Acid-Base Indicators: Color Ranges and pKa Values
The common acid-base indicators with their pKa values, color-change intervals, and acid/base colors. Choose an indicator whose full transition interval lies inside the steep region of the titration curve.
Indicator Color Ranges and pKa
Choose an indicator whose transition range brackets the equivalence-point pH of your titration.
| Methyl orange | 3.7 | 3.1 to 4.4 | Red | Yellow |
| Bromocresol green | 4.7 | 3.8 to 5.4 | Yellow | Blue |
| Methyl red | 5.1 | 4.4 to 6.2 | Red | Yellow |
| Litmus | 6.5 | 4.5 to 8.3 | Red | Blue |
| Bromothymol blue | 7.1 | 6.0 to 7.6 | Yellow | Blue |
| Phenol red | 7.9 | 6.4 to 8.0 | Yellow | Red |
| Phenolphthalein | 9.4 | 8.3 to 10.0 | Colorless | Purple-pink |
| Alizarin yellow R | 11.0 | 10.2 to 12.0 | Yellow | Red |
Important Notes
- An indicator is itself a weak acid, HIn ⇌ H⁺ + In⁻, whose acid and base forms have different colors. The eye sees the color change over roughly pKa ± 1 (the [HIn]/[In⁻] ratio swinging from about 10:1 to 1:10).
- Selection rule: place the indicator's full color-change interval inside the steep region of the titration curve. Weak acid + strong base: phenolphthalein. Weak base + strong acid: methyl red or bromocresol green are close matches, while methyl orange can work when its transition remains inside the steep drop. Strong + strong: almost any indicator on this table works because the curve is nearly vertical through pH 3 to 11.
- Methyl orange changes color well before the equivalence point of a weak-acid titration, making it the classic wrong-indicator example.
- Phenolphthalein's listed transition is its first (colorless to pink, pH 8.3 to 10.0); it fades again in very strongly basic solution.
Sources:
- ChemWhiz-curated pKa values, the pKa column
- Wikipedia, pH indicator, color ranges and colors (CC BY-SA 4.0)