Common Acid-Base (pH) Indicators — Colors and Transition Ranges
| Indicator | pH Range Low | pH Range High | Color in Acid | Color in Base | pKₐ (approx.) | Common Use |
|---|---|---|---|---|---|---|
| Thymol blue (1st transition) | 1.2 | 2.8 | Red | Yellow | 1.7 | Strongly acidic solutions |
| Methyl violet | 0 | 1.6 | Yellow | Violet | 0.8 | Very strong acid detection |
| Methyl orange | 3.1 | 4.4 | Red | Yellow-orange | 3.5 | Strong acid–strong base titrations; strong acid–weak base titrations |
| Bromocresol green | 3.8 | 5.4 | Yellow | Blue | 4.7 | Titrations with equivalence point near pH 4–5 |
| Methyl red | 4.4 | 6.2 | Red | Yellow | 5 | Strong acid–weak base titrations; Kjeldahl nitrogen determination |
| Litmus | 5 | 8 | Red | Blue | 6.5 | General acid/base screening; litmus paper tests |
| Bromothymol blue | 6 | 7.6 | Yellow | Blue | 7.1 | Strong acid–strong base titrations near neutral; aquarium and pool pH testing |
| Phenol red | 6.8 | 8.4 | Yellow | Red | 7.9 | Biological media pH monitoring; swimming pool testing |
| Cresol red | 7.2 | 8.8 | Yellow | Red | 8.3 | Slightly basic solutions |
| Thymol blue (2nd transition) | 8 | 9.6 | Yellow | Blue | 8.9 | Weakly basic solutions |
| Phenolphthalein | 8.2 | 10 | Colorless | Pink/magenta | 9.4 | Strong acid–strong base titrations; weak acid–strong base titrations |
| Thymolphthalein | 9.3 | 10.5 | Colorless | Blue | 10 | Strongly basic solutions; carbonation depth testing in concrete |
| Alizarin yellow R | 10.1 | 12 | Yellow | Orange-red | 11 | Strongly basic titrations |
| Indigo carmine | 11.4 | 13 | Blue | Yellow | 12.2 | Very strongly basic solutions |
| Universal indicator | 1 | 14 | Red (pH 1–3) | Violet (pH 11–14) | — | Full pH range estimation; educational demonstrations |
An acid-base indicator is a weak acid (HIn) or weak base whose protonated and deprotonated forms absorb light at different wavelengths. The color shift spans roughly pKa ± 1 pH unit. Tabulated pKa and transition ranges follow CRC Handbook values at 25 °C in dilute aqueous solution; both shift with temperature, ionic strength, and the presence of mixed solvents. Note that universal indicator is a calibrated mixture of several dyes (commonly methyl red, bromothymol blue, thymol blue, and phenolphthalein) and gives only a rough pH read — it's not a substitute for a single indicator at a sharp endpoint.
Frequently Asked Questions
How do you choose the right indicator for a titration?
Match the indicator's transition range to the pH at the equivalence point of your titration. For strong-acid/strong-base (equivalence at pH 7), bromothymol blue or phenolphthalein both work because the pH shoots through several units in a single drop near equivalence. For weak-acid/strong-base (equivalence above 7, often around 8-9), phenolphthalein is the standard pick. For strong-acid/weak-base (equivalence below 7), use methyl orange or methyl red. Avoid an indicator that changes color in the buffer region — you'll get a fuzzy endpoint instead of a sharp one.
Why does phenolphthalein turn pink in basic solutions?
Below about pH 8.2, phenolphthalein sits in a colorless lactone form. As you add base, it loses two protons and rearranges into a quinoid structure with extended conjugation; that structure absorbs in the green part of the visible spectrum, so the solution looks pink to magenta. Push the pH above about 12 and the dye converts to a triply deprotonated carbinol that's colorless again — useful to know when titrating into very strongly basic solution, because the pink color you watched for can fade as you overshoot.
What is a universal indicator and how does it work?
A universal indicator is a calibrated cocktail of several dyes — typically methyl red, bromothymol blue, thymol blue, and phenolphthalein — that together give a continuous color sweep from red through violet across pH 1-14. Rough color zones: red 1-3, orange 3-5, yellow 5-7, green 7-9, blue 9-11, violet 11-14. It's what's impregnated in pH paper. Treat it as a quick screen for ballpark pH; for an actual titration endpoint or anything more precise than ±1 pH unit, use a single indicator matched to your equivalence point or a pH meter.