Phenolphthalein ( /ˌfiːnɒlfˈθeɪliːn/) is a chemical compound with the formula C20H14O4 and is often written as "HIn" or "phph" in shorthand notation. Often used in titrations, it turns colorless in acidic solutions and pink in basic solutions. If the concentration of indicator is particularly strong, it can appear purple. In strongly basic solutions, phenolphthalein's pink color undergoes a rather slow fading reaction and becomes colorless again. The molecule has four forms:
| Species | H3In+ | H2In | In2− | In(OH)3− |
|---|---|---|---|---|
| Structure | ||||
| Model | ||||
| pH | <0 | 0−8.2 | 8.2−12.0 | >12.0 |
| Conditions | strongly acidic | acidic or near-neutral | basic | strongly basic |
| Color | orange |
|
pink to fuchsia | colorless |
| Image |
The rather slow fading reaction that produces the colorless InOH3− ion is sometimes used in classes for the study of reaction kinetics.
Phenolphthalein is insoluble in water and is usually dissolved in alcohols for use in experiments. It is itself a weak acid, which can lose H+ ions in solution. The phenolphthalein molecule is colorless. However, the phenolphthalein ion is pink. When a base is added to the phenolphthalein, the molecule ⇌ ions equilibrium shifts to the right, leading to more ionization as H+ ions are removed. This is predicted by Le Chatelier's principle.
Read more about Phenolphthalein: Synthesis, Uses, Physiology