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What is Bromothymol Blue?

Bromothymol blue is also called bromothymol sulfone phthalein, and BTB, which is a pH indicator. It is mostly used in applications that hold measuring substances that would have a neutral pH (nearly 7) relatively. As a common use, we can say it for measuring the presence of carbonic acid in a liquid. 

It is typically sold in its solid form because of the sodium salt of the acid indicator. The other name of Bromothymol Blue can be given as 3,3′-Dibromothymolsulfonphthalein and Dibromothymolsulfophthalein.

Bromothymol Blue Structure and Properties

Bromothymol blue is the indicator that acts as a weak acid in a solution. Thus, it can be either in a deprotonated form or protonated form, by appearing blue or yellow, respectively. It is also a bright aquamarine by itself and greenish-blue in a neutral solution. This neutral form deprotonation results in a structure of highly conjugated, considering for the color difference. A deprotonation intermediate mechanism is responsible for the greenish color in the neutral solution.

The bromothymol blue's protonated form has its peak absorption at 427 nm, therefore transmitting yellow light in the acidic solutions. In contrast, the deprotonated form contains its peak absorption at 602 nm, thereby transmitting the blue light in many basic solutions. Besides, the Bromothymol blue of highly acidic is magenta in color.

The bromothymol blue's general carbon skeleton is common to most of the indicators, including thymol blue, bromocresol green, and chlorophenol red.

The presence of a single moderate electron-withdrawing group (which is a bromine atom) and two moderate donating groups (which are alkyl substituents) are completely responsible for the active indication range of bromothymol blue from a pH value of 6.0 to 7.6. While the conjugation is responsible for the nature of the color change range and length, these substituent groups are ultimately responsible for the active range of the indicator.

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Bromothymol blue indicator is sparingly soluble in oil but soluble in ether, water, and alkalis' aqueous solutions. It is also less soluble in nonpolar solvents, including toluene, benzene, and xylene, and it is practically insoluble in the petroleum ether.

Physical Properties of Bromothymol Blue

Let us look at some of the physical properties of bromothymol blue.



Covalently-Bonded Unit



Yellow - in acidic solutions; green - in neutral solutions; blue - in basic solutions

Bromothymol blue ph




Synthesis and Preparation

Bromothymol blue is synthesized by adding elemental bromine to the thymol blue in a solution of glacial acetic acid.

To prepare a solution that is used as a pH indicator, we should dissolve 0.10 g in an 8.0 cm3 N/50 NaOH and then dilute it with water to 250 cm3. To prepare a solution used as an indicator in volumetric work, we should dissolve 0.1 g in the 100 cm3 of 50% (v/v) ethanol.

Uses of Bromothymol Blue

Let us discuss the major uses of bromothymol blue in detail.

Bromothymol blue can be used either to observe photosynthetic activities or as a respiratory indicator (which turns yellow as CO2 is added). A common demonstration of the pH indicator properties of BTB involves exhaling through a tube into the neutral solution of Bromothymol blue. As the carbon dioxide is absorbed from the breath into the solution, the solution changes its color to yellow from green by forming carbonic acid. Therefore, BTB can be used commonly in science classes to demonstrate as, "the more that the muscles are used, the greater the carbon dioxide output.

Bromothymol blue indicator has been used in conjunction with phenol red in monitoring the fungal asparaginase enzyme activity, with the phenol red turning pink. And, the bromothymol blue turns blue by indicating an increase in pH and thus enzyme activity. However, one recent study suggests that the methyl red part is more useful in determining the activity because of the bright yellow ring form in the enzyme activity zone.

It can also be used in the laboratory as a stain of a biological slide. It is already blue at this point, and a few drops are used on the water slide. The cover-slip is also placed on the top of the water droplet, including the specimen in it, mixed with blue coloring. Sometimes, it is also used to define nuclei or cell walls under the microscope.

Bromothymol can be used in obstetrics for detecting the premature rupture of membranes. Typically, the amniotic fluid has a pH value of greater than 7.2, and the bromothymol will thus turn blue when made in contact with the amnion's leaking fluid. Normally, as the vaginal pH is acidic, its blue color indicates the amniotic fluid's presence. The test may result in false-positives in the presence of other alkaline substances, including semen or blood, or the presence of various bacterial vaginosis.

Do You Know?

Chronic Effects of Bromothymol Blue on Humans: 

Bromothymol Blue causes damage to the organs, including mucous membranes, lungs.

Multiple Adverse Effects on Humans:

It is very dangerous for ingestion, inhalation, and skin touch (irritant).

FAQ (Frequently Asked Questions)

1. Explain what the Bromothymol blue indicates the Presence of?

Ans: Bromothymol blue (also called BMB) is an indicator dye that turns yellow when an acid is present. Whereas, when carbon dioxide is added to the solution, it produces carbonic acid, which lowers the solution's pH.

2. What is the Color of Bromothymol Blue When in a Base?

Ans: Bromothymol Blue is a mild acid indicator. It is based on the solution's pH, whether it can be in the acid or base form. This reagent is purple and blue for acidic solutions, green for the neutral solution.

3. What happens if We consume Bromothymol Blue?

Ans: When the Bromothymol blue powder is consumed, it can irritate the eyes and skin, and if swallowed, it results in acute toxicity. Whereas spontaneous, tiny digestion has not seen any problems, but it can cause vomiting, nausea, and even death if it is an excessive level. Drinking water will treat incidental swallowing.

4. Why does the Bromothymol Blue Powder turn out to be Green in Neutral Solutions?

Ans: Bromothymol blue serves as a heavy acid in the solution. This is bluish-green in neutral water - the neutral form's deprotonation results in a highly conjugated structure accounting for the color difference. And, the greenish color in the neutral solution is responsible for a deprotonation process intermediate.

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