Thymol blue (thymolsulfonephthalein) is a brownish-green or reddish-brown crystalline powder that is used as a pH indicator. It is insoluble in water but soluble in alcohol and dilute alkali solutions.

Thymol blue (thymolsulfonephthalein) is a brownish-green or reddish-brown crystalline powder that is used as a pH indicator. It is insoluble in water but soluble in alcohol and dilute alkali solutions. It transitions from red to yellow at pH 1.2–2.8 and from yellow to blue at pH 8.0–9.6.

The invention relates to a method for preparing water-soluble thymol blue, which comprises the following steps of: dissolving sodium hydroxide in distilled water, slowly adding thymol blue into an alkaline solution, stirring at the temperature of between 70 and 90DEG C for 1.5 to 3 hours, filtering a mixture to obtain 

Bromthymol blue changes color over a pH range from 6.0 (yellow) to 7.6 (blue). It is a good indicator of dissolved carbon dioxide (CO2) and other weakly acidic solutions. Low levels of carbon dioxide or acid in solution with bromothymol blue indicator will appear blue.

THYMOL BLUE INDICATOR SOLUTION