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 pKa is within about 1 pH unit of the titration's equivalence-point pH — the color change then lands on the steep rise of the titration curve.
| Indicator | pKa | pH Range | Acid Color | Base Color |
|---|---|---|---|---|
| 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: match pKa(indicator) to the equivalence-point pH. Weak acid + strong base (equivalence pH 8 to 10): phenolphthalein. Weak base + strong acid (equivalence pH 4 to 6): methyl red or bromocresol green. Strong + strong (equivalence pH 7): 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 — 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.
Source: pKa values: ChemWhiz course-canonical (Topic 23). Color ranges and colors: Wikipedia, pH indicator (CC BY-SA 4.0) (CC BY-SA 4.0)