2.8-3.2) to red in a strongly acidic solution.
Cherries and cherry juice is bright red in acidic solution but purple to blue in basic solution.
Curry powder and tumeric are spices that contain a bright yellow pigment called curcumin (which is not an anthocyanin). It turns from yellow at pH 7.4 to red at pH 8.6.
Delphinium petals contain an anthocyanin called delphinin, which changes from bluish red in acid to blue to violet in basic solution.
ExLax tablets contain phenolphthalein. The powdered tablets change from colorless under pH 8.3 to pink to a deep red above pH 9.
Geranium petals contain pelargonin, an anthocyanin which changes from orange-red in acid solution to bluish in basic solution.
Horsechestnut leaves can be ground with alcohol to extract esculin, a fluorescent dye.
Esculin changes from colorless at pH 1.5 to fluorescent blue at pH 2.
Shine a black (ultraviolet) light on the indicator to get the full effect.
Mood lipsticks undergo many interesting pH-related color changes. See a related question for more.
Morning glories contain an anthocyanin called "heavenly blue anthocyanin" which changes from purplish red at pH 6.6 to blue at pH 7.7. See the Canadian Society for Chemistry's Discover Canadian Chemistry site for more.
Onion is an olfactory indicator. The onion odor isn't detectable in strongly basic solutions. Red onion can act as a visual indicator at the same time. It changes from pale red in acid solution to green in basic solution.
Petunia petals contain petunin, an anthocyanin that changes from reddish purple in acid to violet in basic solution.
Poison primrose (Primula sinensis) has both orange and blue flowers. The orange flowers contain a mixture of pelargonins (the same type of pigment found in geraniums). The blue flowers contain malvin (similar to the pigment in blue grapes), which turns from red to purple as a solution changes from acidic to basic.
Poppy flower petals
Purple peonies contain peonin, which changes from reddish purple or magenta in acid solution to deep purple in basic solution.
Red cabbage contains a mixture of anthocyanins and other pigments that indicate a wide range of pH. The photograph at right shows how red cabbage juice changes from deep red at pH 1 to purple at pH 7 to to green at pH 12. At higher pH (13-14) it turns yellow.
Rose petals contain the oxonium salt of cyanin, and they turn blue in basic solution. (The potassium or calcium salt of the same pigment makes cornflowers blue!)
Thyme (extract in alcohol)
Vanilla extract, like onion, is an olfactory indicator. The vanilla odor isn't detectable in strongly basic solution because vanillin exists in ionic form at high pH.
Washing soda, like baking soda,
fizzes when added to an acidic solution. No change occurs in basic solution. The reaction is
CO32-(aq) + 2 H+(aq) = H2O() + CO2(g)
- P. W. Atkins, Molecules ,W. H. Freeman, Oxford, England, (1987).
- K. C. Li, A. C. Wagenknecht, "Anthocyanin Pigments Of Sour Cherries", Journal Of The American Chemical Society, 78, 979 (1956).
- F. J. Francis, J. B. Harborne, W. Barker, "Anthocyanins In The Lowbush Blueberry, Vaccinium Angusifolium",
Journal Of Food Science, 31, 583 (1966).
- T. Fuleki, "The Anthocyanins Of Strawberry, Rhubarb, Radish And Onion", Journal Of Food Science, 34, 365 (1969).
- A. Lukton, C. O. Chichester, G. Mackinney, "Characterization Of A Second Pigment In Strawberries", Nature, 176, 790 (1955).
Author: Fred Senese email@example.com