Acid-Base Indicators

alizarin red S

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

alizarin yellow GG

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

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

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

bromocresol green

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

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

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

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

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

congo red

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)

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)

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

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

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)

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

malachite green

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)

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)

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

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

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

methyl orange

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

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

phenolphthalein

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

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

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

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

resazurin

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

thymolphthalein

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)

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)

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