In aqueous solution, bromocresol green will 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 acid dissociation constant (pK) of this reaction is 4.8. Tap water is sufficiently basic to give a solution of bromocresol green its characteristic blue-green color.
The acid and basic forms of this dye have an isosbestic point in their UV-Visible spectrum, around 515 nm, indicate that the two forms interconvert directly without forming any other substance.
An ethanol solution (0.04 wt%) of bromocresol green has been proposed for TLC staining and is suitable for visualisation of the compounds with functional groups whose pK is below 5.0 (carboxylic acids, sulfonic acids, etc.). These appear as yellow spots on a light or dark blue background; no heating is necessary. Bromophenol blue solution can be used for the same purpose.
The compound is synthesized by bromination of cresol purple (m-cresolsulfonphthalein).