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Acid-Base Indicators

How do acid-base indicators work?
Frequently asked questions about indicators

alizarin red S

color change for alizarin red S
pH scale

Approximate pH range for color change: 4.6-6.0
Color of acid form: yellow
Color of base form: red

alizarin yellow GG

color change for alizarin yellow GG
pH scale

Type: In + H2O InH+ + OH-
pK: 11.0
Approximate pH range for color change: 10.0-12.0
Color of acid form: clear
Color of base form: yellow

alizarin yellow R

color change for alizarin yellow R
pH scale

Also known as: sodium 5-(p-nitrophenylazo)benzenesulfonate
Approximate pH range for color change: 10.1-12.0
Color of acid form: yellow
Color of base form: red

brilliant yellow

color change for brilliant yellow
pH scale

Approximate pH range for color change: 6.6-7.8
Color of acid form: yellow
Color of base form: orange

bromocresol green

color change for bromocresol green
pH scale

Also known as: 3',3'',5',5''-tetrabromo-m-cresol-sulfonephthalein
Type: HIn + H2O In- + H3O+
pK: 4.90
Approximate pH range for color change: 3.8-5.4
Color of acid form: yellow
Color of base form: blue

bromophenol blue

color change for bromophenol blue
pH scale

Also known as: 3',3'',5',5''-tetrabromophenolsulfonephthalein
Type: HIn + H2O In- + H3O+
pK: 4.10
Approximate pH range for color change: 3.0-4.6
Color of acid form: yellow
Color of base form: purple

bromothymol blue

color change for bromothymol blue
pH scale

Also known as: 3',3''-dibromothymolsulfonephtalein
Type: HIn + H2O In- + H3O+
pK: 7.30
Approximate pH range for color change: 6.0-7.6
Color of acid form: yellow
Color of base form: blue

chlorophenol red

color change for chlorophenol red
pH scale

Also known as: 3',3''-dichlorophenolsulfonephthalein
Type: HIn + H2O In- + H3O+
pK: 6.25
Approximate pH range for color change: 4.8-6.4
Color of acid form: yellow
Color of base form: red

clayton yellow

color change for clayton yellow
pH scale

Approximate pH range for color change: 12.2-13.2
Color of acid form: yellow
Color of base form: amber

congo red

color change for congo red
pH scale

Approximate pH range for color change: 3.0-5.0
Color of acid form: blue
Color of base form: red

cresol red (in acid solution)

color change for cresol red (in acid solution)
pH scale

Also known as: o-cresolsulfonephthalein
Approximate pH range for color change: 0.0-1.0
Color of acid form: red
Color of base form: yellow

cresol red (in basic solution)

color change for cresol red (in basic solution)
pH scale

Also known as: o-cresolsulfonephthalein
Approximate pH range for color change: 7.0-8.8
Color of acid form: yellow
Color of base form: red

crystal violet

color change for crystal violet
pH scale

Type: HIn + H2O In- + H3O+
pK: -
Approximate pH range for color change: 0.0-1.8
Color of acid form: yellow
Color of base form: blue

dimethyl yellow

color change for dimethyl yellow
pH scale

Type: In + H2O InH+ + OH-
pK: 3.25
Approximate pH range for color change: 2.9-4.0
Color of acid form: red
Color of base form: yellow

erythrosine (sodium salt)

color change for erythrosine (sodium salt)
pH scale

Approximate pH range for color change: 2.2-3.6
Color of acid form: orange
Color of base form: red

malachite green

color change for malachite green
pH scale

pK: 1.0
Approximate pH range for color change: 0.2-1.8
Color of acid form: yellow
Color of base form: blue-green

metacresol purple (in acid solution)

color change for metacresol purple (in acid solution)
pH scale

Also known as: m-cresol purple, cresol purple, m-cresolsulfonephthalein
pK: ~2.0
Approximate pH range for color change: 1.2-2.8
Color of acid form: red
Color of base form: yellow

metacresol purple (in basic solution)

color change for metacresol purple (in basic solution)
pH scale

Also known as: m-cresol purple, cresol purple, m-cresolsulfonephthalein
Type: HIn + H2O In- + H3O+
pK: 8.32
Approximate pH range for color change: 7.6-9.2
Color of acid form: yellow
Color of base form: purple-red

metanil yellow

color change for metanil yellow
pH scale

Also known as: m-(p-anilinophenylazo)benzene sulfonate
Approximate pH range for color change: 1.2-2.4
Color of acid form: red
Color of base form: yellow

methyl green

color change for methyl green
pH scale

Approximate pH range for color change: 0.2-1.8
Color of acid form: yellow
Color of base form: blue

methyl orange

color change for methyl orange
pH scale

Type: InH+ + H2O In + H3O+
pK: 3.46
Approximate pH range for color change: 3.1-4.4
Color of acid form: red
Color of base form: orange-yellow

methyl red

color change for methyl red
pH scale

Type: InH+ + H2O In + H3O+
pK: 5.00
Approximate pH range for color change: 4.4-6.2
Color of acid form: red
Color of base form: yellow

neutral red

Approximate pH range for color change: 6.8-8.0
Color of acid form: red
Color of base form: amber

phenolphthalein

color change for phenolphthalein
pH scale

Type: HIn + H2O In- + H3O+
pK: 9.5
Approximate pH range for color change: 8.0-9.8
Color of acid form: clear
Color of base form: red-violet

phenol red

color change for phenol red
pH scale

Also known as: phenolsulfonephthalein
Type: HIn + H2O In- + H3O+
pK: 8.00
Approximate pH range for color change: 6.4-8.2
Color of acid form: yellow
Color of base form: red

paramethyl red

Also known as: sodium p-(p-dimethylaminophenylazo)-benzoate
Approximate pH range for color change: 1.0-3.0
Color of acid form: red
Color of base form: yellow

paranitrophenol

Also known as: p-nitrophenol
Type: HIn + H2O In- + H3O+
pK: 7.15
Approximate pH range for color change: 5.6-7.6
Color of acid form: colorless
Color of base form: yellow

quinaldine red

color change for quinaldine red
pH scale

Approximate pH range for color change: 1.0-2.2
Color of acid form: clear
Color of base form: red

resazurin

color change for resazurin
pH scale

Approximate pH range for color change: 3.8-6.4
Color of acid form: orange
Color of base form: violet

thymolphthalein

color change for thymolphthalein
pH scale

Type: HIn + H2O In- + H3O+
pK: 9.7
Approximate pH range for color change: 9.3-10.5
Color of acid form: colorless
Color of base form: blue

thymol blue (in acid solution)

color change for thymol blue (in acid solution)
pH scale

Also known as: thymolsulfonephthalein
Type: HIn + H2O In- + H3O+
pK: 1.65
Approximate pH range for color change: 1.2-2.8
Color of acid form: red
Color of base form: yellow

thymol blue (in basic solution)

color change for thymol blue (in basic solution)
pH scale

Also known as: thymolsulfonephthalein
Type: HIn + H2O In- + H3O+
pK: 9.20
Approximate pH range for color change: 8.0-9.6
Color of acid form: yellow
Color of base form: blue

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Last Revised 12/10/01.URL: http://antoine.frostburg.edu/chem/senese/101/acidbase/indicators.shtml