Indicator colors and how this tool decides them
Acid–base indicators are weak acids or bases that have two differently colored forms.
For a typical indicator written as HIn ⇌ H⁺ + In⁻:
- Acid form (HIn) shows the “lower-pH” color.
- Base form (In⁻) shows the “higher-pH” color.
At equilibrium the color you see is set by the ratio of the two forms, which (like every weak acid/base pair) obeys the Henderson–Hasselbalch relation
\( \displaystyle \text{pH} = \text{p}K_a + \log_{10}\!\big([\text{In}^-]/[\text{HIn}]\big) \).
When pH ≪ pKa, the acid form dominates (lower-pH color); when pH ≫ pKa the base form dominates (higher-pH color).
Transition range & endpoint
The color change is visible over a finite interval—the transition range—which is typically
about one pH unit on either side of pKa:
\(\text{pH} \approx \text{p}K_a \pm 1\).
We show this as “pH range” in the table. The endpoint (the visual midpoint of the change) is taken as the center of that interval,
very close to pKa.
From pOH to pH
If you enter pOH, the calculator converts using \( \text{pH} = \text{p}K_w - \text{pOH} \) with your chosen \(K_w\)
(default \(1.0\times 10^{-14}\) at 25 °C; \(\text{p}K_w\approx 14.00\)).
How the table is computed
- We convert your input to pH (if needed).
- For each indicator we compare pH with its published transition range:
- pH < low → predicted color = lower-pH (acid) color.
- low ≤ pH ≤ high → predicted color = “transition” (a mix of both colors).
- pH > high → predicted color = higher-pH (base) color.
- We also compute a suitability score (distance of pH from the indicator’s endpoint and whether pH falls inside the range) to recommend the top candidates.
Choosing an indicator (practical rules)
- Match the equivalence pH. Pick an indicator whose transition range straddles the expected equivalence point:
- Strong acid vs strong base: endpoint ≈ 7 → e.g., bromothymol blue.
- Weak acid vs strong base: endpoint > 7 → e.g., phenolphthalein, cresol purple.
- Strong acid vs weak base: endpoint < 7 → e.g., methyl orange, methyl red.
- Prefer a narrow range near the target pH for a sharper endpoint.
- Consider solution color & turbidity. If the sample is colored or cloudy, favor indicators with vivid, contrasting colors (phenolphthalein’s colorless→pink is popular).
- Temperature & ionic strength. Indicator pKa (and thus ranges) shift slightly with T and medium; the listed values are for ~25 °C aqueous solutions.
Quick examples
- pH = 5.1: indicators centered near 5 (e.g., bromocresol green 3.8–5.4, methyl red 4.4–6.2) will show a clear change; predicted color is near the middle of their transition.
- pH = 9.2: phenolphthalein (8.2–10.0) and cresol purple (7.4–9.0) are suitable; phenolphthalein will be distinctly pink above ~9.
Reference ranges used by the tool
Typical literature values at ~25 °C (colors are approximate). Exact shades depend on concentration and medium.
| Indicator |
Lower-pH color |
pH range |
Endpoint |
Higher-pH color |
| Thymol blue (1st) | red | 1.2–2.8 | 2.0 | yellow |
| Methyl orange | red | 3.1–4.4 | 3.75 | orange–yellow |
| Bromocresol green | yellow | 3.8–5.4 | 4.6 | blue-green |
| Methyl red | red | 4.4–6.2 | 5.3 | yellow |
| Bromothymol blue | yellow | 6.0–7.6 | 7.1 | blue |
| Phenol red | yellow | 6.4–8.0 | 7.2 | red |
| Cresol purple | yellow | 7.4–9.0 | 8.2 | purple |
| Phenolphthalein | colorless | 8.2–10.0 | 9.1 | pink |
| Thymolphthalein | colorless | 9.3–10.5 | 9.9 | blue |
| Alizarin Yellow R | yellow | 10.1–12.0 | 11.05 | red |
| Litmus (broad) | red | 4.5–8.3 | 6.4 | blue |
Why our “Top indicators” are chosen
We score each indicator by (i) whether the pH lies inside its transition range and (ii) how close pH is to the range midpoint (endpoint).
Indicators with pH inside the range and small distance to the midpoint get the highest scores. Ties are broken by narrower ranges.
Note: The calculator assumes standard aqueous conditions near 25 °C and typical indicator concentrations
(≈10−4–10−3 M). Extremely dilute indicators or non-aqueous solvents may shift the apparent colors and ranges.
