Bromophenol blue is an acid/base indicator. At acidic pH levels (0-3) it makes a yellow solution in water and at more neutral and basic pH levels (4.5-14) it appears as a purple or blue solution. Therefore it is commonly used in chemical reactions when the pH needs to be adjusted...
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Bromophenol blue is an acid/base indicator. At acidic pH levels (0-3) it makes a yellow solution in water and at more neutral and basic pH levels (4.5-14) it appears as a purple or blue solution. Therefore it is commonly used in chemical reactions when the pH needs to be adjusted since the color change very visibly and reliably denotes the change in pH. The link below shows the structure of bromophenol blue. Its reaction with NaOH is an acid/base reaction. The hydroxide ion removes a proton from one of the phenol OH's on the aromatic rings of bromophenol blue to make water and the phenolic anion. This change from phenol to phenolic anion is what changes color. The general equation is below:
C19H10Br4O5S + NaOH <--> H2O + C19H9Br4O5SNa