Acid–Base Indicators

Core Concept

An acid–base indicator is a weak acid (or base) whose acid form and conjugate-base form have different colors. The equilibrium HIn ⇌ H⁺ + In⁻ shifts with pH, so the solution's color reveals the ratio [In⁻]/[HIn]. The color change is not instantaneous — it unfolds over a transition range of roughly pH = pKa ± 1, centered where [In⁻] = [HIn]. Choosing an indicator whose transition range overlaps the steep part of a titration curve lets the observed color change (the endpoint) approximate the stoichiometric equivalence point.

Key Ideas

• Indicator equilibrium: HIn(aq) ⇌ H₃O⁺ + In⁻; HIn and In⁻ have different colors.
• Color determined by the ratio [In⁻]/[HIn], which pH controls through Ka.
• Transition range: approximately pKa(indicator) ± 1 pH unit.
• Midpoint of change: when [In⁻] = [HIn], pH = pKa(indicator); the color is a blend.
• Good endpoint: an indicator whose transition range falls inside the steep vertical region of the titration curve.
• Universal indicators / pH paper: mixtures of several indicators spanning the whole pH scale.

Equations and Variables

• HIn + H₂O ⇌ H₃O⁺ + In⁻; KIn = [H₃O⁺][In⁻]/[HIn]
• pH = pKIn + log([In⁻]/[HIn])
• Transition range ≈ pKIn ± 1
• Endpoint (observed color change) ≈ equivalence point (stoichiometric) when the indicator is well chosen

How It Works

1. The indicator is added in tiny amounts so it doesn't disturb the solution's pH.
2. As pH changes, the equilibrium HIn ⇌ H₃O⁺ + In⁻ shifts. At low pH, HIn dominates (one color); at high pH, In⁻ dominates (the other color).
3. In the transition range, both forms coexist in visible amounts; the eye perceives a gradual blend, typically changing "completely" over about two pH units.
4. For a titration, pick an indicator whose pKIn sits near the equivalence pH. For a strong-acid/strong-base titration (equivalence pH 7), phenolphthalein (range ~8.2–10.0) or bromothymol blue (6.0–7.6) works; for a weak-acid/strong-base titration (equivalence pH > 7), phenolphthalein is the classic choice.

Worked Example

Choose an indicator for titrating CH₃COOH (Ka = 1.8 × 10⁻⁵) with NaOH.

The equivalence point is at pH ≈ 8.72 (the conjugate base CH₃COO⁻ makes the solution basic). The steep part of the curve lies roughly between pH 7 and 11.

• Phenolphthalein: transition range ~8.2–10.0 — falls inside the steep region → good choice; it turns pink right at the equivalence point.
• Methyl orange: range ~3.1–4.4 — far below the equivalence pH → it would change color far too early, giving a large error.

Choose phenolphthalein.

How it works

1. The indicator is added in tiny amounts so it doesn't disturb the solution's pH.
2. As pH changes, the equilibrium HIn ⇌ H₃O⁺ + In⁻ shifts. At low pH, HIn dominates (one color); at high pH, In⁻ dominates (the other color).
3. In the transition range, both forms coexist in visible amounts; the eye perceives a gradual blend, typically changing "completely" over about two pH units.
4. For a titration, pick an indicator whose pKIn sits near the equivalence pH. For a strong-acid/strong-base titration (equivalence pH 7), phenolphthalein (range ~8.2–10.0) or bromothymol blue (6.0–7.6) works; for a weak-acid/strong-base titration (equivalence pH > 7), phenolphthalein is the classic choice.

Common confusions

• "Indicators change color exactly at pH 7." — Each indicator changes around its own pKa; phenolphthalein changes near 8.2–10.0, methyl orange near 3.1–4.4.
• "The endpoint and equivalence point are the same." — The endpoint is where the indicator happens to change; the equivalence point is the stoichiometric truth. Good indicator choice makes them coincide.
• "Indicator color change is instantaneous." — It is gradual over ~2 pH units; that range is what you must place on the steep curve.
• "Add lots of indicator for a clear color." — Indicators are acids/bases themselves; too much perturbs the very pH you are measuring.
• "Phenolphthalein is always the right choice." — For titrating a weak base with strong acid (equivalence pH < 7), methyl orange is better.

Quick review

• HIn ⇌ H₃O⁺ + In⁻, two colors.
• Transition range ≈ pKIn ± 1.
• Midpoint (blend) at pH = pKIn.
• Match indicator range to equivalence pH.
• Endpoint ≈ equivalence point only when well matched.