Acid-Base Titration Indicator Selector: Color Change & Endpoint Error
Choose the titration, calculate the curve and compare indicators by transition range and expected endpoint error. The SVG demo shows the burette and color change as a teaching animation.
Set up the titration
Recommended indicator
Titration curve and transition interval
The colored band shows the selected indicator transition; vertical lines mark equivalence and the transition midpoint.
Indicator comparison
Ranking uses the volume at the transition midpoint and the volume width of the whole transition interval. A range falling in the steep part of the curve gives a small endpoint error.
| # | Indicator | pH range | V error | Transition width | Rating |
|---|
Teaching animation of the color change
Why does an indicator change color?
An acid-base indicator is itself an acid/base pair whose two forms have different colors:
\[\mathrm{HIn \rightleftharpoons H^+ + In^-}\]
For an ideal indicator the Henderson–Hasselbalch relation applies:
\[\mathrm{pH=pK_{a,ind}+\log\frac{[In^-]}{[HIn]}}\]
When the ratio of the two forms changes roughly from 1:10 to 10:1 the color transition becomes visible; this gives the teaching rule pH ≈ pKa ± 1. Correct selection, however, depends on the titration curve: the transition must lie in its steep region.
Model limits
The tool treats aqueous monoprotic titrations at 25 °C with a strong titrant and ideal activities. It does not include polyprotic acids, non-aqueous solvents, precipitation, complexation, ionic-strength effects, indicator kinetics or indicator mixtures. SVG colors are visual approximations for teaching purposes.
Indicator reference table
| Indicator | pKa | pH range | Acid form | Base form |
|---|---|---|---|---|
| Thymol blue (1st transition) | 1.7 | 1.2–2.8 | red | yellow |
| Bromophenol blue | 4.1 | 3.0–4.6 | yellow | blue-violet |
| Methyl orange | 3.7 | 3.1–4.4 | red | yellow-orange |
| Bromocresol green | 4.7 | 3.8–5.4 | yellow | blue |
| Methyl red | 5.0 | 4.2–6.3 | red | yellow |
| Bromocresol purple | 6.1 | 5.2–6.8 | yellow | purple |
| Bromothymol blue | 7.1 | 6.0–7.6 | yellow | blue |
| Phenol red | 7.8 | 6.8–8.4 | yellow | red |
| Cresol red (2nd transition) | 8.2 | 7.2–8.8 | yellow | red |
| Thymol blue (2nd transition) | 8.9 | 8.0–9.6 | yellow | blue |
| Phenolphthalein | 9.6 | 8.3–10.0 | colorless | pink/fuchsia |
| Thymolphthalein | 10.0* | 9.4–10.6 | colorless | blue |
| Alizarin yellow R | 11.0 | 10.1–12.0 | yellow | orange-red |
Transition ranges and colors are based on OpenStax Chemistry 2e and Chemistry LibreTexts tables. Thymolphthalein pKa is used only as a teaching approximation near the midpoint of the stated transition range; ranking still uses the stated transition interval.