Thymol blue is an indicator that changes color from red to yellow in the pH range of 1.2-2.8. Phenolphthalein is an indicator that changes color from colorless to pink in the pH range of 8.2-10.0. Sodium hydroxide is a strong base that will raise the pH of the solution. When sodium hydroxide is added to the mixture of thymol blue and phenolphthalein, the thymol blue will remain yellow and the phenolphthalein will turn pink, resulting in a blue color.
