| Acid-Base Indicators | |
 | |
What is an acid-base indicator?
An acid-base indicator is a weak acid or a weak base. The undissociated
form of the indicator is a different color than the iogenic form of the
indicator. An Indicator does not change color from pure acid to pure
alkaline at specific hydrogen ion concentration, but rather, color
change occurs over a range of hydrogen ion concentrations. This range is
termed the color change interval. It is expressed as a pH range.
How is an indicator
used?
Weak acids are titrated in the presence of indicators which change under
slightly alkaline conditions. Weak bases should be titrated in the
presence of indicators which change under slightly acidic conditions.
What are some common
acid-base indicators?
Several acid-base indicators are listed below, some more than once if
they can be used over multiple pH ranges. Quantity of indicator in
aqueous (aq.) or alcohol (alc.) solution is specified. Tried-and-true
indicators include: thymol blue, tropeolin OO, methyl yellow, methyl
orange, bromphenol blue, bromcresol green, methyl red, bromthymol blue,
phenol red, neutral red, phenolphthalein, thymolphthalein, alizarin
yellow, tropeolin O, nitramine, and trinitrobenzoic acid. Data in this
table are for sodium salts of thymol blue, bromphenol blue,
tetrabromphenol blue, bromcresol green, methyl red, bromthymol blue,
phenol red, and cresol red.
| Indicator | pH Range | Quantity per 10 ml | Acid | Base |
|---|---|---|---|---|
| Thymol Blue | 1.2-2.8 | 1-2 drops 0.1% soln. in aq. | red | yellow |
| Pentamethoxy red | 1.2-2.3 | 1 drop 0.1% soln. in 70% alc. | red-violet | colorless |
| Tropeolin OO | 1.3-3.2 | 1 drop 1% aq. soln. | red | yellow |
| 2,4-Dinitrophenol | 2.4-4.0 | 1-2 drops 0.1% soln. in 50% alc. | colorless | yellow |
| Methyl yellow | 2.9-4.0 | 1 drop 0.1% soln. in 90% alc. | red | yellow |
| Methyl orange | 3.1-4.4 | 1 drop 0.1% aq. soln. | red | orange |
| Bromphenol blue | 3.0-4.6 | 1 drop 0.1% aq. soln. | yellow | blue-violet |
| Tetrabromphenol blue | 3.0-4.6 | 1 drop 0.1% aq. soln. | yellow | blue |
| Alizarin sodium sulfonate | 3.7-5.2 | 1 drop 0.1% aq. soln. | yellow | violet |
| a-Naphthyl red | 3.7-5.0 | 1 drop 0.1% soln. in 70% alc. | red | yellow |
| p-Ethoxychrysoidine | 3.5-5.5 | 1 drop 0.1% aq. soln. | red | yellow |
| Bromcresol green | 4.0-5.6 | 1 drop 0.1% aq. soln. | yellow | blue |
| Methyl red | 4.4-6.2 | 1 drop 0.1% aq. soln. | red | yellow |
| Bromcresol purple | 5.2-6.8 | 1 drop 0.1% aq. soln. | yellow | purple |
| Chlorphenol red | 5.4-6.8 | 1 drop 0.1% aq. soln. | yellow | red |
| Bromphenol blue | 6.2-7.6 | 1 drop 0.1% aq. soln. | yellow | blue |
| p-Nitrophenol | 5.0-7.0 | 1-5 drops 0.1% aq. soln. | colorless | yellow |
| Azolitmin | 5.0-8.0 | 5 drops 0.5% aq. soln. | red | blue |
| Phenol red | 6.4-8.0 | 1 drop 0.1% aq. soln. | yellow | red |
| Neutral red | 6.8-8.0 | 1 drop 0.1% soln. in 70% alc. | red | yellow |
| Rosolic acid | 6.8-8.0 | 1 drop 0.1% soln. in 90% alc. | yellow | red |
| Cresol red | 7.2-8.8 | 1 drop 0.1% aq. soln. | yellow | red |
| a-Naphtholphthalein | 7.3-8.7 | 1-5 drops 0.1% soln. in 70% alc. | rose | green |
| Tropeolin OOO | 7.6-8.9 | 1 drop 0.1% aq. soln. | yellow | rose-red |
| Thymol blue | 8.0-9.6 | 1-5 drops 0.1% aq. soln. | yellow | blue |
| Phenolphthalein | 8.0-10.0 | 1-5 drops 0.1% soln. in 70% alc. | colorless | red |
| a-Naphtholbenzein | 9.0-11.0 | 1-5 drops 0.1% soln. in 90% alc. | yellow | blue |
| Thymolphthalein | 9.4-10.6 | 1 drop 0.1% soln. in 90% alc. | colorless | blue |
| Nile blue | 10.1-11.1 | 1 drop 0.1% aq. soln. | blue | red |
| Alizarin yellow | 10.0-12.0 | 1 drop 0.1% aq. soln. | yellow | lilac |
| Salicyl yellow | 10.0-12.0 | 1-5 drops 0.1% soln. in 90% alc. | yellow | orange-brown |
| Diazo violet | 10.1-12.0 | 1 drop 0.1% aq. soln. | yellow | violet |
| Tropeolin O | 11.0-13.0 | 1 drop 0.1% aq. soln. | yellow | orange-brown |
| Nitramine | 11.0-13.0 | 1-2 drops 0.1% soln in 70% alc. | colorless | orange-brown |
| Poirrier's blue | 11.0-13.0 | 1 drop 0.1% aq. soln. | blue | violet-pink |
| Trinitrobenzoic acid | 12.0-13.4 | 1 drop 0.1% aq. soln. | colorless | orange-red |
Primary References:
Lange's Handbook of Chemistry, 8th Edition, Handbook Publishers
Inc., 1952.
Volumetric Analysis, Kolthoff & Stenge, Interscience
Publishers, Inc., New York, 1942 and 1947.
Related Resources:
Introduction
to Qualitative Analysis
Acids & Bases
Analytical Chemistry
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