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pH Indicators - Demo Video #3
Introduction
Many of our demonstrations involve acid-base reactions; when combined with acid-base (pH) indicators, they can make for entertaining and colorful demos. As mentioned in our home lab activity, there are many sources of acid-base indicators both natural and synthetic.
Acid-base indicators are themselves acids or bases, and they change color depending on the pH of the solution. The existence of acid-base indicators has been known for centuries. For example, in 1664, Robert Boyle was investigating a particular wood (Lignum nephriticum) known for its medicinal properties. In his words: “All Acid Salts … destroy the Blewness of the Infusion of our Wood and all Sulfureous salts … have the Vertue of Restoring it” (https://www.jstor.org/stable/27757238). In essence, he discovered that the color of the wood depended on pH, although the concept of pH was not introduced until 1909 by Sorensen.
We make considerable use of three indicators: phenolphthalein, thymolphthalein, and para-nitrophenol in our outreach presentation. They are colorless in acid, but turn red, blue, and yellow, respectively, in base. This property allows us to “hide” an indicator in the bottom of a cup; when base is added, it turns to its characteristic color. Note that red, blue, and yellow are the three primary colors; thus, we can form any color of the rainbow by proper mixing of the indicators.
Why would we want to hide something in a cup, anyway? It is the basis of many of our demonstrations that involve “chemical magic”. We use this approach in our outreach presentation not because we want to fool the audience, but because we want to get their attention. A chemical magic trick often results in “how did you do that” from the younger members of the audience. That question is the “hook” in discussing the chemistry of the demonstration.
The featured video illustrates the properties of the three indicators and our chemical magic approach to demonstrations.
Our featured molecule is phenolphthalein. It is colorless in solutions from pH 0 – 8. At pH 8 – 13, it forms a pink dianion, and interestingly, at pH > 13, it forms a colorless trianion.
There is another factoid about phenolphthalein, but you can research this yourself. What over-the-counter drug contained phenolphthalein until it was reformulated in the late 1990’s?
Watch our Magic Bottle Demo Video:
