Answer is: Prepare to measure pH change.
For example for strong acid-base titration, sodium hydoxide and hydrochloric can be used.
Balanced chemical reaction: HCl + NaOH → NaCl + H₂O.
In this reaction pH of equivalence point will be always 7.
Equivalence point is the point which there is stoichiometrically equivalent amounts of acid and base.
Chemist can draw pH curve (graph showing the change in pH of a solution, which is being titrated) for titration and determine equivalence point.
Near equivalence point indicator should change color, so we must pick indicator who change color near pH of equivalence point.
