Redetermination of (NH4)2HAsO4

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.

In comparison with the original determination based on Weissenberg film data [Khan et al. (1970). Acta Cryst. B26, [1889][1890][1891][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 (NH 4 ) 2 HPO 4 . The structure of the title compound consists of slightly distorted AsO 3 (OH) and NH 4 tetrahedra, linked into a three-dimensional structure by an extensive network of O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds.

Matthias Weil
Comment (NH 4 ) 2 HAsO 4 is a frequently used precursor material for preparation of arsenate(V) compounds, starting either from (aqueous) solutions or via ceramic routes. The crystal structure of (NH 4 ) 2 HAsO 4 has originally been determined by Khan et al. (1970) based on Weissenberg photographs. In the original study all atoms were refined with isotropic displacement parameters. Since H atoms could not be localized, the authors could make only assumptions with respect to the resulting hydrogen bonding pattern, deduced from N···O and O···O distances. These assumptions included three models: i) the NH 4 ions exhibit rotatory oscillations; ii) the NH 4 ions are in static disorder; iii) each of the two N atoms forms a bifurcated bond in addition to three normal hydrogen bonds (Khan et al., 1970). Somewhat later Khan et al. (1972) showed for the isotypic phosphate analogue (NH 4 ) 2 HPO 4 that dynamic or static disorder can be ruled out for the NH 4 groups and that the ammonium tetrahedra form four classical hydrogen bonds to the PO 3 (OH) groups. The current redetermination of the structure of (NH 4 ) 2 HAsO 4 using modern CCD-based data was intended to shed some light on its hydrogen bonding pattern and to compare the results with the phosphate analogue.
The redetermination confirmed the basic features of the original study, however with the unambiguous localization of all H atoms and, as expected, with higher precision and accuracy. Like in (NH 4 ) 2 HPO 4 , the ammonium groups show no static or dynamic disorder and four normal N-H···O hydrogen bonds are formed between the constituents. The largest difference between the two determinations pertains to the O···O distance of the O-H···O (O1···O4) hydrogen bond. In the original study this distance was determined as 2.669 (13) Å, whereas it is 2.613 (2) Å in this study. The latter matches very well with 2.615 (1) Å for the phosphate analogue for which all H atoms could be localized (Khan et al., 1972). In the latter study it was suggested that the difference between these O···O distances of the phosphate (2.615 (1) Å) and the arsenate (2.669 (13) Å) structure is a consequence of the different size of the P 5+ and the As 5+ ions. However, the current redetermination of (NH 4 ) 2 HAsO 4 shows that the influence of the different sizes for the phosphate (average P-O distance 1.54 Å) and the arsenate (average As-O distance 1.68 Å) tetrahedra can in fact be neglected.   (Khan et al., 1972).
For (NH 4 ) 2 HPO 4 another crystalline polymorph has been described, resulting from hydrolysis of the educt, viz.
ammonium hexafluoridophosphate (Kunz et al., 2010). It would be interesting to know whether an arsenate polymorph isotypic with the phosphate analogue or another polymorph (NH 4 ) 2 HAsO 4 exist as well.

Experimental
Crystals of the title compound were grown from an aqueous solution containing diluted arsenic acid (20% wt ) mixed with a concentrated aqueous solution of ammonia in excess. The solution was kept in a desiccator with CaCl 2 as drying agent.
The first crystals, mostly with a plate-like form, appeared approximately after one week.

Refinement
For better comparison, the same unit cell setting as in the previous determination of (NH 4 ) 2 HAsO 4 (Khan et al., 1970) was used. The current setting is not reduced and can be transformed to the reduced setting by application of the matrix (101, 010, 001). For refinement, the atomic coordinates of the As, O and N atoms (Khan et al., 1972) were used as starting parameters. All H atoms were clearly discernible from difference Fourier maps and were refined freely.