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Acid-Base Indicators

Useful Weak Acids and Bases for Acid-Base Indicators

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These are the colors of cabbage juice as a pH indicator.

These test tubes show the range of colors exhibited by red cabbage juice when it is used as a pH indicator.

V. Belkhir
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.

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.

Table of Common Acid-Base Indicators

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
α-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
α-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
α-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

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