Why Does Phenolphthalein Change Color?

By Josh Baum


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About Phenolphthalein

Phenolphthalein is a mildly acidic compound that has a chemical formula of C20H14O4. It is primarily used as a pH indicator, allowing chemists to easily test whether a substance is an acid or a base. It is also sometimes used as a laxative, though because its effects are harsh and long lasting, it is generally reserved for serious medical situations. The compound was discovered in 1871 by the renowned German chemist Adolf von Baeyer.

Phenolphthalein And The pH Scale

The pH scale falls in the range from zero to 14, in which numbers less than 7 indicates an acidic compound and those greater than 7 indicates an alkaline compound; on this scale, pure water is neutral with a pH of 7. In common practice, chemists use litmus paper to measure the pH of a compound; the paper turns red when dipped in acids and blue in bases. Phenolphthalein works somewhat differently, as it is naturally colorless but turns pink in alkaline solutions. It remains colorless throughout the range of acidic pH levels, but it begins to turn pink at a pH level of 8.2 and continues to a bright purple in stronger alkalines.

How Phenolphthalein Changes Color

This compound’s color change happens through a process called ionization. Ionization occurs when a molecule gains or loses electrons, giving the molecule a negative or positive electric charge. Ionized molecules attract other molecules with the opposite charge and repel those with the same charge. With phenolphtalein, this also affects the molecule’s shape. The combination of shape and electric charge determine how a molecule responds to light. Normally, phenolphtalein is clear because all colors of light pass through it. When exposed to alkaline solutions, it begins to block the blue colors of the spectrum, which turns the light pinkish. The stronger the alkaline solution is, the more the phenolphthalein molecule changes and the darker the its pink hue will be.

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