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Acid and Base pH Indicators

pH range vs. color change for acid and base indicators - together with pKa and structures of the indicators.

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The ranges for the color changes are given in the table below the figure, together with the corresponding pKa value of the indicators.

The molecule structure of each indicator is shown below the table.

See also pKa values of inorganic acids and bases, organic nitrogen compound and alcohols and carboxylic acids.

acid base pH indicators

Trade name pH start pH end pKa low pH color high pH color
Cresol red - step 1 0.2 1.8 1.0 red yellow
Malachite green -step 1 0.2 1.8 1.3 yellow green
Thymol blue - step 1 1.2 2.8 1.7 red yellow
Methyl yellow 2.9 4.0 3.3 red yellow
Methyl orange 3.1 4.4 3.5 red yellow
Bromophenol blue 3.0 4.6 4.1 yellow blue
Congo red 3.0 5.0 blue-violet red
Methyl orange in xylene cyanol (screened) 3.2 4.2 grey-violet green
Bromocresol green 3.8 5.4 4.7 yellow blue
Methyl red 4.4 6.2 5.1 red yellow
Methyl purple 4.8 5.4 purple green
Bromocresol purple 5.2 6.8 6.4 yellow purple
Azolitmin (litmus) 4.5 8.3 red blue
Bromothymol blue 6.0 7.6 7.3 yellow blue
Phenol red 6.4 8.0 8.0 yellow red
Neutral red 6.8 8.0 red yellow
Cresol red -step 2 7.2 8.8 8.5 yellow reddish-purple
Naphtholphthalein 7.3 8.7 pale red greenish-blue
Thymol blue - step 2 8.0 9.6 9.2 yellow blue
Cresolphthalein 8.2 9.8 colorless purple
Phenolphthalein 8.3 10.0 9.5 colorless red-pink
Thymolphthalein 9.0 10.5 9.2 colorless blue
Alizarin yellow R 10.1 12.0 11.2 yellow red
Indigo carmine 11.4 13.0 blue yellow
Malachite green -step 2 11.5 13.2 green colorless

Structure of ph acid base indicator

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