Indicators are weak acids caught in their own equilibrium. The molecule itself shifts between two forms with wildly different colours, and the ratio between them is set entirely by the pH around it. Drag the slider to watch phenolphthalein transform as pH crosses its transition range.
pH 0–14 range showing indicator colour shift
Phenolphthalein has a pKa around 9.3, meaning its transition range sits between pH 8.2 and 10.0. In acid form (below pH 8.2), it's completely colourless—the conjugate base hasn't formed yet. As pH rises past 8.2, the equilibrium shifts and the base form (bright pink) starts appearing. By pH 10, almost all molecules have donated their proton and the solution glows deep magenta. This makes it perfect for strong acid–strong base titrations, where the equivalence point lands around pH 7–9, but completely useless for weak acid titrations where the endpoint might only reach pH 5. The indicator's own pKa determines where it changes, not the strength of the acids you're titrating.
An indicator's pKa sets its transition range, which must overlap the titration's equivalence point or you'll miss the endpoint entirely.