I believe the scientist wants to test for the presence of carbonic acid in the solution. Bromthymol blue is a PH indicator and is mostly used in applications that require measuring substances that would have a relatively neutral pH. A common measure is for the measuring the presence of carbonic acid in a liquid.
It may be used for observing photosynthetic activities, or as a respiratory indicator. As a carbon dioxide is absorbed from the breath into the solution, forming carbonic acid, the solution changes color from green to yellow.
