Phenolphthalein is a weak acid which is often used as an acid-base indicator. Its chemical formula is C20H14O4 (HIn). Phenolphthalein has four color stages in the aqueous solution.

There are 9 test tubes containing the pH indicator phenolphthalein and a solution. The solutions are between pH 3 and 11. The solutions between pH 3 and 7 are colorless. The solutions between pH 8 and 11 change from light pink at pH 8 to dark pink at pH 11.

Figure: The color of phenolphthalein in solution with various pH levels.

When the pH is below 0, the solution is strongly acidic, and the color of the solution is orange. When the pH is between 0 and 0.82, the solution is acidic or near neutral, and the solution is colorless. When the pH is above 13, the solution is basic, and the color of the solution is pink to dark pink.