List of Acid-Base Indicators

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 Indicators Are 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. 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 Langes 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 -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 -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 -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 Poirriers 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