Acid-base indicators: color as a pH signal
Learn how acid-base indicators work, the color-change ranges of phenolphthalein, methyl orange, and litmus, and how to choose the right indicator for a titration.
Quick answer
An acid-base indicator is a weak acid (HIn) whose acid and conjugate-base forms have different colors. It changes color across a ~2 pH-unit range centered on its pKa.
Example: Phenolphthalein is colorless below pH 8.2 and pink above 10.0 — ideal for weak acid–strong base titrations, whose equivalence point lies above 7. Methyl orange (red→yellow, pH 3.1–4.4) suits strong acid–weak base titrations.
What you will learn
- Explain the HIn ⇌ H⁺ + In⁻ equilibrium behind indicator color change.
- State the transition ranges of phenolphthalein, methyl orange, and litmus.
- Match an indicator to a titration type.
- Explain why universal indicator suits estimation but not titration.
The equilibrium behind the color
An indicator is a weak acid with a visible secret — its two forms have different colors:
At low pH, excess H⁺ pushes the equilibrium left: the solution shows HIn's color. At high pH, H⁺ is scarce and the equilibrium shifts right: In⁻'s color appears. The flip happens across roughly pKa ± 1 — a two-unit transition range where the eye sees the blend shift from one color to the other.
The common indicators and their ranges
Three indicators cover most general-chemistry needs:
- Phenolphthalein: colorless → pink, pH 8.2–10.0. The standard for weak acid–strong base titrations.
- Methyl orange: red → yellow, pH 3.1–4.4. The standard for strong acid–weak base titrations.
- Litmus: red → blue, roughly pH 5–8. Good for 'acid or base?' tests; too broad for titrations.
Bromothymol blue (yellow → blue, 6.0–7.6) is the closest to a 'changes at 7' indicator and suits strong–strong titrations. Universal indicator blends several dyes for a full rainbow across pH 1–14 — excellent for estimates, useless for endpoints.
Matching indicator to titration
The rule: the indicator's transition range must fall inside the titration curve's steep vertical jump.
- Strong acid–strong base: jump spans ~pH 4–10 — phenolphthalein, bromothymol blue, or methyl orange all work.
- Weak acid–strong base: jump sits above 7 — phenolphthalein (methyl orange would change too early, before equivalence).
- Strong acid–weak base: jump sits below 7 — methyl orange (phenolphthalein would change too late).
Choosing wrong does not give a 'slightly off' answer — the color change lands on the flat part of the curve, and the endpoint can miss equivalence by milliliters.
Using indicators well
Practical notes: use 2–3 drops — indicators are weak acids that consume titrant, so excess shifts the endpoint. Add indicator to the analyte flask (not the buret). And read the color against a white background in good light; 'first permanent faint pink' (phenolphthalein) is the classic endpoint description because overshooting to deep pink means you passed it.
Common mistakes
Each indicator changes over its own ~2-unit range centered on its pKa: phenolphthalein 8.2–10.0, methyl orange 3.1–4.4. 'Changes at 7' is true of no common indicator — you match the indicator to the titration, not to 7.
Indicators are weak acids/bases that consume titrant themselves. A few drops is the standard; excess indicator shifts the endpoint and muddies the color. More is worse, not better.
The indicator is a spectator — the neutralization proceeds identically without it. Its only job is to make the invisible equivalence point visible.
Real-world applications
Titrations: signaling the endpoint with a color change.
Aquariums and pools: test strips use immobilized indicators.
Soil testing: quick field estimates of soil pH.
Chemistry education: visualizing invisible pH changes.
Frequently asked questions
What makes an indicator change color?
An indicator is itself a weak acid: HIn ⇌ H⁺ + In⁻, with HIn and In⁻ differently colored. At low pH the equilibrium lies left (HIn color); at high pH it lies right (In⁻ color). The color flips across the pH range near its pKa.
How do I pick an indicator for a titration?
Its transition range must bracket the titration's steep pH jump. Phenolphthalein (8.2–10.0) for weak acid–strong base; methyl orange (3.1–4.4) for strong acid–weak base; for strong–strong, almost anything near pH 7 works.
Why is litmus not used for titrations?
Litmus changes around pH 5–8 (red below ~5, blue above ~8) — a wide, fuzzy range. It is fine for rough 'acid or base?' tests but too vague for titrations, where you need the color change inside the steep jump.
What is universal indicator?
Universal indicator is a blend of several indicators that shifts through a rainbow of colors across pH 1–14. It is great for estimating pH by color, but its gradual changes make it useless for pinpointing a titration endpoint.