Bromothymol blue

Bromothymol_blue_deprotonation.svg

Bromothymol blue (also known as bromothymol sulfone phthalein, BTB, and bromothymol blue indicator) is a chemical compound used as a pH indicator. It is a dye belonging to the sulfonephthalein family of dyes.

Chemical Properties

Bromothymol blue has the chemical formula C₂₇H₂₈Br₂O₅S. It is a weak acid and can exist in protonated or deprotonated forms, which are different colors. In its protonated form, bromothymol blue is yellow, and in its deprotonated form, it is blue. The transition between these two forms occurs around a pH of 6.0 to 7.6, making it useful for measuring substances that are near neutral pH.

Uses

Bromothymol blue is commonly used in laboratory settings as a pH indicator. It is particularly useful in applications where a visual indication of pH is required. Some of its uses include:

• Aquarium water testing
• Photosynthesis experiments
• Respiration experiments
• Titration in chemistry

Preparation

Bromothymol blue can be prepared by the bromination of thymol blue. The process involves the reaction of thymol blue with bromine in the presence of a suitable solvent.

Applications in Biology

In biological experiments, bromothymol blue is often used to observe the presence of carbon dioxide. When carbon dioxide dissolves in water, it forms carbonic acid, which lowers the pH of the solution. Bromothymol blue can indicate this change by shifting from blue to yellow.

Safety

Bromothymol blue is generally considered safe to handle in a laboratory setting, but it should be used with appropriate safety precautions, including the use of gloves and eye protection.

See Also

• pH indicator
• Thymol blue
• Phenolphthalein
• Litmus
• Methyl orange