Selecting an indicator is a matching problem: you want the indicator’s visible colour change to occur during the steep pH jump near the equivalence point, not before or after it.

Titration curve with indicator transition ranges overlaid, illustrating how indicator choice must align with the steep pH change near the equivalence point. The horizontal bars represent each indicator’s color-change interval, making it clear why a well-matched indicator yields a sharper, more accurate endpoint for a given titration system. Source

Key ideas for indicator choice

What an indicator does

Indicators establish an equilibrium between two coloured forms, so their behaviour is governed by an acid dissociation constant and a pKa value (for the indicator’s acidic form).

Endpoint vs equivalence point

The goal is a small indicator error, meaning the endpoint pH is as close as possible to the equivalence-point pH.

How pKa controls the colour-change window

An indicator changes colour when both forms are present in comparable amounts; practically, the human eye detects a transition over a pH range of about two units.

From this relationship:

• When $[\text{In}^-]=[\text{HIn}]$, $pH=pK_a$ (midpoint colour).

• A noticeable colour shift typically occurs as the ratio moves from about 1:10 to 10:1, corresponding roughly to $pH \approx pK_a \pm 1$.

Color-change interval chart for common acid–base indicators, showing the approximate pH ranges over which each indicator transitions between its acidic and basic colors. This visual emphasizes that an indicator does not change color at a single pH value, but across a window that is typically about $pK_a \pm 1$. Source

Practical rule for choosing an indicator

Match pKa to the equivalence-point pH

The syllabus requirement is the central selection rule:

• Choose an indicator whose pKa is close to the pH at the equivalence point.

• More precisely, aim for the equivalence-point pH to fall within the indicator’s transition range (approximately $pK_a \pm 1$), so the colour change happens during the steepest part of the titration curve.

Why “close” matters experimentally

An indicator is most accurate when a very small added volume of titrant carries the solution through the indicator’s transition range. If the indicator’s pKa is mismatched:

• The colour change can occur too early (systematic under-titration).

• Or too late (systematic over-titration). Either case produces a larger difference between endpoint and equivalence point, increasing uncertainty in calculated concentrations.

Additional selection considerations (secondary to pKa matching)

• Prefer indicators with a sharp, distinct colour contrast in the relevant pH region.

• Avoid indicators whose transition colour is hard to detect in the sample matrix (e.g., strongly coloured analytes).

• Use only a small amount of indicator; although usually negligible, excessive indicator can slightly perturb pH because it is itself a weak acid/base.