acid–base indicator
/ IN-dih-kay-ter /
Brew red cabbage and you have a kitchen magic trick: a splash of vinegar turns the juice pink, a pinch of baking soda turns it blue-green. That colour-shifting dye is an acid–base indicator — a substance that visibly changes colour depending on how acidic or basic its surroundings are.
An indicator is itself a weak acid (or weak base) whose acid form and base form happen to have different colours. As the pH crosses a certain range, the balance between the two forms tips, and you see the colour switch. Each indicator changes over its own narrow pH band — phenolphthalein near 8–10, methyl orange near 3–4 — so a single dye flags a specific region of the scale.
Indicators matter because they make an invisible chemical quantity, pH, visible at a glance, with no instrument needed. They are the eyes of a titration and the heart of pH paper. The caveat is precision: an indicator only tells you which side of its change-over you are on, so for an exact pH reading a meter beats a colour, and the indicator must be matched to the pH where you expect the change.
Phenolphthalein is colourless in acid but turns bright pink above about pH 8.3. In a titration it stays clear until the very last drop of base tips the solution over, then flushes pink to signal you have arrived.
An indicator's colour flips as pH crosses its narrow change-over band.
Universal indicator is a blend of several dyes whose combined colours sweep smoothly through the whole pH range, giving the familiar rainbow strip — useful for a rough reading, but no single dye in it is precise across all pH values.