indicator
/ IN-dih-kay-ter /
Think of a mood ring that changes colour to signal a change you cannot otherwise see. In a titration, an indicator is a small amount of a special substance added so that a sudden colour change announces the finishing point.
An indicator is a substance, usually a dye, whose colour depends on the chemical condition around it — the acidity, the metal-ion level, or the presence of an oxidiser. As the titration nears completion, that condition swings sharply, and the indicator flips colour, marking the end point for the analyst's eye.
Indicators matter because they turn an invisible chemical balance into something you can simply watch for, making titration fast and equipment-free. The catch is that each indicator changes over its own narrow range, so you must match the indicator to the reaction; the wrong choice shifts the end point away from the true equivalence point.
Phenolphthalein is added to an acid being titrated with base: it stays colourless while acid remains and turns pink the moment the solution becomes slightly basic, signalling the end point.
A dye that flips colour to mark the moment to stop.
Different titration types use different indicator families: acid-base indicators respond to pH, metal-ion indicators to metal concentration, and so on. Add only a tiny amount, since the indicator itself reacts and too much causes its own small error.