Bromothymol Blue (C27H28Br2O5S): Structure & Uses

Bromothymol blue, also known as red synthetic dye, is used as an indicator in determining the pH of a substance. It is a weak acid with the chemical formula C₂₇H₂₈Br₂O₅S. 

• Bromothymol blue is also known as BTB, Dibromothymolsulfophthalein and 3,3′-Dibromothymolsulfonphthalein.
• It is used in applications requiring measuring substances with a relatively neutral pH.
• We can use it to measure the presence of carbonic acid in a liquid. 
• The IUPAC name of bromothymol blue is 3,3-Bis[3-bromo-4-hydroxy-2-methyl-5-(propan-2-yl)phenyl]-2,1λ⁶-benzoxathiole-1,1(3H)-dione.
• BTB can be protonated or deprotonated, thus changing the colour to yellow or blue.
• The pH range for bromothymol blue is 6 to 7.6.
• It is typically sold in solid form as the sodium salt of the acid indicator.

Key Terms:  Bromothymol Blue, Bromothymol Sulfonephthalein, pH Indicator, Organic chemistry, Carbonic Acid, pH, Sodium, Dye, Bromothymol blue Structure

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What is Bromothymol Blue?

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Bromothymol blue or bromothymol sulfonephthalein is used as a pH indicator, which gives a yellow colour in case of acid; in basic solutions, it turns blue, and in neutral solutions, it turns a bluish-green colour.

• It contains cycloheximide, chlortetracycline, and chloramphenicol.
• BTB is a weak acid that can act as an acid or in base form, depending on the pH level of the solution. 
• The dry powder of bromothymol sulfonephthalein is converted into a solution form by mixing sodium hydroxide, ethanol, and water.
• However, the compound is quite toxic and can cause skin and eye irritation if swallowed.
• Bromothymol blue is used by obstetrics to detect premature rupture of membranes.

Bromothymol Blue

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Bromothymol Blue Structure

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Bromothymol blue is a heavy-weight molecule of 625 g/mol, which has a unique structure of three aromatic benzene rings. The first ring is connected with the sulfur atom and has a thym group that has two oxygen atoms bonded by a double bond. 

• Another oxygen atom is attached to the sulfur atom via a single bond. 
• Both the second and third benzene rings have a bromine atom, a methyl group, a tert-butyl group, and an alcohol group attached. 
• The last five carbon atoms on two of the benzene have alcohol, propyl, bromine, methyl and hydrogen atoms attached to them.

In another way, bromothymol blue is a member of 2,1-benzoxazoles, which is 2,1-benzoxathiole 1,1-dioxide in which both of the hydrogens at position three have been substituted by 3-bromo-4-hydroxy-5-isopropyl-2-methylphenyl groups.

Bromothymol Blue Structure

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Properties of Bromothymol Blue

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The physical and chemical properties of Bromothymol Blue are as follows:

Physical Properties of Bromothymol Blue

The physical properties of Bromothymol Blue are as follows:

• Bromothymol Blue is an odourless indicator that is sparingly soluble in water.
• It is a white crystalline solid that has a molecular density of 624.38 g/mol.
• The melting and boiling point of BTB is 202 °C (396 °F; 475 K) and 614.26 °C at 760 mmHg, respectively.
• The pH indicator has a density of 1.25g/cm³.

The physical properties are tabulated below:

Chemical Properties of Bromothymol Blue

The chemical properties of Bromothymol Blue are as follows:

Bromothymol Blue pH Range

Bromothymol Blue has a pH range of 6 to 7.6. It is a member of the class of 2,1-benzoxazoles, which is 2,1-benzoxathiole 1,1-dioxide, in which both of the hydrogens at position three have been substituted by 3-bromo-4-hydroxy-5-isopropyl-2-methyl phenyl groups. 

• Bromothymol Blue is a protonated form with an absorption at 927 nm that makes it turn yellow. 
• Its deprotonated form has an absorption point of 602 nm, turning it into blue light when in basic solution. 
• BTB turns blue when neutral solutions are used. 

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Synthesis and Preparation of Bromothymol Blue

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Bromothymol blue is a large molecule that is converted into an aqueous solution as follows:

Reagents required for this process:

The reagents required are as follows:

• 0.1 grams of bromothymol blue powder
• 16mL of 0.01 N sodium hydroxide
• Distilled Water

Steps followed in the process: 

The steps required for the preparation of Bromothymol Blue are as follows:

• Dissolve the given bromothymol blue powder in sodium hydroxide solution
• After the above process, dilute the solution with water that is not more than 250 mL.

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Bromothymol Blue Uses

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Bromothymol Blue is used for various purposes, including :

• It can be used as an indicator of CO2 level and to observe photosynthetic activities.
• Bromothymol blue is a common demonstration of the pH indicator properties and measures the pH level of any solution.
• This indicator is used in conjunction with phenol red to examine the fungal asparaginase enzyme activity when it turns pink. 
• BTB turns blue indicating an increase in pH, which determines the enzyme activity. 
• According to a recent study, the red part is useful for determining activity because the bright yellow ring forms in the enzyme activity zone.
• Bromothymol blue is also used to detect premature membrane rupture. 
• So, bromothymol turns blue after being in contact with leaking fluid amnion. 
• It is also used in the laboratory as a stain on a biological slide. 

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Things to Remember

• Bromothymol blue is mainly used to measure the pH level of any liquid.
• It turns yellow in acid solution, blue in basic solutions, and green in neutral solution.
• The chemical formula of bromothymol blue is C₂₇H₂₈Br₂O₅S.
• Its pH level ranges from 6.0 to 7.6.
• Bromothymol blue has various usages in finding results about various solutions. 

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