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The title compound, C2H10N2 2+·2HC4O4 −·H2O, a new polymorph of ethyl­enediammonium bis­(hydrogen squarate) monohydrate, was synthesized by slow evaporation of an acid solution. The asymetric unit contains two hydrogen squarate anions, two half-mol­ecules of protonated ethyl­enediamine arranged around a twofold axis and one water mol­ecule. In the crystal, N—H⋯O and O—H⋯O hydrogen bonds between the hydrogen squarate anions, protonated N atoms from the amine group and water mol­ecules lead to a three-dimensional framework. In particular, the cohesion between the squarate groups is ensured by very short intermolecular hydrogen bonds bonds. The title compound crystallized together with the previously reported polymorph [Mathew et al. (2002 ▶). J. Mol. Struct. 641, 263–279].

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Key indicators: single-crystal X-ray study; T = 293 K; mean (C-C) = 0.005 Å; R factor = 0.041; wR factor = 0.103; data-to-parameter ratio = 15.9.