pH meter

pH measurements

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Buffer Solutions

by R.J. Beynon, J.S. Easterby

pH measurements - indicators

pH indicators are usually weak acids or weak bases that change their color depending on their dissociation (protonation) state. Sometimes both forms are colored, sometimes only one. In most cases you may assume that to completely change color of bicolored indicator pH must change by 2 units. However, human eye is more sensitive to some colors than to others, thus some color changes can be perceived over wider pH range.

pH indicators can be used to check pH of the solution, although they are rarely added directly. Only in acid-base titrations indicator should be added to the solution. To check pH it is much more convenient to use pH strips. It is worth of noting here that pH strips are nothing else but pieces of paper impregnated with indicator or a mixture of indicators.

pH indicators - colors and color change pH range
indicator namepH
color
pH
color
Malachite green oxalate0.0
green
2.0
green-blue
Brilliant green0.0
yellow
2.6
green
Eosin yellowish0.0
yellow
3.0
green fluoresc.
Erythrosine B0.0
yellow
3.6
red
Methyl green0.1
yellow
2.3
blue
Methyl violet0.1
yellow
2.7
violet
Picric acid0.2
colourless
1.0
yellow
Cresol red0.2
red
1.8
yellow
Crystal violet0.8
yellow
2.6
blue/violet
m-Cresol purple1.2
red
2.8
yellow
Thymol blue1.2
red
2.8
yellow
p-Xylenol blue1.2
red
2.8
yellow
Eosin, bluish1.4
colourless
2.4
pink fluoresc.
Quinaldine red1.4
colourless
3.2
pink
2,4-Dinitro phenol2.8
colourless
4.7
yellow
4-(Dimethylamino) azobenzol2.9
red
4.0
yellow/orange
Bromochlorophenol blue3.0
yellow
4.6
blue/violet
Bromophenol blue3.0
yellow
4.6
blue/violet
Congo red3.0
blue
5.2
yellow/orange
Methyl orange3.1
red
4.4
yellow/orange
Bromocresol green3.8
yellow
5.4
blue
2,5-Dinitrophenol4.0
colourless
5.8
yellow
Alizarin sulphonic acid4.3
yellow
6.3
violet
Methyl red4.4
red
6.2
yellow/orange
Chlorophenol red4.8
yellow
6.4
purple
Litmus5.0
red
8.0
blue
Bromocresol purple5.2
yellow
6.8
purple
Bromophenol red5.2
orange/yellow
6.8
purple
4-Nitrophenol5.4
colourless
7.5
yellow
Bromoxylenol blue5.7
yellow
7.5
blue
Bromothymol blue6.0
yellow
7.6
blue
Phenol red6.4
yellow
8.2
red/violet
3-Nitrophenol6.6
colourless
8.6
yellow/orange
Neutral red6.8
blue/red
8.0
orange/yellow
Creosol red7.0
orange
8.8
purple
1-Naphtholphthalein7.1
brownish
8.3
blue/green
m-Cresol purple7.4
yellow
9.0
purple
Thymol blue8.0
yellow
9.6
blue
p-Xylenol blue8.0
yellow
9.6
blue
Phenolphthalein8.2
colourless
9.8
red/violet
Thymolphthalein9.3
colourless
10.5
blue
Alkali blue9.4
violet
14.0
pink
Alizarin yellow GG10.0
bright yellow
12.1
brown/yellow
Indigo carmine11.5
blue
13.0
yellow
Epsilon blue11.6
orange
13.0
violet
Titan yellow12.0
yellow
13.0
red