When bromothymol blue is used to detect carbon dioxide, the bromothymol blue aqueous solution changes from blue to green and then to yellow. Bromothymol blue is an acid-base indicator with a color change range of pH 6.0 (yellow) to 7.6 (blue). Ordinary water is neutral, with a pH of about 7, and is almost light green. When carbon dioxide is dissolved, carbonic acid is formed, and carbonic acid is a weak acid, so the pH will not drop too much and turn yellow. Identifying the presence of CO2 is a bit like clarifying lime water, but the accuracy is higher than that of lime water, and there is no interference from SO2.

Bromothymol Blue, also known as bromothymol blue, is an acid-base indicator and adsorption indicator with the chemical formula C27H28O5SBr2. It is easily soluble in ethanol, ether, methanol and dilute Alkaline hydroxide solution. Slightly soluble in benzene, toluene and xylene, slightly soluble in water, almost insoluble in petroleum ether. The English abbreviation is BTB. It is often used as a respiratory reagent for aquatic organisms in biological experiments.

It is often used as a respiratory reagent for aquatic organisms in biological experiments. Organisms can live in water containing this reagent for a short time without any impact. Water with 0.1% BTB solution generally appears light green. If the carbon dioxide content in the water increases and becomes acidic, the water will change from light green to yellow.

Bromothymol blue is a light rose-colored crystalline powder with a melting point of 200~202°C. It is easily soluble in ethanol, ether, methanol and dilute alkali hydroxide solution. Slightly soluble in benzene, toluene and xylene, slightly soluble in water, almost insoluble in petroleum ether. It appears blue in alkaline solution and yellow in acidic solution.