ChemWhiz ReferenceAcids & Bases
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.
01Use this when
Choosing an indicator for a titration whose transition range brackets the equivalence-point pH.
02Conditions
The color change spans about pKa ± 1.
03Watch out
Match the indicator to the equivalence pH, not to 7 (weak/strong titrations do not end at 7).
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.
Verification references: ChemWhiz-curated pKa values; Wikipedia, pH indicator.