On one of many blustery snow days a few weeks ago, Upper School Science Teacher Dr. Meg Yoshioka-Tarver took to the cozy warmth of her kitchen to chop and boil red cabbage for an upcoming science lab. Once cooked, the cabbage was strained and cooled, yielding a purple liquid to be used as a pH indicator solution.
What makes cabbage an indicator? Red cabbage contains anthocyanin, a pigment found in nature that changes color when in contact with acids and bases. The pH affects the structure of the anthocyanin molecule, causing it to absorb and reflect different wavelengths. In neutral substances, like water, anthocyanin turns blue. When an acid, like lemon juice, is added to the water, the anthocyanin molecules change shape and turn pink. When a base is added, the molecules change shape again and turn green or yellow.
Have you ever heard of the term “litmus test?” Litmus, derived from lichens, is one of the most well-known natural pH indicators. Its use has a long history, with records dating back to the 14th century. Some medieval painters used natural dyes, often treated with substances like vinegar and limewater, to create varying watercolor hues.
Sir Robert Boyle formally documented the use of natural dyes as pH indicators in 1664 in his work Experimental History of Colours. By using these indicators, including beet juice and turmeric, he established an early understanding of acids and bases. Red cabbage is popular because it exhibits a broader range of color changes across the pH spectrum.
For Dr. Meg’s AP Chemistry auxiliary class, it was the perfect experiment to create a homemade pH indicator and test the acidity of different household ingredients.
Using beakers, pipettes, and pH paper, students added water to 14 test tubes, then proceeded to meticulously add acid and base solutions of various levels to achieve a sequence of 14 different pH numbers. With a few drops of cabbage indicator solution on their quest to create a perfect rainbow, students could visually test how acidic or basic their solution was and how to tweak the color to nail a visual representation of the pH scale. The room ebbed and flowed with chatter between focused pipetting until they had created their range of colors. It was a balancing act between precision and patience.
Once their rainbow of colors was set, students measured the pH of household products such as vinegar, dish soap, lemon juice, baking soda in water, antacid in water, and bleach.
Check out Dr. Meg’s demo below!
What an engaging way to bridge ancient practices with modern scientific inquiry!