Turning your holiday poinsettia into poinsettia pH indicator is easy and educational. Use the homemade pH indicator to test whether common household chemicals are acids, bases, or neutral. Here are simple instructions for making pH paper test strips and a look at the chemistry of how it works.
You only need a poinsettia plant and water to make pH indicator, but it’s easiest to test chemicals if you make pH paper test strips. Use any porous paper, such as a coffee filter or paper towel.
- Coffee filter
- Heat source
- Home chemicals for pH tests
Poinsettia “flowers” are actually specialized leaves called bracts. Choose a plant that has deep red bracts. Pale flowers will work, but the color won’t be as visible. Some holiday plants are dyed or coated with glitter. It’s probably best to avoid these for the best results.
Make Poinsettia pH Indicator
- Chop the red “flowers” by hand or in a blender. The plant stores the pigment in vacuoles inside the cells, so you need to break open as many cells as possible to extract the color.
- Add just enough water to barely cover the plant matter.
- Heat the mixture over a stove or in a microwave until the color bleeds into the water. The colored water is the poinsettia pH indicator.
For pH indicator, pass the mash through a coffee filter or paper towel to filter out the solids. The colored liquid stays good in the refrigerator for several days or indefinitely in the freezer.
To make pH paper test strips, soak a coffee filter or paper towel in the liquid. Let the paper dry before cutting it into strips. As long as they are kept dry, the test strips remain good indefinitely.
Test the pH of Common Chemicals
Usually, you’ll get red/pink color under acidic conditions, purple color when the pH is neutral, and blue/green/yellow as the pH becomes increasingly alkaline. The acidic color is almost always pink or red, but the alkaline color could be more blue or more greenish-yellow. If you like, you can make a color chart by testing chemicals with known pH values.
Test a chemical by dipping a toothpick or cotton swab into it and applying it to a test strip. Don’t dip the test strip into the container of the chemical. Dissolve solids in a small amount of water in order to test them.
Good home chemicals to try include:
- Lemon juice (acid)
- Vinegar (acid)
- Your water (?)
- Baking soda (base)
- Laundry detergent (base)
pH indicators work with water-based liquids. What do you think will happen if you try to test the pH of vegetable oil?
Make Color Change Paper
You can use poinsettia pH indicator to make color-change paper. Soak an entire sheet of paper in the liquid and let it dry. Paint on the paper with a paintbrush, toothpick, or swab dipped in common household chemicals. The colors you get depend on the products you use.
How Poinsettia pH Indicator Works
Most colored flowers make good pH indicators because the colors are due to water-soluble pigments called anthocyanins. Plants use anthocyanins for photosynthesis, to attract pollinators, and to protect against temperature changes. The functional groups on the molecules (indicated by the letter R) tend to be -H, -OH, and -OCH3 groups, which easily deprotonate at higher pH values. Changing the pH also leads to ring-opening reactions, which change the color of the molecule.
When used as pH indicators, the color range depends on the presence of other molecules. But, usually anthocyanins are pink or red under acidic conditions (pH < 7), purple in neutral solution (pH = 7), blue to green under alkaline conditions (pH > 7), and change to yellowish or colorless under very alkaline conditions as the pigment molecule is reduced.
If you don’t have access to a poinsettia, there are many possible substitutes. Red cabbage juice, purple grape juice, or red wine are readily available alternatives that yield vibrant colors.
Poinsettias actually aren’t toxic, but you shouldn’t taste the pH indicator or allow pets to drink it. According to Poison Control, poinsettias can irritate skin and cause an upset stomach if eaten. Similarly, common home chemicals should be handled with care, such as laundry detergent and vinegar. Avoid skin contact with home chemicals and wash your hands after your exploration.
- Fossen, T.; et al. (1998). “Colour and Stability of Pure Anthocyanins Influenced by pH including the Alkaline Region.” Food Chemistry. 63(4): 435-440.
- Lapidot, T.; et al. (1999). “pH Dependent Forms of Red Wine Anthocyanins as Antioxidants.” J. Agric. Food Chem. 47: 67-70.
- Seltzer, Erica D.; Spinner, MaryAnne. “Poinsettia Facts“. The Poinsettia Pages. University of Illinois Extension.