Phenolphthalein is colorless at low pH and pink or red at higher pH. How does it appear at pH below 8.3 and above 8.3?

• A. pink, blue
• B. blue, red
• C. colorless, pink or red
• D. red, blue

Answer: (C)

Phenolphthalein is a pH indicator that changes color as the solution becomes more basic. In acidic or neutral solutions it is colorless, and as the pH rises into the basic range it develops color, starting as a faint pink and becoming deeper pink or red as the alkalinity increases. The transition occurs around pH 8.2 to 10, with 8.3 commonly used as a practical turning point. So, at pH below 8.3 it appears colorless, and at pH above 8.3 it appears pink or red, depending on how high the pH is.