Acid-Base Indicators

A series of different liquids, being Ph-tested with universal indicator paper
Dorling Kindersley/Getty Images

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. The 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

IndicatorpH RangeQuantity per 10 mlAcidBase
Thymol Blue1.2-2.81-2 drops 0.1% soln. in aq.redyellow
Pentamethoxy red1.2-2.31 drop 0.1% soln. in 70% alc.red-violetcolorless
Tropeolin OO1.3-3.21 drop 1% aq. soln.redyellow
2,4-Dinitrophenol2.4-4.01-2 drops 0.1% soln. in 50% alc.colorlessyellow
Methyl yellow2.9-4.01 drop 0.1% soln. in 90% alc.redyellow
Methyl orange3.1-4.41 drop 0.1% aq. soln.redorange
Bromphenol blue3.0-4.61 drop 0.1% aq. soln.yellowblue-violet
Tetrabromphenol blue3.0-4.61 drop 0.1% aq. soln.yellowblue
Alizarin sodium sulfonate3.7-5.21 drop 0.1% aq. soln.yellowviolet
α-Naphthyl red3.7-5.01 drop 0.1% soln. in 70% alc.redyellow
p-Ethoxychrysoidine3.5-5.51 drop 0.1% aq. soln.redyellow
Bromcresol green4.0-5.61 drop 0.1% aq. soln.yellowblue
Methyl red4.4-6.21 drop 0.1% aq. soln.redyellow
Bromcresol purple5.2-6.81 drop 0.1% aq. soln.yellowpurple
Chlorphenol red5.4-6.81 drop 0.1% aq. soln.yellowred
Bromphenol blue6.2-7.61 drop 0.1% aq. soln.yellowblue
p-Nitrophenol5.0-7.01-5 drops 0.1% aq. soln.colorlessyellow
Azolitmin5.0-8.05 drops 0.5% aq. soln.redblue
Phenol red6.4-8.01 drop 0.1% aq. soln.yellowred
Neutral red6.8-8.01 drop 0.1% soln. in 70% alc.redyellow
Rosolic acid6.8-8.01 drop 0.1% soln. in 90% alc.yellowred
Cresol red7.2-8.81 drop 0.1% aq. soln.yellowred
α-Naphtholphthalein7.3-8.71-5 drops 0.1% soln. in 70% alc.rosegreen
Tropeolin OOO7.6-8.91 drop 0.1% aq. soln.yellowrose-red
Thymol blue8.0-9.61-5 drops 0.1% aq. soln.yellowblue
Phenolphthalein8.0-10.01-5 drops 0.1% soln. in 70% alc.colorlessred
α-Naphtholbenzein9.0-11.01-5 drops 0.1% soln. in 90% alc.yellowblue
Thymolphthalein9.4-10.61 drop 0.1% soln. in 90% alc.colorlessblue
Nile blue10.1-11.11 drop 0.1% aq. soln.bluered
Alizarin yellow10.0-12.01 drop 0.1% aq. soln.yellowlilac
Salicyl yellow10.0-12.01-5 drops 0.1% soln. in 90% alc.yelloworange-brown
Diazo violet10.1-12.01 drop 0.1% aq. soln.yellowviolet
Tropeolin O11.0-13.01 drop 0.1% aq. soln.yelloworange-brown
Nitramine11.0-13.01-2 drops 0.1% soln in 70% alc.colorlessorange-brown
Poirrier's blue11.0-13.01 drop 0.1% aq. soln.blueviolet-pink
Trinitrobenzoic acid12.0-13.41 drop 0.1% aq. soln.colorlessorange-red