Dhristi JEE 2022-24

What is Bromothymol Blue?

Bromothymol blue is also called bromothymol sulfonephthalein, 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 names of Bromothymol Blue can be given as 3,3′- Dibromothy molsulfon phthalein and Di Bromothymol Sulfo Phthalein.

Bromothymol Blue Formula

Bromothymol blue is a relatively large molecule and has a weight of 625 g/mol. The chemical formula of this compound comes out to be C27H28Br2O5S. Bromothymol blue has a unique structure that consists of three aromatic benzene rings. The first benzene ring has a thym group connected to it along with a sulfur atom that has two oxygen atoms bonded to it via double bonds. Another oxygen atom is attached to the sulfur atom via a single bond. The second and third benzene rings each have a bromine atom, an alcohol group, a tert-butyl group, and a methyl group attached.

The name 'bromothymol' is the common name of the compound. The most scientifically accurate naming system is the IUPAC nomenclature. The IUPAC name of this compound is 4, 4 - (1, 1 - Dioxide-3H-2 , 1 - benzoxathiole - 3 , 3-diyl ) bis ( 2 - Bromo - 6 - isopropyl - 3- methyl phenol.

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 highly conjugated, considering 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 which is highly acidic is magenta.

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.

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 petroleum ether.

Physical Properties of Bromothymol Blue

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

Odor- Odorless

Covalently - Bonded Unit 1

Appearance - Yellow - in acidic solutions; green - in neutral solutions; blue - in basic solutions Bromothymol blue ph. 

Synthesis and Preparation

We have already studied how bromothymol blue is a large molecule that consists of three benzene rings. These aforementioned benzene rings have two bromine atoms, one sulfur atom, and an alcohol group connected to them. In its true form, bromothymol blue is a powder. This makes it difficult to mix it in with the sample for pH testing. This is why we try to first convert the compound into an aqueous solution.

Reagents required for this process:

  • 0.1 grams of bromothymol blue powder

  • 16mL of 0.01 N sodium hydroxide

  • Distilled Water

Steps involved in the process: 

  • Dissolve the given bromothymol blue powder in sodium hydroxide solution

  • Dilute the solution further with water up to 250 mL


If you follow the measurements given above, you will make a 0.04% bromothymol blue solution. The entire solution has a volume of 250 mL, and there is 0.1 g of bromothymol blue in the solution. 0.1 divided by 250 is equal to 0.0004 or 0.04%. 0.04% is the most common concentration used here but 0.01% is also used sometimes. To make the solution change to a concentration of 0.01%, all you need to do is increase the amount of water you add. Then, proceed with the calculations in the manner demonstrated below:

0.1/x = 0.0001

0.1 = 0.0001x

0.1/0.0001 = x

Thus, x is equal to 1000. 

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 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 that, "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 coverslip 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.

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).

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FAQs (Frequently Asked Questions)

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

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. This change in pH helps us to determine a lot of things and has a plethora of clinical applications. In the field of obstetrics and gynecology, it is used to detect the premature rupture of membranes. The amniotic fluid typically has a pH that is greater than 7.2 which implies that bromothymol will therefore turn blue when brought in contact with any fluid leaking from the amniotic sac.

2. What is the color of Bromothymol Blue when in a base?

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. We have been able to identify that bromothymol blue is a weak acid but it is capable of being in both acid or base form, depending on the pH of the solution. To sum it up, bromothymol blue is yellow in acidic solutions, blue in basic solutions, and green in neutral solutions. 

3. What happens if we consume Bromothymol Blue?

It is extremely important to note that bromothymol blue is not a chemical that is safe for consumption. If and 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 at an excessive level. Drinking water will treat incidental swallowing. Thus, you can expect gastrointestinal irritation with nausea, vomiting, and diarrhea in case of accidental consumption. 

4. Why does the Bromothymol Blue Powder turn out to be green in neutral solutions?

Bromothymol blue turns different colors in different solutions, depending on the pH of the mixture. It 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. This deprotonated form of the compound has its peak absorption wavelength fixed at 427 nm, therefore transmitting yellow light in all acidic solutions.

5. How can students learn more about compounds like bromothymol blue in Chemistry?

Lakhs of students rely on Vedantu to help them with all academic issues. You will come across several chemical compounds in both Organic and Inorganic chemistry. You will need to remember the formulas of at least the basic chemical compounds to score well in your exams. Check out this detailed article on the Definition and Types of Chemical Compounds to know more. Expert teachers at Vedantu strive to make learning accessible and fun for all. Go to their website and make use of their free online resources.