The pH scale measures the probability of a substance to release protons (or H ions+) or receive them. Many molecules, including dyes, change their structure by accepting protons from an acidic environment (one that readily releases them), or by donating them to a basic environment (predisposed to receive them). The pH test is a key part of many chemistry and biology experiments. It can be done by coating strips of paper with dyes that can take on a different color when in contact with an acid or a base.
Steps
Method 1 of 2: Create a Homemade Cabbage pH Test Strip
Step 1. Chop some red cabbage
You will need to cut about 1/4 of a basket of red cabbage and put it in the blender. You're going to get the chemicals out of the cabbage to coat the paper strips. These substances are known as anthocyanins and are found in plants such as cabbage, roses and berries. In a neutral environment (pH 7), they take on a purple color, but change color when exposed to acids (pH 7).
- You can do the same with berries, roses, and other plants that contain anthocyanin.
- This method does not work with kale, which does not contain anthocyanin.
Step 2. Put the cabbage in boiling water
You will need about half a liter of water, which you can boil on the stove or in the microwave. Pour it directly into the blender that contains the cabbage to get the chemicals you need out of the plant.
Step 3. Turn on the blender
For best results, you need to blend the water with the cabbage. Continue until the solution turns a dark purple color. The color change indicates that you have managed to extract the chemicals you need (anthocyanins) from the cabbage and dissolve them in hot water. Let the liquid in the blender cool for at least 10 minutes before continuing.
Step 4. Strain the mixture with a colander
You need to remove all the cabbage pieces from the colored solution. If you don't have a strainer, you can also use filter paper, which may take longer, however. Once the solution is filtered, you can throw the cabbage.
Step 5. Add isopropyl alcohol to the solution
Add about 50ml of alcohol to protect it from bacteria. Alcohol can alter the color of the solution; in this case, add vinegar until it turns dark purple again.
If necessary or if you prefer, you can substitute isopropyl alcohol with ethanol
Step 6. Pour the solution into a pot or bowl
You will need a container large enough to submerge the paper. Choose a stain resistant one, because anthocyanins are very dye. The best options are ceramic or glass bowls.
Step 7. Dip the paper into the solution
Make sure you get it to the bottom so that all corners and edges of the paper are wet. For this step, it's a good idea to wear gloves.
Step 8. Let the paper dry on a towel
Find an environment free of acidic or basic vapors. Wait until the paper is completely dry before continuing. If possible, let it sit overnight.
Step 9. Cut the paper into strips
This way you can test various samples. You can choose your preferred size for the strips, but usually a good reference is the length of your index finger. This way, you can dip the strip into a sample without getting your hands wet.
Step 10. Use the strips to test the pH of various solutions
You can try liquids in your home, such as orange juice, water, and milk. You can also mix multiple substances to test, such as water and baking soda. This way, you will have many samples on hand to measure.
Step 11. Store the strips in a cool, dry environment
You should close them in an airtight container until it's time to use them. This way, you will protect them from contamination by any acidic or basic gases. Also, you shouldn't leave them exposed to direct sunlight, which can discolour them.
Method 2 of 2: Create a Homemade Litmus Paper
Step 1. Get dry litmus powder
Litmus is a compound derived from lichens, fungi that form a symbiotic relationship with algae or with cyanobacteria capable of carrying out photosynthesis. You can buy it online or at local stores that sell chemicals.
If you are an experienced chemist, it is possible to make litmus powder yourself. However, the operation is quite complex and involves the addition of many substances, such as quicklime and potash to grind the lichens. Furthermore, fermentation takes a few weeks
Step 2. Dissolve the litmus in water
Make sure you mix the solution and heat the powder if it doesn't dissolve well. Litmus powder must dissolve completely. The resulting solution should have a purple-blue color.
Step 3. Dip white acid-free drawing paper into the litmus solution
Wet all edges and corners of the paper with the solution. This way, you will have the whole area of the paper available as a litmus test and you will get the most accurate results. There is no need to let the paper soak if you make sure you wet it completely.
Step 4. Let the paper dry
It should dry naturally in the air, but make sure you don't leave it exposed to acidic or basic vapors. These vapors could contaminate the strips and make measurements inaccurate. You should also make sure you store it in a dry, dark place to prevent contamination and discoloration.
Step 5. Use litmus paper to test the acidity of a solution
Blue litmus papers turn red on contact with an acid. Remember that this transformation does not indicate the potency of an acid or the basicity of a solution. If you don't notice any changes, the solution is neutral or basic, but not acidic.
You can make red litmus paper (which turns blue when exposed to a base) by adding acid to the solution before soaking the paper
Advice
- You can cut the paper into strips either before or after wetting it with the solution. Just avoid doing it when it's wet.
- You can use the universal indicator to compare the measurement of your strip with others made with the same solution. This way, you will get an idea of the actual pH value.
- Use only distilled or filtered water.
Warnings
- Store the strips in a cool, dry, dark, and airtight container.
- Handle the strips only when your hands are clean and dry.
- Handle acids very carefully and only under the supervision of a responsible person, such as the science teacher, during a school project. Wear appropriate protective clothing when handling hazardous substances.
