Bromothymol blue
Bromothymol blue (BTB), chemically known as bromothymol sulfone phthalein, is a pH indicator widely used in various applications, particularly those involving substances with a relatively neutral pH, close to 7. This compound is typically sold in its solid form as the sodium salt of the acid indicator[1].
Structure and Properties
BTB functions as a weak acid in solution, capable of existing in either a protonated or deprotonated state. These states correspond to different colors: yellow when protonated and blue when deprotonated. By itself, BTB is bright aquamarine, but it appears greenish-blue in a neutral solution. The transition between its protonated and deprotonated forms, which occurs between pH 6.0 and 7.6, is responsible for its utility as a pH indicator. The color change is attributed to the deprotonation of the neutral form, leading to a highly conjugated structure that significantly influences its color. The protonated form of BTB absorbs peak light at 427 nm, transmitting yellow light in acidic solutions, while the deprotonated form absorbs peak light at 602 nm, transmitting blue light in more basic solutions. In highly acidic conditions, BTB turns magenta[1].
The general carbon skeleton of BTB is shared with other indicators like chlorophenol red, thymol blue, and bromocresol green. Its active range and the nature of its color change are influenced by its conjugated structure and the presence of substituent groups, including a moderate electron-withdrawing group (bromine atom) and two moderate donating groups (alkyl substituents)[1].
Synthesis and Preparation
BTB is synthesized through the addition of elemental bromine to thymol blue in a solution of glacial acetic acid. For use as a pH indicator, a solution can be prepared by dissolving 0.10 g of BTB in 8.0 cm3 of N/50 NaOH and diluting it to 250 cm3 with water. Alternatively, for volumetric work, 0.1 g can be dissolved in 100 cm3 of 50% (v/v) ethanol[1].
Applications
BTB's primary use is as a pH indicator, especially for measuring the presence of carbonic acid in liquids. It is also employed in observing photosynthetic activities, as a respiratory indicator (changing to yellow as CO2 is added), and in science education to demonstrate CO2 output from increased muscle use. Additionally, BTB is used in conjunction with phenol red for monitoring fungal asparaginase enzyme activity, although recent studies suggest methyl red might be more effective for certain applications. In the laboratory, BTB serves as a biological slide stain, helping to define cell walls or nuclei under the microscope. In obstetrics, it is used for detecting premature rupture of membranes, as amniotic fluid, which typically has a pH above 7.2, will turn BTB blue, distinguishing it from the normally acidic vaginal pH. However, false positives may occur in the presence of other alkaline substances such as blood or semen, or in cases of bacterial vaginosis[1].
In summary, bromothymol blue is a versatile pH indicator with a range of applications in scientific research, education, and clinical settings, owing to its ability to visually demonstrate changes in pH through color transitions.
Citations:
[1] https://en.wikipedia.org/wiki/Bromothymol_blue
[2] https://www.labxchange.org/library/items/lb:LabXchange:6c09438a:html:1
[3] https://www.amazon.com/bromothymol-blue-indicator-solution/s?k=bromothymol+blue+indicator+solution
[4] https://www.sciencecompany.com/Bromothymol-Blue-pH-Indicator-1-oz-P6363
[5] https://www.sigmaaldrich.com/US/en/product/sial/114413