The atomic structure of Bromothymol Blue

Bromothymol Blue is a pH indicator.


Bromothymol blue acts as a weak acid in a water solution. It can thus be in protonated (dissolved in acid) or deprotonated (dissolved in an alkaline) form, appearing yellow or blue respectively. It is bright aquamarine or greenish-blue in a neutral solution. Bromothymol blue may be used for observing photosynthetic activities, or as a respiratory indicator (turns yellow as Carbon Dioxide is added). A common demonstration of Bromothymol Blue's pH indicator properties involves exhaling through a tube into a neutral solution of Bromothymol Blue. As carbon dioxide is absorbed from the breath into the solution, forming carbonic acid, the solution changes color from green to yellow.

pH indicator

800px-Bleu bromothymol teintes

* Magenta - Concentrated Hydrochloric acid * Yellow - Normal acids * Green - Neutral * Blue - Alkalines

In concentrated HCl, Bromothymol Blue turns pink, but in normal acids, it turns yellow. It will remain green in a neutral solution anywhere between pH 6.0 - 7.6 Bromothymol Blue, hence its name, turns blue in all alkalines above pH 7.6 .

Color pH Example
Magenta < 0 Concentrated Hydrochloric acid
Yellow 0 - 6 Lemon Juice
Green 6 - 7.6 Water
Blue > 7.6 Washing-up liquid