Growing Chemistry: Plant Compounds for Studying pH

Each Tuesday we generally celebrate plants with “Seed of the Week” posts. Today we’ll look at chemicals found in plants for Chemistry Week.

Plants contain an encyclopedia of different chemicals, which they use for growth, communication, and defense. For this lesson we are going to look at three different classes of plant chemicals that can be used to study the pH of acids and bases.

A Quick Introduction to Acids and Bases

People have known for centuries that acids:

• Taste sour (like lemons)
• Dissolve or corrode metals
• Turn blue litmus paper red

On the other hand, bases:

• Taste bitter (like caffeine in coffee)
• Feel slippery
• Turn red litmus paper blue

Note:  Scientists know:  never taste, drink or eat anything from a science experiment!

Chemists have created a scale to measure how acidic or basic a substance is, called the pH scale. Although no one knows for sure how the name pH came to be, it is acceptable to think of pH as the “power” of hydronium ions, or “how many hydronium ions are present.” (Hydrodium ions are technically H₃O⁺ ions, but are often written as H ⁺.) It is a logarithmic scale, which means that lower numbers, like 1 or 2, indicate a larger number of hydronium ions, and that a substance is more acidic.

Gather at least some of the following possible acids or bases to test:

• Lemon and/or orange juice
• Apple juice
• Soda (pop)
• Vinegar
• Baking soda (mix with water to moisten)
• Dish and/or laundry determent
• Shampoo
• Milk of Magnesia
• Coffee
• Salt
• Cream of Tartar (in water to moisten)

Add any other household materials you might be curious about, but only under adult supervision.

Note: Bleach doesn’t work well because it interferes with the dye molecules. Also, adding vinegar to baking soda is fine, but do not mix strong acids and bases.

Gather small clear containers such a plastic cups or clean baby food jars to carry out the experiments in. Label each container when you add a substance to be tested so you remember which is which.

Plant-Based pH Indicators

Certain molecules and substances change color when exposed to specific acidic or basic conditions, and thus can be used as pH indicators. Examples of  pH indicators from plants include: 

1. Litmus is a complex of dyes extracted from certain lichens. 

Litmus paper is a classic for exploring acids and bases. All you need to do is dip the litmus paper strip into the liquid to be tested. If the blue litmus paper turns red, it is an acid. If the red (pink) litmus paper turns blue, it is a base. If neither changes, the substance you are testing is neutral.

Litmus paper is available online or in science supply stores. Pool supply and aquarium supply stores may also carry it.

2. Anthocyanins- derived from red or purple fruits and berries, such as:  red cabbage!

I know, I know, red cabbage indicator is all over the Internet and has been probably over-exposed, but that is because it is easy and works for both acids and bases. If you haven’t done this, it really is fun. Even if you have done it before, pull it out again and try something new with it. Using red cabbage as a pH indicator works well for an activity with mixed-age groups because each age may achieve a different level of understanding. If you are really tired of red cabbage, try the juicing beets (the root part), or berries instead.

All you need is red cabbage from the grocery store and a blender.

Some recipes call for boiling the cabbage but that is smelly and unnecessary. Simply grind up the fresh red cabbage leaves in small batches with just enough water to allow the blender to work properly. Pour the batches together in a pitcher (which can be placed in the refrigerator for use later in the day if necessary.) You can also strain the slurry through a strainer to remove the solids.

Pour about 1/3 cup of the red cabbage juice into testing containers such as clear glasses or plastic cups. Mix in about a Tablespoon of one of the testing compounds into one of the containers and label it. Does the color of the liquid change? Have fun admiring the wild colors you can make.

Try another material in the next container.

Questions to try:

• Does the color change more if you add more test material?
• Can you figure out which colors indicate acids and which indicate bases?
• Does the temperature of the liquid matter? Hot or cold?
• How does the color change when you add baking soda to vinegar with the cabbage juice already in it? Can you add enough vinegar to turn the color back? How much does it take?
• What happens when you dilute the test mixture with water?

What is happening? The pigment molecules in the red cabbage juice change shape, and thus color, when in the presence of acids or bases. Lemon juice, vinegar and soda are acids; detergents and soaps are bases. In the above photograph, from left to right are:  red cabbage juice in water, dish detergent, baking soda in water and lemon juice.

Edit:  Sarah of Share It! Science blog just stopped by with a link to a post about using the anthocyanins in poinsettia bracts (the red ones) as a pH indicator.

3. The third pH indicator is curcumin, which is found in turmeric.

Gather:

• Paper towel
• Isopropyl (rubbing) alcohol
• Paper plate or waxed paper
• Turmeric (sold in the spice aisle of most groceries)

Watch out, turmeric will stain like crazy! Wear old clothes and do this outdoors, if possible.

Cut paper toweling into strips. Mix a few teaspoons turmeric with isopropyl or rubbing alcohol in a small container. Dip the thin strips of paper towel into the solution. Pull out and allow the paper towel strips to dry on a disposable or stain-resistant surface, such as a paper plate or sheet of wax paper. The alcohol should evaporate fairly quickly.

One the strip are dry, dip them into test substances placed in small containers, as discussed above.

 Note: Turmeric is bright golden yellow below pH 7.4, orange above pH 7.4 and bright red above pH 8.6.

Extra turmeric strips can be stored in a plastic bag for later use (up to months).

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Check our Chemistry Week Table of Contents for more children’s chemistry activities.