Why Does Phenolphthalein Change Color?

••• Dick Luria/Photodisc/Getty Images

Most people know the terms acidic or alkaline from common household substances, but the function of pH indicators is much more advanced. One such indicator, phenolphthalein, is usually colorless but ranges from pink to purple when exposed to alkaline solutions.

TL;DR (Too Long; Didn't Read)

Phenolphthalein turns pink when exposed to substances above a pH of 8.2- and becomes purple at even higher pH values. This color change is a result of ionization, which alters the shape and charge of phenolphthalein molecules. This causes it to block the blue light spectrum when exposed to alkaline substances, producing a pink to purple hue.

What is phenolphthalein?

In 1871, renowned German chemist Adolf von Baeyer discovered phenolphthalein, a mildly acidic compound that has a chemical formula of C20H14O4. This compound primarily serves as a pH indicator, allowing chemists to easily test whether a substance is an acid or a base. In the past, medical providers also employed phenolphthalein as a laxative, but its harsh side effects and potential as a carcinogen (cancer-causing agent) prompted the Food and Drug Administration to ban it for this use in 1999.

Phenolphthalein and the pH Scale

The pH scale runs from 0 to 14 with acidic substances registering less than 7 on the scale and alkaline substances registering above 7 on the scale. A reading of 7 indicates a neutral pH like pure water. In common practice, chemists use litmus paper to measure the pH of a compound; the paper turns red when dipped in acids and blue when dipped in bases.

Phenolphthalein works somewhat differently as it is naturally colorless but turns pink in alkaline solutions. The compounds remains colorless throughout the range of acidic pH levels but 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 pink hue will be.

References

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  • Dick Luria/Photodisc/Getty Images

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