The title compound, C(8)H(8)N(2)O(4), is almost planar (r.m.s. deviation = 0.037 Å) and an intra-molecular N-H⋯O hydrogen bond generates an S(6) ring. In the crystal, inversion dimers linked by pairs of O-H⋯O hydrogen bonds generate R(2) (2)(8) loops. Inter-molecular N-H⋯O hydrogen bonds (involving the same H atom that forms the intra-molecular hydrogen bond) link the dimers into infinite sheet...

The asymmetric unit of the title compound, C(7)H(5)ClO(3), contains two mol-ecules; both feature an intra-molecular O-H⋯O hydrogen bond, which generates an S(6) ring. In the crystal, both mol-ecules form inversion dimers linked by pairs of O-H⋯O hydrogen bonds with R(2) (2)(8) ring motifs. The dimers are inter-linked by C-H⋯O inter-actions.

In the title compound, C(5)H(5)NO(3), the mol-ecule lies on a crystallographic mirror plane with one half-mol-ecule in the asymmetric unit. An intramolecular C-H⋯O inter-action is present. In the crystal, strong inter-molecular O-H⋯N hydrogen bonds result in the formation of a linear chain structure along [100], and there are also weak C-H⋯O hydrogen bonds between the chains which help to stabi...