What are the pH ranges of phenolphthalein, methyl orange and litmus?

Phenolphthalein changes over pH 8.2–10.0: colourless below 8.2, pale pink in the range and deep pink-red above 10, so it stays colourless in acidic and neutral solutions. Methyl orange changes over 3.1–4.4: red below 3.1, orange in between and yellow above 4.4. Litmus changes over about 5.0–8.0: red in acid, purple near neutral and blue in alkali.

Why is an indicator’s range about pKIn ± 1?

For HIn ⇌ H⁺ + In⁻, pH = pKIn + log([In⁻]/[HIn]). You see only the base colour once [In⁻]/[HIn] reaches about 10 and only the acid colour once it falls to about 1/10, which corresponds to pH = pKIn + 1 and pKIn − 1. Measured ranges are not always symmetric because the eye is more sensitive to some colours; methyl orange has pKIn 3.45 but changes over 3.1–4.4.

How do I choose an indicator for a titration?

Pick one whose colour-change range lies wholly or partly inside the steep jump of the titration curve, ideally with pKIn close to the equivalence pH. For 0.1 M strong acid–strong base the jump is pH 4.3–9.7, so methyl orange, methyl red and phenolphthalein all work. A weak acid titrated with NaOH has a basic equivalence point (about 8.7 for acetic acid), so use phenolphthalein; a weak base titrated with HCl has an acidic one (about 5.3 for ammonia), so use methyl orange or methyl red. Litmus is never used because its change is too gradual.

How accurate are universal indicator and red-cabbage colours?

Universal indicator and universal pH paper are mixtures of several indicators that run red–orange–yellow–green–blue–violet from pH 0 to 14 and read to about one pH unit; always compare with the chart supplied with the paper. Red-cabbage juice contains anthocyanins that go red in acid, purple near neutral, blue and green in alkali and yellow in strong alkali. Its exact shades depend on the plant and the extract strength, so it gives only a rough pH.

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🌈Acid-Base Indicator Chart - pH Ranges, Colours, Titration Choice & Theory

An acid–base indicator is a weak acid whose acid form HIn and base form In⁻ have different colours: HIn ⇌ H⁺ + In⁻, and pH = pKIn + log([In⁻]/[HIn]). The eye sees a pure colour only when one form is about ten times the other, so the theoretical colour-change range is pKIn ± 1. This tool lists 15 common indicators with their transition ranges, colours and pKIn: methyl violet 0.0–1.6, thymol blue 1.2–2.8 (red → yellow) and 8.0–9.6 (yellow → blue), methyl yellow, methyl orange 3.1–4.4 (red → yellow), bromophenol blue 3.0–4.6, bromocresol green 3.8–5.4, methyl red 4.4–6.2, litmus 5.0–8.0 (red → blue), bromothymol blue 6.0–7.6, neutral red, phenol red 6.8–8.4, phenolphthalein 8.2–10.0 (colourless → pink), thymolphthalein 9.3–10.5 and alizarin yellow R 10.1–12.0. Drag the pH slider to see each indicator’s colour, whether it shows its acid, intermediate or base colour, and the fraction present as In⁻; a band chart puts every range on one 0–14 scale. At pH 9.0, for example, phenolphthalein has [In⁻]/[HIn] = 10^(9.0 − 9.1) ≈ 0.79, so 44 % is In⁻ and the solution is pale pink; at pH 3.7 methyl orange is about 64 % In⁻ and looks orange. The titration helper works out the ±0.1 % jump range and equivalence pH for strong acid–strong base, weak acid–strong base and strong acid–weak base titrations (0.1 M NaOH into acetic acid: jump 7.76–9.70, equivalence 8.73) or a custom range, and rates each indicator. Universal-indicator and pH-paper colours, red-cabbage colours and mixed indicators such as bromocresol green–methyl red are included.

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How to use Acid–base indicators

1Drag the pH slider or pick a typical solution to see every indicator’s colour and the band chart
2In “Choose for a titration” pick the titration type and enter the concentration and pKa/pKb (or a custom jump range) to see which indicators are suitable
3In “Theory & calculator” choose an indicator to get [In⁻]/[HIn], the fractions of both forms and the δ–pH curve

FAQ

What are the pH ranges of phenolphthalein, methyl orange and litmus?: Phenolphthalein changes over pH 8.2–10.0: colourless below 8.2, pale pink in the range and deep pink-red above 10, so it stays colourless in acidic and neutral solutions. Methyl orange changes over 3.1–4.4: red below 3.1, orange in between and yellow above 4.4. Litmus changes over about 5.0–8.0: red in acid, purple near neutral and blue in alkali.
Why is an indicator’s range about pKIn ± 1?: For HIn ⇌ H⁺ + In⁻, pH = pKIn + log([In⁻]/[HIn]). You see only the base colour once [In⁻]/[HIn] reaches about 10 and only the acid colour once it falls to about 1/10, which corresponds to pH = pKIn + 1 and pKIn − 1. Measured ranges are not always symmetric because the eye is more sensitive to some colours; methyl orange has pKIn 3.45 but changes over 3.1–4.4.
How do I choose an indicator for a titration?: Pick one whose colour-change range lies wholly or partly inside the steep jump of the titration curve, ideally with pKIn close to the equivalence pH. For 0.1 M strong acid–strong base the jump is pH 4.3–9.7, so methyl orange, methyl red and phenolphthalein all work. A weak acid titrated with NaOH has a basic equivalence point (about 8.7 for acetic acid), so use phenolphthalein; a weak base titrated with HCl has an acidic one (about 5.3 for ammonia), so use methyl orange or methyl red. Litmus is never used because its change is too gradual.
How accurate are universal indicator and red-cabbage colours?: Universal indicator and universal pH paper are mixtures of several indicators that run red–orange–yellow–green–blue–violet from pH 0 to 14 and read to about one pH unit; always compare with the chart supplied with the paper. Red-cabbage juice contains anthocyanins that go red in acid, purple near neutral, blue and green in alkali and yellow in strong alkali. Its exact shades depend on the plant and the extract strength, so it gives only a rough pH.

Features

15 common indicators with transition range, acid/base colours and pKIn
pH 0–14 slider showing each indicator’s colour and acid/intermediate/base state
Band chart of all ranges, coloured by subtractive (Beer–Lambert) mixing
Jump range, equivalence pH and indicator rating for strong and weak acid–base titrations
[In⁻]/[HIn], δ(HIn), δ(In⁻) and the distribution curve for any indicator
Universal indicator / pH-paper colour chart and red-cabbage natural indicator
Four mixed indicators with change point, colours and typical use
Key school facts on litmus, phenolphthalein, methyl orange and pH paper

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