The structure of phenolphthalein. Note the spelling: "hth". Phenolphthalein is an indicator. In acidic and slightly alkaline solutions (pH < 8.5) phenolphthalein is colorless and in more alkaline solutions it is beautifully deep red. Can you account for this change in color? Take a look at the structure discussed in problem 59 on p. 584 in Bruice's text. This compound works in a very similar way.