acid-base indicator
/ AS-id BAYS IN-dih-kay-ter /
Think of mood rings that change colour with warmth, except this one reads acidity. Drop a little into a solution and its colour announces whether the liquid is acidic or basic, no instrument required. An acid-base indicator is a dye that wears one colour as an acid and a different colour as a base.
Chemically, an indicator is itself a weak acid or weak base whose protonated and deprotonated forms happen to absorb light differently, so they look like different colours. As the surrounding pH rises through the indicator's own pKa, the dye shifts from mostly the acid-coloured form to mostly the base-coloured form. The change is visible over a span of roughly two pH units centred on that pKa, called the transition range.
Indicators matter because they give a free, instant readout of when a titration has reached its endpoint, without a meter. The honest caveat is that the colour change marks the indicator's own transition, not the true equivalence point — choosing an indicator whose transition range straddles the titration's steep pH jump is what keeps the small gap between endpoint and equivalence point, the titration error, negligibly tiny.
Phenolphthalein is colourless in acid but turns bright pink above about pH 8.3; a single drop in a flask of acid stays clear until the very last addition of base, then flushes pink to signal the endpoint.
The colour flips across the indicator's transition range, not at a single pH.
Match the indicator to the titration, not to neutrality. For a weak-acid-strong-base titration the equivalence point is basic, so phenolphthalein fits, while methyl orange, which changes around pH 4, would signal far too early.