Bromocresol Purple

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Bromocresol Purple

Bromocresol Purple (BCP) is a pH sensitive triphenylmethane dye that belongs to a class of dyes called sulfonephthaleins which are very well known Ph indicators.

Bromocresol purple or 5′,5″-dibromo-o-cresolsulfophthalein is a pH indicator that possesses yellow color while at a low pH 5.2 and changes color to violet at pH6.8 and above.


The structure of bromocresol purple changes with pH. The change in the acidity levels causes a shift in the equilibrium between two different colored structures. In near-neutral or alkaline solution, the chemical has a sulfonate structure that gives purple color to the solution and with the decrease in the pH, it converts to a sultone (cyclic sulfonic ester) that gives yellow color to the solution.


It is used for staining of Saccharomyces cerevisiae (commonly known as Baker’s Yeast). It is mainly used as a fluorescent stain at a pH below 5.2 for detecting yeast cells with plasma membrane damage. It detects and counts the dead cells with plasma membrane damage developed as an assay of killer toxin activity. Cells with proper cell membrane are not stained, while the damaged ones turn into blue-grey color. The Borocomocresol Purple test is as precise as the plating test.


Bromocresol Purple is also used in laboratories to measure albumin and this method is considered beneficial over methods in which Bromocresol green is used.


It is also used as an additive in photographic processing to acid stop baths to indicate that the bath has reached neutral pH and needs to be replaced.


Bromocresol purple milk solids glucose agar is used as a medium used to distinguish dermatophytes from bacteria and other organisms in cases of ringworm fungus infestation in cattle and other animals.







Chemical FormulaC21H16Br2O5S
Molar Mass540.22 g·mol−1


AppearancePurple Powder




Melting Point241 to 242 °C (466 to 468 °F; 514 to 515 K) (decomposition)


Solubility in water< 0.1 %


CAS No. :115 – 40 – 2
Chemical Formula :C21H16Br2O5S
Molecular Weight :540.24
Melting Point :240° c
Dye Content :~95%
Transition Range :pH 5.2 – 6.8 Greenish Yellow to Purple
Absorption Max(pH5.2) :427 – 431 nm
Absorption Max(pH6.8) :588 – 590 nm
Absorptivity(1%,1cm)pH 5.2 :400 – 450
Absorptivity(1%,1cm)pH 6.8 :1000 – 1100
Loss On Drying :<1%