Choosing an Indicator for Titration

Introduction

Universal indicator is excellent for estimating pH, so why is it almost never used in a titration? A titration depends on spotting the exact drop at which the acid is neutralised. An indicator that drifts gradually through red, orange, yellow, green and blue gives no single moment to stop. Titrations need a single indicator with one clear, sudden colour change that happens right in the steep part of the pH curve. This page explains how to choose one.

Core explanation

What makes a good titration indicator. A good indicator has two clearly different colours and changes between them over a narrow pH range, typically about 1.5–2 pH units. If that range falls inside the vertical section of the pH curve, a single drop of titrant pushes the pH right through it, and the colour switches instantly.

Two common indicators.

Indicator Colour in acid Colour in alkali Approximate change range --- --- --- --- Methyl orange red yellow pH 3.1–4.4 Phenolphthalein colourless pink pH 8.3–10.0 Litmus red blue pH 5–8 (gradual)

Why universal indicator fails. Universal indicator is a mixture of several indicators designed to show many different colours across the whole pH range. During a titration it passes through a series of shades, and it is hard to decide which shade counts as "neutral". Litmus is also a poor choice because its colour change is gradual and the purple intermediate is hard to judge.

Matching the indicator to the curve. For a strong acid and strong alkali , the vertical section runs from about pH 3 to pH 11. Both methyl orange (3.1–4.4) and phenolphthalein (8.3–10) change within this range, so either gives the same titre to within a drop.

For a weak acid and strong alkali (for example ethanoic acid with sodium hydroxide), the vertical section is shorter and higher, roughly pH 7 to 11. Phenolphthalein fits; methyl orange would change far too early, while plenty of acid remained.

For a strong acid and weak alkali (for example hydrochloric acid with ammonia solution), the vertical section is lower, roughly pH 3 to 7. Methyl orange fits; phenolphthalein would not change until well after the end point.

For a weak acid and weak alkali , there is no sharp vertical section at all, so no simple indicator gives a clear end point; a pH meter is used instead.

Practical details. Only two or three drops of indicator are used, because indicators are themselves weak acids and a large amount would react with the titrant. The colour change is easiest to judge from colourless to pink (phenolphthalein) or from yellow to the first orange tint (methyl orange).

Step-by-step reasoning

To choose an indicator:

1. Identify the acid and alkali as strong or weak. 2. Sketch or recall the vertical section of the pH curve. 3. Compare it with each indicator's colour-change range. 4. Choose an indicator whose whole range lies inside the vertical section.

Visual explanation

Draw a pH curve and shade two horizontal bands across it: one at pH 3.1–4.4 for methyl orange and one at pH 8.3–10 for phenolphthalein. Any band that crosses the vertical cliff of the curve marks a suitable indicator. The indicator simulation shows each colour change as the pH passes through the band.

Real-world analogy

A good indicator is like a light switch rather than a dimmer. A dimmer changes slowly, so you cannot say when the light is "on". A switch flips at one precise point — which is exactly what you need to decide when to close the burette tap.

Real-world example

In food testing laboratories, the acidity of vinegar or fruit juice, which contains weak acids, is found by titration with sodium hydroxide using phenolphthalein. For strongly coloured samples such as red wine, where a colour change is hard to see, a pH meter is used instead.

Why?

Why does a narrow range give a sharp change? In the vertical section, one drop changes pH by several units. If the indicator's whole range is only about 1.5 units wide and lies inside that jump, the single drop carries the solution from one colour to the other at once.

Common misconception

"Phenolphthalein turns pink exactly at pH 7." Phenolphthalein stays colourless until about pH 8.3. It still works for strong acid–strong alkali titrations because the pH jumps from about 4 to 10 within a drop or two.

Worked example

Question: Choose an indicator for titrating ethanoic acid with sodium hydroxide solution, and explain.

Reasoning: Ethanoic acid is weak and sodium hydroxide is strong, so the vertical section is at about pH 7–11. Phenolphthalein changes at 8.3–10, inside that range; methyl orange changes at 3.1–4.4, outside it.

Answer: Phenolphthalein, because its colour change range lies within the steep part of the pH curve.

Quick check

1. What colour is phenolphthalein in acid, and what colour is it in alkali? Answer: Colourless in acid and pink in alkali.

Exam focus

Learn the colours of methyl orange and phenolphthalein in acid and alkali, and be ready to explain why universal indicator is not used for titrations. Higher-level questions ask you to choose an indicator from a table of ranges and a pH curve.

Advanced insight

An indicator is a weak acid, HIn, whose un-ionised form and ion, In⁻, have different colours: HIn ⇌ H⁺ + In⁻. Adding H⁺ shifts the balance to HIn; removing H⁺ shifts it to In⁻. The colour changes when the two forms are present in similar amounts, which happens near a pH set by the indicator's own acid strength.

Summary

A titration indicator must change colour sharply over a narrow pH range lying within the vertical section of the pH curve. Methyl orange (red to yellow, pH 3.1–4.4) suits strong acid–weak alkali titrations; phenolphthalein (colourless to pink, pH 8.3–10) suits weak acid–strong alkali. Either works for strong–strong. Universal indicator changes gradually and is unsuitable.

Practice questions

1. Why is universal indicator not used in titrations? Answer: It changes colour gradually through many shades, so there is no single sharp colour change to mark the end point. 2. State the colour of methyl orange in acid and in alkali. Answer: Red in acid and yellow in alkali. 3. Which indicator should be used to titrate hydrochloric acid with ammonia solution? Explain. Answer: Methyl orange, because the vertical section is at low pH (about 3–7), which includes its change range of 3.1–4.4. 4. Why are only a few drops of indicator added? Answer: Indicators are weak acids themselves, so a large amount would react with the titrant and affect the titre; a few drops also give a clear colour.