Bromothymol blue

Bromothymol Blue (also known as dibromothymolsulfonephthalein, Bromthymol Blue, and BTB) is a chemical indicator for weak acids and bases. The chemical is also used for observing photosynthetic activities or respiratory indicators (turns yellow as CO2 is added).

Bromothymol Blue acts as a weak acid in solution and therefore can be in acid or base forms which appear yellow and blue respectively. It is green in neutral solution. It is typically sold in solid form as the sodium salt of the acid indicator. It also finds occasional use in the laboratory as a biological slide stain. At this point it is already blue, and a drop or two is used on a water slide. The cover slip is placed on top of the water droplet and the specimen in it, with the blue coloring mixed in. It is sometimes used to define cell walls or nuclei under the microscope.

Bromothymol Blue is mostly used in measuring substances that would have relatively low acidic or basic levels (near a neutral pH). It is often used in pools, fish tanks, or measuring the presence of carbonic acid in a liquid. Addition: Bromthymol is used in Obstetrics for detection of presumed premature rupture of membranes:


 * C27H28Br2O5S H+ + C27H27Br2O5S−

The pKa for bromothymol blue is 7.10.

A common demonstration of BTB's pH indicator properties involves exhaling through a tube into a neutral solution of BTB. As carbon dioxide is absorbed from the breath into the solution, forming carbonic acid, the solution changes color from green to yellow.

Indicator colors


