Make Litmus Solution

Here is a familiar science lesson. I include it here because it correlates nicely with a previous lesson about using alkali to process corn. An alkali is a caustic or corrosive substance such as lye or powdered lime that is added to soil. As a caustic substance, it can help break down the coatings on corn. As lime, it helps lower or neutralize the acid in some soils. Thus, two important uses of alkalies are to break down difficult material, or to react with acids.

A gentle alkali is baking soda. A dangerous alkali is lye. The difference between the two is their position on the pH scale. The pH scale rates the acidity or alkalinity of a material. High pH can burn your skin and mucous membranes. Low pH can be used in baking. But how do you know the pH of a substance? Litmus paper, purchased at lab supply stores or some garden centers, will give you a reading. However, you can make your own litmus solution. This lesson shows you how.

Commercial litmus paper is soaked in a solution of water and lichen species. When you dip the paper into a liquid to be tested, the paper changes color. Match the color with the key that comes with the litmus paper and you’ll discover the pH value, ranging from 0 to 14. Low numbers are for acids, high numbers are alkali, 7 is neutral. The more extreme the number, the higher the acidity or alkalinity. Different litmus papers have different color values, but the numbers are standard.

Here is a simple pH scale:

Make Your Own Litmus Solution or Paper

Chop or tear leaves of purple cabbage. Put into a large pot and cover with water. (Note: the water source could make a difference. Tap water is generally neutral, which is what you want. Well water may start off being alkaline or acidic, which will affect the results of your homemade pH test.) Bring the water to a boil; then allow it to cool. Strain out the cabbage leaves and discard. Keep the water, which will be blue. This is your litmus solution.

Distribute the solution among several clear glasses. Keep one for comparison. This will be your “neutral”, unchanged solution and it represents #7 on the pH scale. Add a small amount of a substance you wish to test, into each of the other cups and stir. The color will change due to the acidity or alkalinity of each substance. Pink to deep red are acid; green to yellow are alkaline, also known as “basic.” Arrange the cups in order of color on each side of the neutral cup and you have your own litmus color scale. Be sure to label each cup.

From left: vinegar, orange juice, neutral, hardwood ash, calcium hydroxide, lye

Make litmus paper

Soak paper towel strips in the solution and allow them to dry. The color changes are not as dramatic as the litmus solution. You can drop liquids onto the paper, or dip the paper into liquids you want to test. If you want to test a powder, or soil, mix it with water first. Again, make sure your water is neutral.

Standardizing your tests

It is impossible to be completely accurate with the cabbage juice indicator, but you can approximate the standard pH test. Test the items on the standard test (graphic above) and note the color you get with the cabbage juice indicator. Either take a photo of your solution and then write the pH number on the photograph, or try to match the color with crayons/colored pencils. Keep these for future reference.

Don’t be limited by the items on the standard pH scale! Try all sorts of powders and liquids you find around your house. So much fun!

An experiment that didn’t turn out the way we anticipated (but we learned something important):

I taught this subject to a group of Kabul university chemistry students. The class was held in a hotel conference center. I instructed students to get a large pot of boiling water from the hotel kitchen. The hotel kitchen had just finished serving lunch to students and staff. Students were delighted to learn they could make their own litmus solution because litmus paper was difficult to come by, but purple cabbage was readily available from street vendors. The indicator solution was rapidly made, but the test colors kept coming out wrong. Everything was testing too alkaline. I decided the human dishwashers in the kitchen hadn’t rinsed the pot sufficiently after washing. It still had soap residue in it, which is alkaline. I was somewhat insulting of their care in rinsing pots and thought ruefully of the lunch we had just eaten, cooked in those same pots. Then it occurred to me that the well water serving the kitchen – and all of Kabul – might be alkaline itself. I checked this with local geologists who told me that the famously good Kabul water came from wells sunk deep into limestone layers. The limestone filters the water but adds alkali to it. Then I asked a physician whether this could be a cause of kidney stones that many residents had. “Yes,” he said. “Alkaline water can cause them.”

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