Bromocresol green

Bromocresol green (3,3',5,5'-tetrabromo-m-cresolsulfonphthalein) is a dye of the triphenylmethane family (triarylmethane dyes), which is used as a pH indicator and as a tracking dye for DNA agarose gel electrophoresis. It can be used in its free acid form (light brown solid), or as a sodium salt (dark green solid).

In aqueous solution, both solids ionize to give the monoanionic form (yellow), that further deprotonates at higher pH to give the dianionic form (blue), which is stabilized by resonance:



The pK (pKa) of this reaction is 4.9.

The acid and basic form of this dye have an isosbestic point in their spectra, around 515 nm.