Surprises in a ‘Simple' System: 2,4‐Diaminobenzenesulfonic Acid

The search for the polymorphic forms of 2,4‐diaminobenzenesulfonic acid (DBSA), known to exist since 1880, has revealed a surprisingly rich solid‐state system for such a simple molecule. A monohydrate, a dimoiric hydrate, an anhydrate and two polymorphic forms of the hydrochloride of this material have thus far been prepared. Their characterization by microscopic and thermal methods, FT‐IR spectroscopy, and single‐crystal structure determination are described.