Phenolphthalein, which is an acid-base pH indicator, is a weak acid. For this reason, this indicator is colorless in the presence of acids that are also associated with a high concentration of hydrogen ions. In the presence of a basic solution, phenolphthalein changes to a pink color because bases are associated with a concentration of hydroxide ions.
The presence of an excess of either hydrogen or hydroxide ions in solutions accounts for these color shifts. When using phenolphthalein as an indicator, the shift in color from colorless to a light pink occurs at a pH of 8.2. For a pH that ranges from 8.3 and 10, the solution the color is a dark pink. Basic solutions that are at the upper end of the pH scale, which is 14.0, can be a violet color.
The pH scale ranges for 0.0 to 14.0, where 7.0 is a neutral substance. Above this neutral point are basic solutions, while below this point are acidic solutions.