8-Hydroxy-7-nitroquinoline-5-sulfonic acid monohydrate.

The title compound, C9H6N2O6S.H2O, is zwitterionic (i.e. 8-hydroxy-7-nitroquinolinium-5-sulfonate monohydrate) with a deprotonated sulfonic group and a protonated quinoline N atom which interacts with the hydroxy O atom. The H atom of the hydroxy group is distal to the N atom and is involved in an interaction with an O atom of the nitro group, which is oriented at 32.2(1) degrees with respect to the quinoline ring system.