Acid-base indicators are coloured substances which they themselves are weak acids. They can work as indicators because the co-acid and base forms have different colours.
A common indicator used in acid/alkai titrations is phenol phthalein. This is a white organic solid. In its ionised form it is colourless in solution. Its co-base is pink in colour. If we represent its formula as
We can write an expression for
At the end point of a titration
By consulting a table of
So for phenol phthalein
Here is a video of an indicator lab!
video from: Noel Pauller