A reddish-brown powder which can be dissolved in water or alcohol that has the chemical formula of C21H18O5S. The dissolved compound has different colors depending on the pH; when the pH is 7.2 it is yellow, when the pH is 8.8 it is red, and when the pH is 2-3 it is orangish. As a result, it is used as an acid-base indicator. It is also used as a stain in the study of histology.