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The title compound, C4H12N22+·2C8H9N2O4−·H2O, is a hydrated proton-transfer compound obtained from 2-propyl-1H-imidazole-4,5-dicarboxylic acid and piperazine. The asymmetric unit contains one half-cation, one anion and half a water molecule. There is a centre of inversion at the centre of the cation ring and the water molecule O atom lies on a twofold rotation axis. In the crystal, intermolecular N—H...O and N—H...N hydrogen bonds help to construct a three-dimensional framework. Almost symmetrical, intramolecular O—H...O interactions are also observed.