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Bromothymol blue is a pH indicator commonly used to measure substances with a near-neutral pH (around 7). It is often used to detect the presence of carbonic acid in liquids and is typically sold as the sodium salt in solid form.
Structure and Properties

BTB acts as a weak acid and exists in protonated (yellow) or deprotonated (blue) forms. In a neutral solution, it appears greenish-blue. The protonated form absorbs light at 427 nm, transmitting yellow light, while the deprotonated form absorbs at 602 nm, transmitting blue light. In strongly acidic conditions, it turns magenta. The presence of bromine (an electron-withdrawing group) and alkyl substituents determines its active pH range from 6.0 to 7.6.
🏭⚗️ Synthesis and Preparation

Bromothymol blue is synthesized by adding elemental bromine to thymol blue in glacial acetic acid. To prepare a working indicator solution, dissolve 0.10 g of BTB in 8.0 cm³ of 0.02 N NaOH and dilute to 250 cm³ with water. For volumetric uses, dissolve 0.1 g in 100 cm³ of 50% ethanol.
🧪 Applications
• Monitoring photosynthetic or respiratory activity: As CO2 is added, the color changes from green to yellow, demonstrating CO2 production during muscle activity.
• Enzyme activity assays: Used alongside phenol red to observe fungal asparaginase activity, indicated by color changes.
• Biological staining: Helps visualize cell walls or nuclei under a microscope by staining samples blue.