Application
pH indicators are frequently employed in titrations in analytical chemistry and biology to determine the extent of a chemical reaction. Because of the subjective choice (determination) of color, pH indicators are susceptible to imprecise readings. For applications requiring precise measurement of pH, a pH meter is frequently used. Sometimes a blend of different indicators is used to achieve several smooth color changes over a wide range of pH values. These commercial indicators (e.g., universal indicator and Hydrion papers) are used when only rough knowledge of pH is necessary.
Tabulated below are several common laboratory pH indicators. Indicators usually exhibit intermediate colors at pH values inside the listed transition range. For example, phenol red exhibits an orange color between pH 6.8 and pH 8.4. The transition range may shift slightly depending on the concentration of the indicator in the solution and on the temperature at which it is used.
| Indicator | Low pH color | Transition pH range | High pH color |
|---|---|---|---|
| Gentian violet (Methyl violet 10B) | yellow | 0.0–2.0 | blue-violet |
| Leucomalachite green (first transition) | yellow | 0.0–2.0 | green |
| Leucomalachite green (second transition) | green | 11.6–14 | colorless |
| Thymol blue (first transition) | red | 1.2–2.8 | yellow |
| Thymol blue (second transition) | yellow | 8.0–9.6 | blue |
| Methyl yellow | red | 2.9–4.0 | yellow |
| Bromophenol blue | yellow | 3.0–4.6 | purple |
| Congo red | blue-violet | 3.0–5.0 | red |
| Methyl orange | red | 3.1–4.4 | yellow |
| Screened methyl orange (first transition) | red | 0.0–3.2 | grey |
| Screened methyl orange (second transition) | grey | 3.2–4.2 | green |
| Bromocresol green | yellow | 3.8–5.4 | blue |
| Methyl red | red | 4.4–6.2 | yellow |
| Azolitmin | red | 4.5–8.3 | blue |
| Bromocresol purple | yellow | 5.2–6.8 | purple |
| Bromothymol blue | yellow | 6.0–7.6 | blue |
| Phenol red | yellow | 6.4–8.0 | red |
| Neutral red | red | 6.8–8.0 | yellow |
| Naphtholphthalein | colorless to reddish | 7.3–8.7 | greenish to blue |
| Cresol Red | yellow | 7.2–8.8 | reddish-purple |
| Phenolphthalein | colorless | 8.3–10.0 | fuchsia |
| Thymolphthalein | colorless | 9.3–10.5 | blue |
| Alizarine Yellow R | yellow | 10.2–12.0 | red |
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