phenolphthalein

Meaning: Phenolphthalein is a weak-acid indicator, HIn, whose protonated and deprotonated forms differ in color. HIn is colorless and In⁻ is pink. Adding base shifts the equilibrium HIn ⇌ H⁺ + In⁻ toward In⁻, so the solution turns pink. The change happens over a narrow window, roughly pH 8 to 10, centered near the indicator's pKa.

Equation: pH = pKa + log([In⁻]/[HIn]), where pH and pKa (−log Ka of the indicator) are unitless and the concentrations are in mol/L. The color visibly shifts when the ratio of the two forms moves from about 1:10 to 10:1, which is about pKa ± 1.

On the MCAT: A titration passage asks which indicator to use. Choose one whose transition range includes the pH at the equivalence point. For a weak acid with a strong base the equivalence point is above 7, so phenolphthalein works. For a strong acid with a strong base it also works, because the pH jumps steeply near equivalence. For a weak base with a strong acid the equivalence point is below 7, so an indicator such as methyl orange or methyl red fits better.

Common confusion: The color change marks the endpoint, not the equivalence point. The equivalence point is where moles of acid equal moles of base. A well-chosen indicator makes the two nearly coincide. Also, phenolphthalein is pink in base and colorless in acid, not the reverse.