Bromophenol blue

Meaning: Bromophenol blue is a weak acid, HIn, whose conjugate base, In⁻, has a different color. The acid form is yellow and the base form is blue, so the color shows which form predominates. The color change happens over a range of about pH 3.0 to 4.6, centered near its pKa of about 4. Below the range the solution looks yellow, above it blue, and in between it is a green-ish mixture.

Equation: pH = pKa + log([In⁻]/[HIn]). pH and pKa (= −log Ka) are dimensionless, and the brackets are concentrations in mol/L. A color change is visible when the ratio is roughly 1/10 to 10, so the transition range is about pKa ± 1.

On the MCAT: A passage lists indicators with their ranges and gives a titration curve. You pick the indicator whose range falls on the steep, near-vertical part of the curve at the equivalence point. Bromophenol blue has an acidic range, so it fits a strong acid–weak base titration, whose equivalence point is below 7. It is a poor choice for a weak acid–strong base titration, whose equivalence point is above 7. You may also be asked its color at a given pH.

Common confusion: Students assume an indicator changes color at pH 7 or exactly at equivalence. It changes near its own pKa. The endpoint approximates the equivalence point only if the indicator's range matches the curve. At pH 7, bromophenol blue is already fully blue.