Experimental Crystal data

R factor = 0.022; wR factor = 0.055; data-to-parameter ratio = 16.6. In comparison with the original determination based on Weissenberg film data [Khan et al. (1970). Acta Cryst. B26, 1889–1892], the current redetermination of diammonium hydrogenarsenate(V) reveals all atoms with anisotropic displacement parameters and all H atoms localized. This allowed an unambiguous assignment of the hydrogen-bonding pattern, which is similar to that of the isotypic phosphate analogue (NH4)2HPO4. The structure of the title compound consists of slightly distorted AsO3(OH) and NH4 tetrahedra, linked into a three-dimensional structure by an extensive network of O—H O and N—H O hydrogen bonds. Related literature For the previous determination of (NH4)2HAsO4, see: Khan et al. (1970). The arsenate compound is isotypic with the phos-phate analogue (NH4)2HPO4 (Khan et al., 1972), for which another modification with Z0 = 2 has also recently been described (Kunz et al., 2010)