Bromothymol blue

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Bromothymol blue
CAS number 76-59-5 YesY
PubChem 6450
ChemSpider 6208 YesY
Jmol-3D images Image 1
Molecular formula C27H28Br2O5S
Molar mass 624.38 g mol−1
Density 1.25 g/cm3
Melting point 202 °C (396 °F; 475 K)
Acidity (pKa) 7.10
MSDS External MSDS
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oil Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Supplementary data page
Structure and
n, εr, etc.
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Bromothymol blue (also known as bromothymol sulfone phthalein, Bromthymol Blue, and BTB) is a pH indicator for weak acids and bases. It is mostly used in applications that require measuring substances that would have a relatively neutral pH (near 7), such as managing the pH of pools and fish tanks. A common use is for measuring the presence of carbonic acid in a liquid. It is typically sold in solid form as the sodium salt of the acid indicator.

Bromothymol Blue (pH indicator)
below pH 6.0 above pH 7.6
6.0 7.6


Bromothymol blue acts as a weak acid in solution. It can thus be in protonated or deprotonated form, appearing yellow or blue respectively. It is bluish green in neutral solution. The deprotonation of the neutral form results in a highly conjugated structure, accounting for the difference in color.

Structure at different pH ranges

Bromothymol blue is sparingly soluble in water, but soluble in alcohol, ether and in aqueous solutions of alkalis. It is less soluble in non-polar solvents such as benzene, toluene and xylene, and practically insoluble in petroleum ether.[1]


To prepare a solution for use as pH indicator, dissolve 0.10g in 8.0 cm3 N/50 NaOH and dilute with water to 250 cm3. To prepare a solution for use as indicator in volumetric work, dissolve 0.1g in 100 cm3 of 50% (v/v) ethanol.[1]


The different colors of bromothymol blue. From left to right: blank, pH 4, pH 6.2, pH 6.6, pH 7.2, pH 7.5, pH 8, pH 12.

Bromothymol blue may be used for observing photosynthetic activities, or as a respiratory indicator (turns yellow as CO2 is added).[2][3] A common demonstration of BTB's pH indicator properties involves exhaling through a tube into a neutral solution of BTB. As carbon dioxide is absorbed from the breath into the solution, forming carbonic acid, the solution changes color from green to yellow. Thus, BTB is commonly used in science classes to demonstrate that the more that muscles are used, the greater the CO2 output.

It may also be used in the laboratory as a biological slide stain. At this point it is already blue, and a drop or two is used on a water slide. The cover slip is placed on top of the water droplet and the specimen in it, with the blue coloring mixed in.[clarification needed] It is sometimes used to define cell walls or nuclei under the microscope.

Bromothymol is used in obstetrics for detecting premature rupture of membranes. Amniotic fluid typically has a pH > 7.2, bromothymol will therefore turn blue when brought in contact with fluid leaking from the amnion. As vaginal pH normally is acidic, the blue color indicates the presence of amniotic fluid. The test may be false-positive in the presence of other alkaline substances such as blood, semen, or in the presence of bacterial vaginosis.

See also[edit]


  1. ^ a b O'Neil, Maryadele J (2006). The Merck Index. Merck Research Laboratory. p. 1445. ISBN 978-0-911910-00-1. 
  2. ^ Sabnis R. W. (2007). Handbook of Acid-Base Indicators. CRC Press. ISBN 0-8493-8218-1. 
  3. ^ Sabnis R. W. (2010). Handbook of Biological Dyes and Stains: Synthesis and Industrial Applications (1st ed.). Wiley. ISBN 0-470-40753-0. 

External links[edit]