A pH indicator measures whether a substance (usually a liquid) is acidic or basic, and some indicators can also measure how acidic or basic it is. When the indicator comes in contact with a substance, the indicator's color corresponds to that substance's approximate pH, which is a scale of zero to fourteen (with zero being the most acidic, seven being neutral, and fourteen being the most basic) that indicates the acidity or basicity of a substance.
These instructions explain the simple process of how to make an indicator solution from the leaves of a red cabbage (also called purple cabbage) and how to make indicator paper by soaking a coffee filter in the indicator solution. [Note: A solution in this sense simply means a substance dissolved in water. In this case, the chemicals with indicator properties in the cabbage leaves are dissolved in boiling water.] The indicator paper works in the same way as the indicator solution itself: the paper changes color when it comes in contact with an acidic or basic substance. Some optional steps at the end of the instructions (steps 9-14) explain how to use some household items to test the indicator solution and paper to see how they change color.
About these instructions:
These instructions can be divided into three parts:
1) Making a pH indicator by boiling cabbage leaves in water (see steps 1-5).
2) Making indicator paper by soaking a coffee filter in the indicator solution and letting it dry, then cutting it into strips (see steps 6-8).
3) This part is optional and is not necessary for actually making the indicator solution and paper. Both the paper and the indicator solution itself can be tested with common household items (see steps 9-14). Vinegar (which is mildly acidic) and baking soda (which is mildly basic when dissolved in water) are used as examples in these instructions.
Approximately 10-15 minutes to make the indicator (plus an additional 25-30 to allow it to cool), then about 15-30 minutes to make the indicator paper (the time needed depends on how the paper is dried). Finally, allow 15-20 minutes for testing the indicator solution and indicator paper with vinegar and baking soda (if desired). Total time (including testing): approximately 60-90 minutes.
The total cost of a head of red cabbage and a pack of coffee filters is approximately $5-7.
Makes approximately two cups of indicator solution. The number of indicator paper strips varies depending on the size they are cut.
1. Two leaves of red (purple) cabbage
A head of red cabbage can be purchased at most grocery stores.
2. One or more coffee filters
A package of these can also be purchased at most grocery stores.
3. Medium-sized pan
This should be big enough so that the cabbage leaves will lie on the bottom (although they do not have to lie completely flat).
4. Measuring cup
For measuring out two cups of water.
5. Kitchen tongs
For safe removal of the cabbage leaves from the boiling water.
For cutting the coffee filter into strips after it has been soaked and dried.
7. One or more glasses (or beakers if available)
For testing the indicator with vinegar and/or baking soda.
Tap water works fine, but distilled water may also be used.
9. Paper towels or paper plates
These are useful for the used cabbage leaves, for laying out the coffee filter after it has been soaked, and for preventing the liquid indicator from making a mess on countertops/tables.
The following supplies are needed if you wish to test the indicator solution and indicator paper:
1. White vinegar
This can be purchased at most grocery stores. This will be used to test the pH indicator and indicator paper.
2. Baking soda
Another common grocery store item. This will also be used to test the indicator.
For making a baking soda solution and for stirring during the testing of the indicator solution.
Use care when boiling water and when removing the cabbage leaves from the water. Make sure to let the indicator solution cool before using it. The indicator solution and the vinegar and baking soda are all non-toxic and can be poured down the sink after use. If your hands or eyes come in contact with any of these, simply rinse them thoroughly with water.
The indicator solution can stain your clothes, so it is a good idea to wear an apron (or lab coat if available) when handling the indicator. If the indicator does get on your clothes, try using a stain remover and then washing them in cold water.
Pour two cups of tap or distilled water into the pan.
The water should cover the bottom of the pan and be approximately an inch deep. The depth can be tested in the next step after the cabbage leaves have been added. If the water is too shallow, try using a smaller pan. If it is too deep, use a larger pan.
The leaves will probably tear when you remove them; that is fine. When placed in the water, the leaves do not have to be completely submerged, but put as much of each leaf underwater as possible. If the bottom of the pan is significantly smaller than the size of the leaves when they are laid horizontally, a larger pan should probably be used.
Set your stove to high heat. Once the water starts bubbling, the heat can be lowered slightly (start your 5-minute timer at this time). It usually takes approximately 5 minutes for the water to reach a boil. During and after boiling, you may notice a slight bad smell; this is to be expected.
Use caution with the hot pan and make sure the water is relatively cooled before moving to the next step (it can still be warm, but it should not be hot). The water in the pan should now appear dark purple or dark blue, depending on the pH of the water (tap water tends to be slightly basic, so if you used tap water, your solution may be bluish). It may not be the same color as in the photos shown here; that is fine. The leaves should also appear less purple, with a more bluish green color. If the water color has not changed, try boiling it a bit longer.
The cabbage leaves can be thrown away. The color of the water should be more evident now that the leaves have been removed. This is the finished indicator solution. Make sure that the solution is no longer hot before moving to the next step.
Place the coffee filter in the leftover indicator solution in the pan and press it down until it is completely submerged, then let it soak for about 5 minutes.
When the indicator solution soaks into the filter, indicator paper is made.
The filter can be hung with a clothespin if a suitable place to hang it is available (this is the fastest method) or it can be laid flat to dry (see photo), although this method may take additional time to dry.The paper will probably look slightly blue, but that is fine.
The strips of indicator paper can be any size, but a practical size might be approximately 0.5 inches (1.3 centimeters) by 2 inches (5.1 centimeters). The indicator strips are now ready for use.
Both the indicator solution and the indicator paper should turn pink or red when put in contact with an acidic substance, and blue or greenish blue when in contact with a basic substance. The following steps show how to test the indicator solution and paper with vinegar and baking soda to observe these colors.
Pour about 1/4 cup of the indicator solution from the pot into the measuring cup.
This will be used for testing the indicator solution with vinegar and baking soda.
Any amount is fine, but about 1/4 cup of vinegar works well. This will serve as an acidic solution for testing the indicator solution and indicator paper.
This will serve as a basic solution to test the indicator solution and paper.
The strip dipped in the vinegar solution should turn pink, indicating that the vinegar is acidic, and the strip dipped in the baking soda solution should turn a more definite shade of blue, indicating that the baking soda is basic (see photo above). The colors of the strips do not have to match the photo exactly, but they should be similar.
The amount of indicator added does not have to be a certain amount, it just needs to be enough to observe a color change (in other words, as soon as you see pink, you can stop adding indicator). The vinegar in the glass should turn pink or pinkish red (depending on how much indicator was added), which indicates that it is mildly acidic. The color does not have to exactly match the photos above, but it should be similar.
The solution should turn blue, indicating that the baking soda is mildly basic. [Note: The indicator is probably already blue if you used tap water (since tap water is usually slightly basic), so you may not notice much of a color change here.] Again, you do not have to add a specific amount of indicator, just stop as soon as you see blue. As before, the color does not have to match the photo exactly.
Clean-up and storage:
You can discard both the vinegar and baking soda test solutions by pouring them down the sink. The indicator paper strips can be stored in a plastic bag (a Ziploc bag for example) or in a box. The indicator solution should be stored in an airtight and watertight container to prevent spills (a Tupperware container with a lid usually works well for this, or any sort of bottle with a screw cap can be used).
More uses for the indicator solution and paper:
Vinegar and baking soda are just two of many substances whose approximate pH can be measured with the red cabbage indicator. The indicator solution works well mainly for clear liquids, such as the vinegar and baking soda solutions in these instructions. However, the indicator paper can be used for a variety of other substances, whether they are colorless or not. Some household items that are mildly acidic or basic and can be tested with the indicator paper include:
- Orange juice -- acidic
- Apple (cut a slice or take a bite, then press the indicator paper against the wet part inside) -- acidic
- All-purpose cleaner, such as Windex or 409 -- basic
- Bar soap (shave some off the bar, put the shavings in a watertight container, and shake vigorously) -- basic
- Soda pop -- acidic
The indicator solution and paper can also be used to test strongly acidic substances (the indicator solution or paper should turn red) and strongly basic solutions (the indicator solution or paper should turn greenish blue or greenish yellow). However, examples of these are not given in these instructions because of the additional safety precautions that must be taken (strong acids and bases can cause chemical burns).
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