Growing up, I attended a really rural public school with less than 300 students K-12. While I had lots of one-on-one attention from the teachers, like most places, money for any type of activity was limited. So my teachers improvised. The first time I remember REALLY using the scientific method was in the 6th grade for one of these improvised experiments. Our science teacher, Mrs. Berry, asked all of us to bring in a pair of plain white socks or a plain white t-shirt, which we forgot about until she brought them back to us dyed purple!
As we all looked at her in confusion, she lined up different liquids on the counter. These liquids were commonly found in homes (lemon juice, vinegar, laundry soap, etc), and she asked which we thought were acids and which we thought were bases. In the previous week, we learned the pH scale and she explained how acids have more H+ ions (protons) and bases have more OH– (hydroxyl). She also mentioned that we could guess if something was acidic by a sour taste, and basic by a bitter taste. Since some liquids are poisonous, that’s not a good way to test things! Because of that, she turned our clothes into a fun tie-dye pH indicator! She had all of us guess which solutions were acids and bases, and then started our experiment.
We watched in amazement as she squirted lemon juice on the shirt in front of her, and a pink mark appeared on the fabric. This, she said, indicated an acid, about pH 2-3, thus it had lots of H+ ions. Then she added some strong laundry soap solution to a different part of the shirt, and it turned green-yellow. This soap was basic, around pH 10-12, with lots of OH–.
Then we were free to test out other solutions we had available: toothpaste solution, baby soap, different diluted cleaners, a dissolved antacid, and solutions of different foods we commonly ate. We had to hypothesize which solutions were acids and bases, how strongly acidic or basic they were, and what color this would turn the pH indicator. The fun lab day ended with all of us taking home fun dyed shirts and socks, along with a much better understanding of the scientific property of pH and the chemical nature of things both natural and man-made.
It wasn’t until much later, when I was about to graduate from high school, that I learned how Mrs. Berry was able to make such a great experiment for us with a limited budget. It turns out what we used as a pH indicator was red cabbage juice, made from red cabbage boiled in water! This, in addition to the other great lessons she taught us, left a strong impression on me. In the eight years since then, I’ve been using this science hack (and others) for outreach projects. Back in November, the Science ACEs did an outreach booth at the Houston Mini Maker Faire where we included a “magic painting with pH activity.”
How does the red cabbage juice act as a pH indicator?
The purple color of red cabbage comes from flavin, which is a member of a special group of pigments called anthocyanins. These special pigments change color when they gain or lose hydrogen atoms to solution. In acidic solutions, the anthocyanin can gain a proton, while in basic solutions it donates (loses) a proton to the OH– ions. These changes to the pigment alter its shape just a little bit, but it is enough to change the wavelength of light that reflects off of it, changing the color we see!
- Food preparation or lab gloves (unless you want purple hands!)
- A large knife
- One head of purple cabbage, the darker purple the better
- A large stock pot
- Storage container for the cabbage juice
- Different solutions for testing pH
- Test tubes or other similar clear container for experiments
- Thick coffee filters or NEUTRAL craft paper (optional)
Preparation of Red Cabbage Juice (needs to be done by an adult):
- Place 4 cups of water into the stock pot and set it to boil. While you wait, go to step2.
- Wearing gloves, chop the entire head of lettuce into 1-inch pieces.
- Once the water is boiling, add all of the lettuce to the pot, mixing thoroughly.
- Turn off the heat, and let the lettuce steep in the water until it is cool, usually about 2 hours. Periodically go in and stir the juice, squishing the lettuce against the sides. Within a few minutes, the water should turn purple, but by the end of 2 hours, it should be a very deep purple.
- Strain the juice through a strainer and store the juice in your refrigerator up to 2 weeks until use.
Experimental design: (can also make pH filter paper strips, but they must be dry before use)
- Give each student or group of students several test tubes or clear cups with a small amount of red cabbage juice. In separate containers give them several household solutions to test.
- Have them arrange the household solutions in order from what they think will be the most acidic to the most basic.
- Add the household solutions drop-wise to a container of red cabbage juice until a color change happens. Pink means acidic and green means basic.
- Were they right?
- What would happen if they added a basic solution to an acidic solution and tested it? This is only good for very rough titration for the acidity or alkalinity of a solution (measuring how acidic or basic the solution is), but they will get the idea; make sure they don’t mix things that could cause toxic chemical reactions, such as some household cleaners. Never mix bleach and ammonia, for example.
- If lemon juice was used, what can they infer about how acids would taste? Can they come up with some other things they think would be acidic? (Don’t drink the lab experiment!)
To make pH strips, the thick “hipster” coffee filters work well. You can also buy filter paper like that used in breweries or a thick craft paper so long as the word “neutral” is somewhere in the paper description. Otherwise your paper will be a different color to start as the paper will be acidic or basic itself, and it may not give results as accurately. The dried filter paper can keep for years so long as it is kept sealed and dry once it is dried initially. Paper will draw up liquid well if it is very porous and not treated with anything. Dry it on cookie sheets or waxed paper for 24-48h, until completely dry. One quarter sheet of paper was enough to cut into strips to do a single lab class. Just use it as you would normal pH paper. You can also dye fabric in a similar fashion as the paper, but note that the dye is not permanent through multiple washings!
Final cost for thick coffee filters and a head of purple lettuce? Less than $10, yielding about 2-3 cups of red cabbage juice, enough to do a couple of small classes (20 or so students using small 1 mL volumes per test) worth of liquid experiments or make 150 quarter sheets of paper (yielding up to 150 class experiments).
Biotechie is a third-year graduate student studying metabolism and cell biology. She is also the social media manager for Twitter @ScienceACEs and facebook.com/ScienceACEs. Her career goals include academic research as well as science education and advocacy. When she is not in the lab, she can be found reading, exploring the city, or baking awesome snacks for her fellow Science ACEs. Follow Biotechie on twitter @biotech_babe.