The natural pH indicators are molecules responsible for the colorations observed in plants, fruits or flowers and some vegetables. They consist in their vast majority of a family of organic compounds called anthocyanins, which are identified in nature by their blue, red and purple colors.
Therefore, if we have a reddish fruit, it is likely that its color is due to a set of anthocyanins. These have a molecular structure that allows them to absorb photons, reflecting wavelengths that our eyes receive and the brain interprets as color. When there is a variation in pH, their structures change and, therefore, the color we perceive.
The color of hydrangeas varies depending on the acidity or alkalinity of the soils where your plants grow. Source: Pixnio.
A well-known example is that of hydrangeas. In acid soils, rich in aluminum and other metallic cations, they present bluish petals (upper image). As the acidity of the soil is neutralized or diminished, the hydrangeas show purple hues, finally having reddish or pink petals in basic soils.
Several of the colors that we witness in fruits or flowers are sensitive to changes in pH; however, the color shift (change) suffered by such natural colorants is not always favorable for their use as acid-base indicators in a laboratory.
Dye extraction
The benefit of working with natural indicators is that experiments can be done on home safety and with simple materials and ingredients. First, the colorant must be removed by using a solvent. This can be water or rubbing alcohol.
The purpose of said extraction is to have a colorful solution, to which the pH will be varied by adding everyday products.
There are numerous methods to obtain it. One of them is to leave the skins of the fruits, flowers, stems, etc., soaking in a container with water or alcohol overnight. The process will have a better performance if the material from which the colorant will be extracted is previously crushed; either in a mortar, crumbling it with your hands or blending it.
Another method, much faster, consists of resting the material in a cauldron with boiling water for half an hour. Finally, using a sieve or sieve (or filter paper if available) we proceed to remove the remaining solids and preserve the liquid or colored solution.
We will then have our indicator, which will be subject to citric acid, vinegar, sodium bicarbonate and detergents. In this way, its color change can be seen from acidic pH values (pH <7) to basic (pH> 7).
Natural pH indicators
Turmeric
Since the colors are not due to a single colorant, but to a mixture of ten or more of them, the natural indicators should be mentioned depending on which fruit or flowers they make up.
So, we start with turmeric powder, whose orange color is characteristic of this vegetable and is due to its curcuminoid content. An extraction of the turmeric will produce a yellowish solution. If vinegar is added to it, it will become colorless, the same as if citric acid (from lemon juice, passion fruit, etc.) were added.
Meanwhile, the turmeric solution will change from yellow to red if baking soda, detergent or soapy water is added to it. This experiment can be done in any kitchen or on a table near the pantry.
Beets
Beet juice (already sifted) is one of the best-known natural indicators in children's experiments. From pH 2 to 9, it shows reddish colors, becoming more and more purple as the alkalinity of the medium increases. However, in strongly alkaline media, it has a yellow color.
Therefore, beet juice would be a good indicator to detect sudden increases in alkalinity.
Purple cabbage
Red cabbage juice is even better known and used than beet juice. Its color variations are more obvious and useful, so presumably it consists of a more complex mixture of anthocyanins.
Freshly prepared at neutral pH, it exhibits its typical purple color. If acid is added, it will turn red. Meanwhile, if a base is added, it will turn blue (pH = 8), green (pH = 9-13), and finally, yellow (pH = 14).
Berries
Some berries, such as cherries, blueberries, blackberries and grapes (although the latter are also considered fruits), have anthocyanin content that make them natural acid-base indicators.
For example, grape juice behaves in a similar way to beet or cabbage juice; however, the colors obtained are different and less varied. At acid pH, grape juice turns yellow-orange; while in basic pH, it has olive green colors, which intensify as the pH approaches 14.
For cherries and blackberries, the color changes are from red (acid) to blue or purple (basic). In contrast, the anthocyanins in blueberries behave like those in purple cabbage; both indicators generate very similar colors, with the difference that the blueberry solutions do not turn blue at any time.
Flower petals
Flower petals can also be crushed to prepare aqueous or alcoholic solutions thereof. Roses, for example, produce colorless solutions at neutral pH. But in acidic media (pH <4), they turn pink, while in basic media (pH> 8), they turn to greenish-yellowish colorations.
On the other hand, the solutions prepared with hibiscus petals remain orange until pH 7, when they begin to turn dark purple at alkaline pH values.
The experiment can be repeated and analyzed with petals of geraniums, petunias, peonies, morning glories, among other flowers.
Onions
By peeling the skins of fruits or vegetables, solutions are prepared which are re-evaluated as natural indicators. With onions, for example, a solution of pinkish tones is obtained, accentuated in acidic pH, and that little by little turn yellowish green in basic pH.
The basic solutions of onions are not only characterized by being greenish in color, but they are also odorless. They no longer smell like onions.
Similarly, you can proceed with the skins of pears, plums, peaches, apples and turnips. However, your resulting indicators will not be compared to those of purple cabbage or blueberry.
References
- Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
- Dr. Erica. (2016, April 22). Natural pH indicator: cabbage vs. beets. Recovered from: rosieresearch.com
- Helmenstine, Anne Marie, Ph.D. (September 16, 2019). Home and Garden pH Indicators. Recovered from: thoughtco.com
- Joanna Klein. (March 02, 2016). A Quick, Colorful Change for a Morning Glory. The New York Times. Recovered from: nytimes.com
- Wikipedia. (2020). pH indicator. Recovered from: en.wikipedia.org