Acid-Base Indicators: Choosing the Right One for a Titration

By Aniket Bhardwaj · 18 October 2026 · Updated 18 October 2026 · Class 11–12 Practical Chemistry

In a titration, the colour change of the indicator tells you when to stop. If you choose the wrong indicator, the colour changes at the wrong volume and your answer is wrong, even if every calculation is perfect. Class 11 and 12 practicals, and viva questions, often ask: “Why phenolphthalein for this titration and methyl orange for that one?” The answer comes from the pH at the equivalence point.

A pH number line shows the equivalence point at 8.72 inside phenolphthalein’s 8.3–10.0 range, while methyl orange’s 3.1–4.4 range lies before it. — Phenolphthalein fits the weak acid–strong base equivalence point at pH 8.72.

The rule

An indicator HIn changes colour over a range of about: pH = pKin ± 1
Choose an indicator whose range lies on the steep part of the titration curve, around the equivalence-point pH.

What this means

Equivalence point – the moment when the acid and base have reacted in exactly the amounts given by the balanced equation.
End point – the moment when the indicator changes colour. We want end point ≈ equivalence point.
pKin – pKa of the indicator, which is itself a weak acid. The colour changes when [HIn] and [In⁻] are comparable.
Steep part – near the equivalence point the pH changes by several units for a single drop of titrant. Any indicator that changes inside this jump gives an accurate result.

Common indicators (approximate ranges)

Indicator	Approximate pH range	Colour change
Methyl orange	3.1 – 4.4	red → yellow
Methyl red	4.4 – 6.2	red → yellow
Phenolphthalein	8.3 – 10.0	colourless → pink

Textbooks quote slightly different limits (for example 8.2–10.0 for phenolphthalein). Use the values in your own book for answers.

Worked examples: find the equivalence pH, then choose

Example 1: strong acid + strong base. 25.0 mL of 0.100 M HCl is titrated with 0.100 M NaOH. Find the pH near the equivalence point (25.0 mL).

At 24.9 mL of NaOH added: acid left = (25.0 − 24.9) × 0.100 = 0.0100 mmol in 49.9 mL. [H⁺] = 0.0100/49.9 = 2.004 × 10⁻⁴ M, so pH = 3.70.
At 25.0 mL: only NaCl is present, pH = 7.00.
At 25.1 mL: base left = 0.0100 mmol in 50.1 mL. [OH⁻] = 1.996 × 10⁻⁴ M, pOH = 3.70, pH = 10.30.
The pH jumps from 3.70 to 10.30 for just 0.2 mL. Methyl orange, methyl red and phenolphthalein all change inside this jump, so any of them works.

Example 2: weak acid + strong base. 25.0 mL of 0.100 M acetic acid (Ka = 1.8 × 10⁻⁵) with 0.100 M NaOH. Find the pH at equivalence.

At 25.0 mL NaOH, all acid becomes sodium acetate. Total volume = 50.0 mL, so [CH₃COO⁻] = 2.50 mmol / 50.0 mL = 0.0500 M.
Kb of acetate = Kw/Ka = 1.0 × 10⁻¹⁴ / 1.8 × 10⁻⁵ = 5.56 × 10⁻¹⁰.
[OH⁻] = √(5.56 × 10⁻¹⁰ × 0.0500) = √(2.78 × 10⁻¹¹) = 5.27 × 10⁻⁶ M.
pOH = 5.28, so pH = 8.72.
The equivalence point is on the basic side. Phenolphthalein (8.3–10.0) fits. Methyl orange (3.1–4.4) would change colour long before, giving a volume that is too small.

Example 3: weak base + strong acid. 25.0 mL of 0.100 M NH₃ (Kb = 1.8 × 10⁻⁵) with 0.100 M HCl. Find the pH at equivalence.

At 25.0 mL HCl, all NH₃ becomes NH₄⁺ at 2.50 mmol / 50.0 mL = 0.0500 M.
Ka of NH₄⁺ = Kw/Kb = 5.56 × 10⁻¹⁰.
[H⁺] = √(5.56 × 10⁻¹⁰ × 0.0500) = 5.27 × 10⁻⁶ M, so pH = 5.28.
The equivalence point is acidic. Methyl red (4.4–6.2) fits; methyl orange (3.1–4.4) is the usual school choice for a strong-acid/weak-base titration. Phenolphthalein would turn too early or fail here.

Example 4: half-way point. In Example 2, after 12.5 mL of NaOH, half of the acetic acid is converted. Then [CH₃COOH] = [CH₃COO⁻], and pH = pKa = −log(1.8 × 10⁻⁵) = 4.74. This is a useful way to find the Ka of a weak acid from a titration graph.

Summary table

Titration	pH at equivalence	Suitable indicator
Strong acid + strong base	7.00	Any of the three above
Weak acid + strong base	Above 7 (8.72 in Example 2)	Phenolphthalein
Strong acid + weak base	Below 7 (5.28 in Example 3)	Methyl red / methyl orange
Weak acid + weak base	No sharp jump	No simple indicator works well; use a pH meter

Mistakes to avoid

Saying the equivalence point is always pH 7. It is 7 only when both acid and base are strong.
Confusing end point with equivalence point. The indicator gives the end point; we hope it is close to equivalence.
Forgetting that the total volume doubles at equivalence (25 + 25 = 50 mL), so the salt concentration is halved.
Adding too much indicator. Indicators are weak acids or bases themselves; two or three drops are enough.
Using the pH = pKa shortcut at the wrong point: it works only at the half-way point of a weak acid.

Exam relevance

Where	What to expect
Class 11–12 practical exam and viva	Which indicator for which titration, and why. Your school or board will state the exact experiments.
Written papers	Short answers on equivalence pH and the hydrolysis of the salt formed. See your latest syllabus for the chapter.

Use the pH tool to check the pH of a solution at any point on your titration.

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