How to Make pH Paper

store-bought pH paper

pH paper is a pH indicator that measures the acidity and alkalinity of various substances by causing a chemical reaction that produced specific color changes to the chemical in the paper. pH is an acronym for "power of hydrogen" because it measures acids and alkaline (bases) using a mathematical logarithm of the concentration of hydrogen ions in substances. Acids have more hydrogen ions (H+), while bases have more hydroxyl or hydroxide ions (OH-), and neutral substances, such as water, have an equal number of both ions. pH is measured on a simplified scale of 0 to 14, with pure water being a neutral 7, substances below 7 being acids, and substances above a 7 being bases. This guide shows how to make a crude homemade pH paper with red cabbage solution and coffee filters. (See Resources below)

Things You'll Need

• Red cabbage leaves 1-quart cooking pot Water 1-quart bowl Strainer White coffee filters Scissors Glue Calibrated pH meter Store-bought pH (found in aquatic pet stores) or litmus paper Acidic and basic solutions to test

Instructions

1. Prepare a red cabbage indicator solution. Take a medium-sized head of red cabbage and cut it into 1-inch pieces. Place the cabbage into a cooking pot, cover with water, and heat it on a stove on medium heat for 30 minutes at a low boil. Turn the heat off and allow it to cool. Pour the liquid through a strainer and into a bowl. It should be a blue-violet color.

2. Make the pH paper. Take a few white coffee filters and soak them in the red cabbage solution for 30 minutes. Remove the filters, shake off the excess solution, lay them out in a single layer on a clean surface or paper towel and allow them to dry completely. The drying can be sped up with a hair dryer or by baking them on a cookie sheet in an oven at a low temperature (150-200 degrees) until dry. Cut the filters into thin strips about 3 inches long. To use, dip an edge of the paper into a substance, and the paper changes color.

3. Calibrate the pH paper. Get store-bought pH or litmus paper and an electronic pH meter, which all give accurate pH measurements. Use these devices as a base with which to compare to the red cabbage pH paper color changes. The easiest way to do this is to make a chart of common substances containing acids or bases from 0 (most acidic) to 14 (most basic). Label column 1 as "Known pH", column 2 as "Substance", column 3 as "pH or litmus paper", column 4 as "pH (pH Meter)", and column 5 as "pH (red cabbage paper)". Enter column 1 and 2 data using these as examples: 0 battery acid, 1 sulfuric acid, 2 vinegar, 3 orange juice, 4 tomato juice, 5 black coffee, 6 milk, 7 pure water, 8 eggs, 9 baking soda, 10 milk of magnesia, 11 ammonia, 12 soapy water, 13 bleach, 14 liquid drain cleaner. Gather some or all of these substances or others as referenced on many science websites. Test each substance with the store-bought pH or litmus paper, cut a small piece of each color-changed paper and glue it onto the correct row on the chart. Test the substances with a calibrated pH meter and write each numeral measurement in the proper column. Test the substances with the red cabbage paper, cut a small piece of each color-changed paper and glue it onto the correct row on the chart. Use the chart to determine the general pH of other substances using the red cabbage paper. (See Resources below)

Tips & Warnings

• The cards will keep for months in a closed container. The cabbage juice will keep for weeks if refrigerated. Expect the color changes to change over time, requiring recalibration.

• Use caution when handling strong acids and bases; avoid skin contact and inhaling fumes. Do not mix strong acids and bases because their interaction can be combustible or create dangerous fumes.

Resources

• Experiments with Acids and Bases
• pH indicator