Bromothymol Blue (CAS 76-59-5)

Bromothymol Blue
  The water quality monitoring engineer at a municipal treatment plant in Chengdu had been running the same colorimetric pH test for six years when she noticed something odd: two bottles of bromothymol blue solution, purchased a month apart from different suppliers, were giving slightly different transition endpoints in the pH 6.5 range. One shifted cleanly from yellow to green at pH 6.8; the other lagged behind and the color change was muddier, more olive than the clear blue-green she expected. She sent samples to the provincial laboratory for purity testing. The batch with the clean transition came back at 99.2% by spectrophotometric assay; the lagging one was 96.8%, with a minor impurity peak suggesting incomplete sulfonation during synthesis. That difference of 2.4 percentage points was enough to shift the apparent pKa of the indicator by nearly 0.3 units, which in practice meant her dissolved oxygen readings, cross-referenced against pH data, were systematically off during the shoulder seasons when influent pH hovered near 7. The experience turned her into a careful reader of certificate of analysis documents, and she never again assumed that one vial labeled “bromothymol blue” was functionally identical to another.
   ♦Bromothymol blue is a sulfonophthalein pH indicator that transitions from yellow at pH 6.0 to blue at pH 7.6, with a clean green midpoint at approximately pH 7.0. The molecule consists of a triphenylmethane-derived scaffold with two thymol groups flanking the central carbon, plus two bromine atoms and a sulfonate group that confer both the chromophoric properties and the aqueous solubility. The color change mechanism involves protonation and deprotonation of the phenolic oxygen, which shifts the electron distribution across the entire conjugated ring system and changes the wavelength of maximum absorbance from around 430 nm in the acidic form to around 617 nm in the basic form. This shift spans the visible spectrum in a range that human color vision detects readily, which is part of why bromothymol blue became one of the most widely used indicators for near-neutral pH work in analytical chemistry, biochemistry, and environmental monitoring.
  In biological laboratories, bromothymol blue appears in culture media to monitor metabolic activity through CO2 production. Cell culture flasks containing bicarbonate-buffered media show yellow coloration when cells are actively metabolizing and producing CO2, pulling the pH below 7. A cell biology technician at a Guangzhou CRO company used this color shift as a coarse viability check before running flow cytometry: if a flask was unexpectedly yellow by 8 AM, she flagged it for inspection before spending two hours on the cytometer. Microbiology labs use bromothymol blue in differential media — blood agar supplements, fermentation substrates for coliform detection — where the color shift serves to classify organisms by their metabolic products. Aquatic ecotoxicology researchers also use it in standardized Daphnia magna toxicity assays, where its color change tracks pH-dependent mortality endpoints in crustacean bioassay protocols.
  The dye’s industrial uses extend well beyond the laboratory. Manufacturers of pH indicator strips and papers incorporate bromothymol blue into cellulose matrices, often in combination with other indicators to extend the readable pH range. The compound appears in swimming pool test kits, hydroponic nutrient solution monitors, and soil amendment testing kits sold to agricultural distributors — applications where a clear color signal readable by a non-specialist is more practical than an electrode. A distributor of agricultural testing supplies in Malaysia sources multi-indicator strips that incorporate bromothymol blue alongside methyl red and cresol red, allowing farmers to assess soil pH over a range from 4.5 to 9.5 with a single test strip. The bromothymol blue component handles the critical 6.0 to 7.6 window that most crop soil recommendations target.
Bromothymol Blue
Shandong Lookchemical Co., Ltd. supplies Bromothymol Blue (CAS 76-59-5) for analytical, biological, and industrial uses. Available in standard powder form with purity suitable for indicator applications, each batch is tested by UV-Vis spectrophotometry to verify the absorption maxima of both the acid and base forms align with specification. Export documentation includes batch certificate of analysis, SDS, and REACH-relevant compliance statements for EU customers. Samples are available; standard orders ship within 7 to 12 business days. Inquiries can be sent to sales01@sdlookchemical.com or placed through sdlookchem.com.
 

Related News Recommendations

Our Factory Equipment