Monday, July 11, 2016

Making A Natural pH Indicator - STEAM Program

Here's a quick and easy STEAM program that falls under the "Kitchen Chemistry" heading. This experiment uses mostly things that people might have in their kitchen already, or could be easily obtained the next time you're grocery shopping.

In this experiment we will make a natural pH indicator from purple cabbage. The pigments that give purple cabbage its color are in the anthocyanin family, a group of color-changing, water-soluble cyclic compounds. They may appear red, purple, or green depending upon the pH. pH-sensitive anthocyanins are also found in other brightly colored fruits, vegetable, and flowers, but purple cabbage has pigments that cover the broadest range of pH and produces the most color changes.

[For a quick chemistry refresher, pH is a numeric scale that rates the acidity or alkalinity of a solution, based on the concentration of protons. Neutral solutions have a pH of 7.0, acidic solutions have a pH below 7.0, and alkaline solutions have a pH greater than 7.0. Acids like to donate protons (in the form of hydrogen ions) and bases like to accept protons.]

To Make Indicator Solution: 

  1. Chop 1/4 of a head of purple cabbage (about 2 cups).
  2. Place in blender with enough boiling water to cover.
  3. Carefully pulse blender to very finely chop cabbage (cover lid with towel and hold).
  4. Let steep for about 10 minutes.
  5. Strain solution through wire mesh strainer to remove large particles.
  6. Strain through coffee filter placed in wire mesh strainer to remove very fine particulates.
  7. Pour in storage container and allow to settle and cool.
  8. Solution may be stored in refrigerator up to one week.
The resulting solution will be about the color of diluted grape juice, the intensity and exact shade will vary depending on the amount, exact pH, and temperature of the water used. If the solution is very dark, so that you can't see through it at all, dilute with more water. This is about what it should look like, but a little lighter is okay, too:

Now, we are ready to experiment! 

Pour some of your indicator liquid into each of several small clear cups, glasses, or test tubes. Try to keep the amounts consistent.

Gather various household solutions for testing. The solutions should be clear and almost colorless for best results, and safe for kids to handle in small amounts. Suggestions for acidic solutions: distilled vinegar, lemon juice, clear carbonated beverage, apple juice, white grape juice, cream of tartar dissolved in water. Suggested alkaline solutions: baking soda, antacid tablets (Tums), washing soda, ammonia (demo only). As you can see, kid-friendly acidic solutions are much more readily available than basic ones. Ammonia must be handled carefully, and only by an adult (or mature teens with gloves and safety glasses in a well-ventilated area).

Test the solutions by pouring some of the desired test solution into one of the indicator solutions and observe the color change. Pinkish-purple is slightly acidic, pink is moderately acidic, and red is very acidic. Blue is slightly alkaline, green is moderately alkaline, and yellow is very alkaline. Yellow cannot be achieved with kid-friendly solutions, but an adult can do a demonstration using ammonia. It will appear light green at first, but over several minutes will become almost yellow. Be sure to keep one container with just the cabbage juice extract as your neutral control. Arrange your test containers in order of most acidic to most alkaline.

*Click on picture to see full-size version*
Left to right: distilled vinegar, apple juice, lemon juice, cream of tartar, Sprite,
neutral control, baking soda, Chlorox Clean-Up, washing soda, ammonia.

In the picture below you can see how the alkaline solutions continued to change color over time, with the washing powder and ammonia solutions become more yellow:

*Click on picture for full-size version*

For a library program, I would make up a large batch of the cabbage extract ahead of time, and just make one small batch as a demonstration. For the solutions to be tested, I would have the powders already dissolved in water (about 1 teaspoon/1 Cup), and have numbered cups (be sure you have a key) with them already set out at each place, but be sure to instruct them not to touch until told. Then distribute the cabbage juice.

Don't let them know what is magical about the cabbage juice ahead of time, just tell them to add the first solution (preferably once of the stronger acids or bases to get a dramatic change) and watch their reactions. Then explain how it works before testing the rest of the solutions, one at a time. Be sure to tell them NOT to mix solutions!

To dispose of your experiment, pour out the ammonia first, diluting with plenty of running water. Then dispose of all the others, pouring one at a time down the sink with the water running. For convenience, you may pool the smaller quantities of like solutions in one container to transfer to the sink.

You can also try making your own "litmus" paper by saturating filter paper or a coffee filter with the cabbage extract, allowing to dry, and cutting into strips. This works better with a more concentrated extract, so use less water in your preparation. Then either dip into test solution, or drip drops of test solution onto the strips. Try extracting anthocyanins from other colored fruits, vegetable, and/or flowers, and see what color changes they produce.

This could be a shorter program by itself, or combined with other "Kitchen Chemistry" experiments in a longer program and/or for older or more advanced groups. Display fiction books featuring mad-scientists, magic vegetables, or cabbage; as well as non-fiction books with simple at-home experiments and biographies of chemists for check-out.

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