Join Date: Jan 2010
Location: Overland Park, KS
You're right -- it has to do with targeting a specific CO2 level. The generally accepted "target" value of CO2 concentration is 30ppm. Since CO2 changes the acidity of a solution, and bromthymol blue is sensitive to a narrow range of pH, this works to our advantage. By shooting for 30ppm CO2, the pH of water is such that bromthymol blue needs to be in a 4dKH solution to get the easily-seen green color. If a different indicator were used, it's likely the solution concentration would change as well.