Bromothymol blue is a dye commonly known as bromothymol sulfonephthalein or BTB that is used as a pH indicator. Bromothymol blue is a low-acid compound. Depending on the pH of the solution, it might be acidic or basic.
It is a weak acid that can be in the form of an acid or a base depending on the pH of the solution. Bromothymol blue has a pKa of 7.1 and a pH range of 6.0 to 7.6. It is commonly used in acid-base indicators to detect the endpoint of the reaction which turns yellow in acid and blue in basic, and green in a neutral. It is also generally used to detect the presence of carbonic acid in a sample.
How does the bromothymol blue indicator work?
In a solution, bromothymol blue acts as a weak acid. It may appear yellow or blue depending on whether it is protonated or precipitated. In a neutral solution, it is brilliant aquamarine and greenish-blue.
The color difference is due to the deprotonation of the neutral form, resulting in a highly conjugated structure. The greenish color in neutral solution is due to an intermediate of the precipitation mechanism.
The acid-base indicator is explained by two theories: Ostwald's and the quinonoid theory. An acid-base indicator is a type of indicator in which the analyte emits a visual signal that confirms the presence of an acid or a basic.
References:
- Horobin R. W. and Kiernan J. A., 2002, 10th ed., CONN'S Biological Stains, A Handbook of Dyes, Stains
- INDIAN PHARMACOPOEIA 2018, VOL-I, 4.3(957-958)
- Wikipedia contributors. (2022, January 25). Bromothymol blue. In Wikipedia, The Free Encyclopedia.
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