An acid-base indicator is a dye or mixture of dye that marks the end-point of the titration. The indicator has a sudden and obvious color change at the pH corresponding to the end-point of the titration. A good indicator must have the following properties:

• The color change must be easily detected.
• The color change must be rapid.
• The indicator works by reacting with one (or both) of the solutions used in the titration, which is why the volume of indicator used should be small.
• For accuracy, the pK_in of an indicator must be close to the pH at the equivalence point of the reaction.

The logarithmic acid dissociation constant (pK_in) determines the pH at which the indicator changes color (end-point).

Read more about various types of acid-base indicators here.