Bromothymol blue

Bromothymol blue is a pH-sensitive dye that appears yellow in acidic solutions, blue in alkaline solutions, and green near neutral pH.

Acid–base indicator: A substance whose observable property changes over a defined pH range. Bromothymol blue is a visual indicator whose changing color signals acidity or alkalinity.
pH indicator: A chemical that changes a measurable property over a range of solution acidity or alkalinity. Bromothymol blue is used to estimate pH through a visible color change.
Thymol blue: A sulfonephthalein pH indicator with two color transitions at different acidity ranges. It is a related indicator with a different structure and transition ranges.
Phenolphthalein: A pH indicator that is colorless in acidic solution and pink in alkaline solution. Its transition occurs at higher pH, making it better suited to different titration endpoints.
Acid–base equilibrium: A reversible reaction between protonated and deprotonated forms of an acid–base pair. The dye’s two colored forms coexist in a pH-dependent equilibrium.
Acid–base titration: A quantitative method that determines an acid or base concentration through neutralization. The indicator can mark endpoints when a titration’s pH changes through its transition range.
Sulfonephthalein: A family of synthetic dyes used chiefly as acid–base indicators. Bromothymol blue belongs to this indicator-dye family.
Methyl orange: An acid–base indicator that shifts from red in acidic solution to yellow at higher pH. Its acidic transition range contrasts with bromothymol blue’s near-neutral range.
Henderson–Hasselbalch equation: An equation relating a buffer’s pH to the pKa and ratio of conjugate base to acid. It relates the dye’s color transition to the ratio of its ionic forms.
Carbon dioxide indicator solution: A solution containing an indicator whose color responds to carbon dioxide-driven pH changes. Dissolved carbon dioxide acidifies the solution and shifts bromothymol blue toward yellow.

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