Bromothymol blue

Bromothymol blue

Bromothymol blue, also known as bromthymol blue, is a chemical indicator for weak acids and bases. Following are given its colours at different solution pH values.

Alkaline (basic) - Blue (pH > 7)
Neutral - Green (pH = 7)
Acid - Yellow (pH < 7)

The bromothymol blue acts as a weak acid in solution and therefore can be in acid or base forms which appear yellow and blue respectively:

C₂₁H₁₅NaBr₂O₃S ↔ H⁺ + C₂₁H₁₄NaBr₂O₃S-

The pKa for bromothymol blue is 7.

Formula: C₂₁H₁₅NaBr₂O₃S
Alternative name: Dibromothymolsulfonephthalein, BTB