Formula of phenolphthalein at very low pH
<a href="//commons.wikimedia.org/wiki/User:Master_Uegly" title="User:Master Uegly">Master Uegly</a>
2012-02-22
own work under usage of <a href="//commons.wikimedia.org/wiki/File:Phenolphthalein-mid-pH-2D-skeletal.svg" title="File:Phenolphthalein-mid-pH-2D-skeletal.svg">Phenolphthalein-mid-pH-2D-skeletal.svg</a>