Phenolsulfonphthalein (phenol red) is a weak acid with pKₐ=8.0 at 20°C (68°F). a solution of phenol red is used as a pH indicator, often in cell culture. Its color exhibits a gradual transition from yellow to red (over the pH range of 6.8─8.2). And above pH 8.2, phenol red turns to a bright pink color.