The pKa for the weak acid bromothymol blue is 7.10. Bromothymol blue is a large chemical that has the molecular formula C27H28Br2O5S and a molar mass of 624.38 grams per mole. It has a melting point of 202 degrees Celsius. It is sold as a solid, but it is mixed with either sodium hydroxide and water or ethanol to produce a usable solution.
Like other compounds with multiple conjugated double bonds, bromothymol blue is colorful in some solutions. This feature allows it to be used as a pH indicator solution. A majority of bromothymol blue molecules are protonated when the pH falls below 6.0, causing the solution to turn yellow. When the pH of a solution containing bromothymol blue climbs above 7.6, the majority of bromothymol blue molecules cease to be protonated and the solution changes from yellow to blue.
Due to the pH ranges that bromothymol blue reacts to, it is best used as an indicator for weak acids and bases like carbonic acid and ammonia. Bromothymol blue can also be used in microscopy as a stain for glass slides.
Medically, bromothymol blue is used in obstetrics as an indicator for premature rupturing of the amnionic sac, which can be fatal to the fetus if the rupture is not realized early and repaired.