Phenolphthalein

Phenolphthalein is a chemical compound with the formula C20H14O4. It is a member of a group of substances called phthalein dyes. This compound is most famous for its role as a pH indicator. An indicator is a substance that changes color to show the acidity or alkalinity of a solution. Scientists use it during acid-base titrations to find the exact point when a reaction is complete.

The way phenolphthalein changes color depends on its chemical structure. It is a weak acid, meaning it can lose hydrogen ions, known as H+ ions, in a solution. The specific color you see depends on the pH level of the liquid. In acidic or near-neutral conditions, the molecule exists in a lactone form called H2In. This form is colorless.

When the solution becomes basic, the molecule loses two hydrogen ions. This creates a doubly deprotonated ion called In2-. This specific form is what produces the bright pink or fuchsia color.

Extreme pH levels cause even more dramatic changes. In very strong acids, such as concentrated sulfuric acid, the molecule undergoes protonation. This creates a stabilized trityl cation known as H3In+. This form appears orange-red.

If the solution is extremely basic, with a pH greater than 13, the pink color actually disappears. The molecule is converted into a form called In(OH)3-. This happens because the conjugated system, which allows the molecule to absorb light and show color, is broken.

The history of phenolphthalein is tied to both chemistry and medicine. Adolf von Baeyer discovered how to synthesize the compound in 1871. He created it through the condensation of phthalic anhydride with two equivalents of phenol under acidic conditions.

In 1900, the Hungarian government attempted to use the chemical to label genuine local white wine. They wanted a label that would not change the wine's taste but would change color if a base was added. However, they discovered that ingesting the substance caused diarrhea. A pharmacist named Max Kiss heard about this in New York and launched the product Ex-Lax in 1906.

Because of its color-changing properties, phenolphthalein has many practical uses. In construction, it helps check the health of concrete. Concrete is naturally basic because of the calcium hydroxide formed when Portland cement reacts with water. When concrete reacts with carbon dioxide from the air, its pH drops to between 8.5 and 9. If a 1% phenolphthalein solution is applied to normal concrete, it turns bright pink. If it stays colorless, it proves the concrete has undergone carbonation.

It is also used in forensic science through the Kastle–Meyer test. This test identifies substances that might contain blood. A colorless, reduced form called phenolphthalin is used. A sample is collected, and drops of alcohol, phenolphthalin, and hydrogen peroxide are added. If hemoglobin is present, the sample turns pink immediately. This reaction works for the blood of almost all vertebrates, though more testing is needed to confirm if the blood is human.

Finally, phenolphthalein appears in education and entertainment. In chemistry classes, students study reaction kinetics by watching the pink color fade in highly basic solutions. In toys, it is used in disappearing inks. The ink is mixed with sodium hydroxide, which reacts with carbon dioxide in the air to lower the pH. This causes the pink color to fade. It is also used in "magic" hair dyes, where a hydroxide spray makes hidden patterns appear.

While it was used as a stimulant laxative for over a century, it is being removed from many products. This is due to concerns that it may be carcinogenic, as it is listed by the IARC as possibly carcinogenic to humans.