In acidic solutions, phenolphthalein exists in its protonated form, which is colorless. The protonated form of phenolphthalein has a chemical structure where the hydroxyl group (-OH) on the central carbon atom is protonated, forming an -OH2+ group. This protonation prevents the formation of the colored anion.
When phenolphthalein is added to an alkaline solution, the high concentration of hydroxide ions (OH-) causes the proton on the hydroxyl group to be removed. This deprotonation leads to the formation of the phenolphthalein anion, which has a pink-red color. The phenolphthalein anion has a delocalized negative charge that extends over the molecule, resulting in the absorption of light in the visible region of the spectrum, giving it the pink-red color.
The color change of phenolphthalein in alkaline pH is a result of the chemical equilibrium that exists between the protonated and deprotonated forms of the compound. At higher pH values, the equilibrium shifts towards the deprotonated form, leading to a more intense pink-red color.
In summary, phenolphthalein changes color in alkaline pH because of the deprotonation of the hydroxyl group, which leads to the formation of the colored phenolphthalein anion. This change in molecular structure results in the absorption of light in the visible region, giving the solution a pink-red color.
