Bromocresol green

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Bromocresol green
Systematic (IUPAC) name
2,6-dibromo-4-[7-(3,5-dibromo-4-hydroxy-2-methyl- phenyl)-9,9-dioxo-8-oxa-9λ6-thiabicyclo[4.3.0]nona- 1,3,5-trien-7-yl]-3-methyl-phenol
Identifiers
PubChem	6451
Chemical data
Formula	C₂₁H₁₄Br₄O₅S
Molar mass	698.02
SMILES	Cc1c(Br)c(O)c(Br)cc1C2(OS(=O)(=O) c3ccccc23)c4cc(Br)c(O)c(Br)c4C
Complete data

Bromocresol Green (3,3',5,5'-tetrabromo-m-cresolsulfonphthalein)(BCG) 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). It is also an inhibitor of the prostaglandin E₂ transport protein.

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 4.7.

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

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_a is below 5.0 (carboxylic acids, sulfonic acids etc.). These appear as yellow spots on light or dark blue background; no heating is necessary. Bromophenol Blue solution can be used for the same purspose.

The compound is synthesized by bromination of Cresol Purple (m-cresolsulfonphthalein).