Phenolphthalein is an organic compound commonly used as an acid-base indicator. Its behavior varies depending on whether it is in an acidic or basic medium.
In an acidic medium: Phenolphthalein remains colorless in acidic solutions with a pH below approximately 8.2. This is because in an acidic environment, the phenolphthalein molecule exists in its protonated form (HIn), which is colorless. The presence of excess hydrogen ions (H+) prevents the deprotonation of phenolphthalein, thus maintaining its colorless appearance.
In a basic medium: When the pH of the solution rises above approximately 8.2, phenolphthalein undergoes deprotonation and forms its negatively charged ion (In-). In this deprotonated form, phenolphthalein exhibits a vibrant pink or magenta color. The deprotonation occurs due to the abundance of hydroxide ions (OH-) in basic solutions, which react with the hydrogen ions in the phenolphthalein molecule, causing it to lose a proton and form the colored ion.
It's important to note that the exact pH at which phenolphthalein undergoes its color change may vary slightly depending on factors such as concentration and temperature. However, the transition from colorless to pink/magenta typically occurs in the pH range of 8.2 to 10.0.
Phenolphthalein is commonly used in titration experiments to indicate the endpoint of an acid-base reaction. The color change of phenolphthalein serves as a visual indicator for determining the completion of the reaction or the equivalence point.
