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What Are the Medical Uses of Phenolphthalein?

By Erik J.J. Goserud
Updated Mar 03, 2024
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Phenolphthalein is an organic compound summarized chemically by the formula C20H14O4. As the formula suggests, this molecule contains 20 carbon atoms, 14 hydrogen atoms and four oxygen atoms. Phenolphthalein is most notably used as an acid/base indicator and was formerly popularized as a component of laxative agents. This molecule is most accurately used as a basic indicator because of a discrepancy in its acidic and strongly basic reactions.

As an acid base indicator, phenolphthalein changes color depending on the chemical nature of the substance to which it is exposed. If phenolphthalein is exposed to an acidic substance, it will turn colorless. When exposed to a more basic substance, this particular indicator becomes almost pink. The extreme to which the tested substance is either basic or acidic affects the way that it alters the indicator, and in the case of phenolphthalein, a strongly acidic reaction causes an orange or deep purple color, and a strongly basic substance results in a colorless appearance.

The reasons that indicators change color can be explained chemically. Every substance in existence appears and functions the way it does because of its chemical nature. A change in this nature, such as the charge of the molecule, therefore can greatly affect its physical properties. An acid, or a chemical that is prone to releasing positively charged ions, creates a more positively charged form of phenolphthalein, hence a specific color change. Conversely, a base absorbs excess positive ions, which might also change an indicator in a specific fashion.

Phenolphthalein, which was discovered in 1871 by German chemist Adolf Van Baeyer, is usually prepared in acid. This is because it is not very water soluble and tends to mix more completely in an alcohol-based solution. When used as a laxative agent, its mechanism was a mild irritation to the small intestine and a more significant irritation to the large intestine, promoting bowel movements. This bowel irritation has a typical onset of six to eight hours and has been known to last three or four days.

Medical research suggests that the use of phenolphthalein as a laxative is unsafe because of its carcinogenic affects. Most modern laxatives therefore do not contain this substance, because of the potential dangers associated with it. Less severely adverse affects of this laxative are skin rash or kidney irritation. Research has been conducted regarding the specific mechanisms of its carcinogenic effects as well as potential other uses of phenolphthalein.

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