Purpurin is a natural dye and, along with alizarin is found in madder root (Rubia tinctorum L.), although it appears to form during storage. It is very similar to alizarin red S, and is used for much the same purpose as it also forms bright red lakes with calcium.
R. D. Lillie.
Conn's Biological Stains
Williams & Wilkins, Baltimore, MD., U.S.A.
Aldrich chemical catalogue, 1992
Aldrich Chemical Company, Milwaukee, WI, USA.
Susan Budavari, Editor, (1996)
The Merck Index, Ed. 12
Merck & Co., Inc., Whitehouse Station, NJ, USA