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ACID-BASE INDICATORS A. K. Covington The first part of this table lists some common acid-base indica- tors in alphabetical order along with the approximate pH range(s) at which a color change occurs. Following this is a table of the same indicators ordered by pH range, which includes the nature of the color change, instructions on preparation of the indicator solu- tion, and the acid dissociation constant p K , when available. The color code is: C = colorless A = amber B/G = blue-green Pk = pink Y = yellow V = violet R = red B = blue P = purple O = orange Reference Bishop, E., Ed., Indicators , Pergamon, Oxford, 1972.
Indicator pH Range Alizarin 5.6-7.2; 11.0-12.4 Alizarin Red S 4.6-6.0 Alizarin Yellow R 10.1-12.0 Benzopurpurine 4B 2.2-4.2 4,4’-Bis(2-amino-1- naphthylazo)-2,2’- stilbenedisulfonic acid 3.0-4.0 4,4’-Bis(4-amino-1- naphthylazo)-2,2’- stilbenedisulfonic acid 8.0-9.0 Brilliant Yellow 6.6-7.8 Bromocresol Green 3.8-5.4 Bromocresol Purple 5.2-6.8 Bromophenol Blue 3.0-4.6 Bromothymol Blue 6.0-7.6 Chlorophenol Red 5.2-6.8 Clayton Yellow 12.2-13.2 Congo Red 3.0-5.0 o -Cresolphthalein 8.2-9.8 Cresol Red 0.0-1.0; 7.0-8.8 Crystal Violet 0.0-1.8 Curcumin (Turmaric) 7.4-8.6 p -(2,4-Dihydroxyphenylazo) benzenesulfonic acid, sodium salt 11.4-12.6 p -Dimethylaminoazobenzene 2.8-4.4 4-(4-Dimethylamino-1-naphylazo)-3- methoxybenzenesulfonic acid 3.5-4.8 2-( p -Dimethylamino-phenylazo)pyridine 0.2-1.8; 4.4-5.6 N,N -Dimethyl- p -( m -tolylazo)aniline 2.6-4.8 2,4-Dinitrophenol 2.0-4.7 2-(2,4 Dinitrophenylazo)-1-naphthol-3,6- disulfonic acid, disodium salt 6.0-7.0 6,8-Dinitro-2,4-(1 H )quinazolinedione 6.4-8.0 Erythrosin, disodium salt 2.2-3.6 4-( p -Ethoxyphenylazo)- m -phenylene-diamine monohydrochloride 4.4-5.8 Indicator pH Range Ethyl bis(2,4-dimethylphenyl)ethanoate 8.4-9.6 Ethyl Orange 3.4-4.8 Ethyl Red 4.0-5.8 Ethyl Violet 0.0-2.4 5,5’-Indigodisulfonic acid, disodium salt 11.4-13.0 Malachite Green 0.2-1.8 Metacresol Purple 1.2-2.8; 7.4-9.0 Metanil Yellow 1.2-2.4 Methyl Green 0.2-1.8 Methyl Orange 3.2-4.4 Methyl Red 4.8-6.0 Methyl Violet 0.0-1.6 p -Naphtholbenzein 8.2-10.0 Neutral Red 6.8-8.0 p -Nitrophenol 5.4-6.6 m -Nitrophenol 6.8-8.6 Orange IV 1.4-2.8 Paramethyl Red 1.0-3.0 Phenolphthalein 8.2-10.0 Phenol Red 6.6-8.0 4-Phenylazodiphenylamine 1.2-2.6 4-Phenylazo-1-naphthylamine 4.0-5.6 Propyl Red 4.8-6.6 Quinaldine Red 1.4-3.2 Resazurin 3.8-6.4 Resorcin Blue 4.4-6.2 Tetrabromophenolphthalein ethyl ester, potassium salt 3.0-4.2 Thymol Blue 1.2-2.8; 8.0-9.6 Thymolphthalein 9.4-10.6 4- o -Tolylazo- o -toluidine 1.4-2.8 1,3,5-Trinitrobenzene 12.0-14.0 2,4,6-Trinitrotoluene 11.5-13.0 Turmaric 7.4-8.6 8-15
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8-16 Acid-Base Indicators Color change pH range Indicator p K Preparation 0.0-1.0 R-Y Cresol Red 0.1 g in 26.2 mL 0.01 M NaOH + 223.8 mL water 0.0-1.6 Y-B Methyl Violet 0.01-0.05% in water 0.0-1.8 Y-B Crystal Violet 0.02% in water 0.0-2.4 Y-B Ethyl Violet 0.1 g in 50 mL 50% v/v methanol-water 0.2-1.8 Y-B/G Malachite Green 1.3 water 0.2-1.8 Y-B Methyl Green 0.1% in water 0.2-1.8 Y-R 2-( p -Dimethylaminophenylazo)pyridine 0.1% in ethanol 1.0-3.0 R-Y Paramethyl Red ethanol 1.2-2.4 R-Y Metanil Yellow 0.01% in water 1.2-2.6 R-Y 4-Phenylazodiphenylamine 0.01 g in 1 mL 1 M HCl + 50 mL ethanol + 49 mL water 1.2-2.8 R-Y Thymol Blue 1.65 0.1 g in 21.5 mL 0.01 M NaOH + 228.5 mL water 1.2-2.8 R-Y Metacresol Purple 1.51 0.1 g in 26.2 mL 0.01 M NaOH + 223.8 mL water 1.4-2.8 R-Y Orange IV 0.01% in water 1.4-2.8 O-Y 4- o -Tolylazo- o -toluidine water 1.4-3.2 C-R Quinaldine Red 2.63 1% in ethanol 2.0-4.7 C-Y 2,4-Dinitrophenol 3.96 sat.
solution in water 2.2-3.6 O-R Erythrosin, disodium salt 0.1% in water 2.2-4.2 V-R Benzopurpurine 4B 0.1% in water 2.6-4.8 R-Y N,N -Dimethyl- p -( m -tolylazo)aniline 0.1% in water 2.8-4.4 R-Y p -Dimethylaminoazobenzene 0.1 g in 100 mL 90% v/v ethanol-water 3.0-4.0 P-R 4,4’-Bis(2-amino-1-naphthylazo)-2,2’- 0.1 g in 5.9 mL 0.05 M NaOH + 94.1 mL water stilbenedisulfonic acid 3.0-4.2 Y-B Tetrabromophenolphthalein ethyl ester, 0.1% in ethanol potassium salt 3.0-4.6 Y-B Bromophenol Blue 4.10 0.1 g in 14.9 mL 0.01 M NaOH + 235.1 mL water 3.0-5.0 B-R Congo Red 0.1% in water 3.2-4.4 R-Y Methyl Orange 3.46 0.1% in water 3.4-4.8 R-Y Ethyl Orange 4.34 0.05-0.2% in water or aqueous ethanol 3.5-4.8 V-Y 4-(4-Dimethylamino-1-naphylazo)-3- methoxybenzenesulfonic acid 0.1% in 60% ethanol-water 3.8-5.4 Y-B Bromocresol Green 4.90 0.1 g in 14.3 mL 0.01 M NaOH + 235.7 mL water 3.8-6.4 O-V Resazurin water 4.0-5.6 R-Y 4-Phenylazo-1-naphthylamine 0.1% in ethanol 4.0-5.8 C-R Ethyl Red 5.42 0.1 g in 100 mL 50% v/v methanol-water 0.1% in ethanol 4.4-5.6 R-Y 2-( p -Dimethylaminophenylazo)pyridine 0.1% in ethanol 4.4-5.8 O-Y 4-( p -Ethoxyphenylazo)- m -phenylene- diamine monohydrochloride 0.1% in water 4.4-6.2 R-B Resorcin Blue 0.2% in ethanol 4.6-6.0 Y-R Alizarin Red S water 4.8-6.0 R-Y Methyl Red 5.00 0.02 g in 100 mL 60% v/v ethanol-water 4.8-6.6 R-Y Propyl Red 5.48 ethanol 5.2-6.8 Y-P Bromocresol Purple 6.40 0.1 g in 18.5 mL 0.01 M NaOH + 231.5 mL water 5.2-6.8 Y-R Chlorophenol Red 6.25 0.1 g in 23.6 mL 0.01 M NaOH + 226.4 mL water 5.4-6.6 C-Y p -Nitrophenol 7.15 0.1% in water 5.6-7.2 Y-R Alizarin 0.1% in methanol 6.0-7.0 Y-B 2-(2,4-Dinitrophenylazo)-1-naphthol-3,6- disulfonic acid, disodium salt 0.1% in water 6.0-7.6 Y-B Bromothymol Blue 7.30 0.1 g in 16 mL 0.01 M NaOH + 234 mL water 6.4-8.0 C-Y 6,8-Dinitro-2,4-(1 H )quinazolinedione 25 g in 115 mL 1 M NaOH + 50 mL water at 100°C 6.6-7.8 Y-R Brilliant Yellow 1% in water 6.6-8.0 Y-R Phenol Red 8.00 0.1 g in 28.2 mL 0.01 M NaOH + 221.8 mL water 6.8-8.0 R-A Neutral Red 0.01 g in 100 mL 50% v/v ethanol-water 6.8-8.6 C-Y m -Nitrophenol 8.28 0.3% in water 7.0-8.8 Y-R Cresol Red 8.46 0.1 g in 26.2 mL 0.01 M NaOH + 223.8 mL water 7.4-8.6 Y-R Turmaric (Curcumin) ethanol 7.4-9.0 Y-P Metacresol Purple 8.3 0.1 g in 26.2 mL 0.01 M NaOH + 223.8 mL water 8.0-9.0 B-R 4,4’-Bis(4-amino-1-naphthylazo)-2,2’- stilbenedisulfonic acid 0.1 g in 5.9 mL 0.05 M NaOH + 94.1 mL water 8.0-9.6 Y-B Thymol Blue 9.20 0.1 g in 21.5 mL 0.01 M NaOH + 228.5 mL water 8.2-10.0 O-B p -Naphtholbenzein 1% in dil.
alkali
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