Acid–Base Indicator
A weak acid or base whose conjugate forms have distinctly different colours, used to signal the end point of a titration.
Formula / equation
HIn ⇌ H⁺ + In⁻
Unit
Dimensionless
In depth
The indicator changes colour as the pH passes through its own pKa, because the ratio of the two coloured forms swings from ten to one in favour of the acid form to ten to one in favour of the base form. Only a drop or two is used, so it does not disturb the titration.
Examples in the real world
Litmus, phenolphthalein, methyl orange, and bromothymol blue for near-neutral end points.