How is the appropriate indicator for an acid/base titration chosen?

The appropriate indicator for an acid/base titration is chosen by considering the pKa of the indicator in relation to the pH at the expected equivalence point of the titration. Specifically, the pKa of the indicator should fall within the range of the expected change in pH at the equivalence point, typically within one pH unit above or below that point. This allows the indicator to exhibit a visible color change precisely when the solution transitions from an acidic to a neutral or basic state, facilitating accurate titration results.

In a titration, the equivalence point is where the moles of acid equal the moles of base, and the solution's pH changes significantly. Thus, selecting an indicator whose pKa is closely aligned with this transition ensures that it will change color at the right moment, indicating that the equivalence point has been reached. This choice is critical for determining the endpoint of the titration with clarity and precision.