Crystal structure of sodium dihydrogen arsenate

The crystal structure of Na(H2AsO4) is isotypic with that of Na(H2PO4). The two structures are compared with the aid of the COMPSTRU program.

Single crystals of the title compound, Na(H 2 AsO 4 ), were obtained by partial neutralization of arsenic acid with sodium hydroxide in aqueous solution. The crystal structure of Na(H 2 AsO 4 ) is isotypic with the phosphate analogue and the asymmetric unit consists of two sodium cations and two tetrahedral H 2 AsO 4 À anions. Each of the sodium cations is surrounded by six O atoms of five H 2 AsO 4 À groups, defining distorted octahedral coordination spheres. In the extended structure, the sodium cations and dihydrogen arsenate anions are arranged in the form of layers lying parallel to (010). Strong hydrogen bonds [range of OÁ Á ÁO distances 2.500 (3)-2.643 (3) Å ] between adjacent H 2 AsO 4 À anions are observed within and perpendicular to the layers. The isotypic structure of Na(H 2 PO 4 ) is comparatively discussed.
Besides the Na:As 1:1 phase Na(H 2 AsO 4 )(H 2 O) another 1:1 phase, Na(H 2 AsO 4 ), has been reported but without an additional water molecule (Fehé r & Morgenstern, 1937). To our surprise, a detailed structural investigation of this salt has not yet been reported. Therefore, we started crystal growth experiments and determined its structure and report here on the results.

Structural commentary
The crystal structure of Na(H 2 AsO 4 ) is isotypic with that of Na(H 2 PO 4 ) (Catti & Ferraris, 1974). The asymmetric unit of Na(H 2 AsO 4 ) comprises two Na + cations and two tetrahedral AsO 2 (OH) 2 À groups. The Na1 + cation shows a narrow Na-O bond-length distribution in the range 2.337 (2) to 2.498 (2) Å with a distorted octahedron as the corresponding coordination polyhedron. The bond-valence sum (Brown, 2002) for the Na1 + cation amounts to 1.15 valence units. The surrounding of the Na2 + cation is much more distorted, with a bond-length range from 2.338 (2) to 2.769 (3) Å under consideration of a sixfold coordination (bond-valence sum 0.92 valence units). There is an additional remote oxygen atom at a distance of 3.000 (3) Å from Na2 + . Its contribution of 0.04 valence units to the bond-valence sum might be considered as too low for a significant interaction, and therefore the first coordination sphere of Na2 + is discussed as that of a considerably distorted octahedron. The two dihydrogen arsenate groups show the usual differences (Weil, 2000(Weil, , 2016 between As-O and As-(OH) bonds, with two significantly shorter As-O bonds [mean 1.659 (8) Å ] and two longer As-(OH) bonds [1.723 (12) Å ].
In the crystal structure of Na(H 2 AsO 4 ) the AsO 2 (OH) 2 tetrahedra are arranged in layers lying parallel to (010) with the Na + cations approximately on the same level ( Fig. 1).
Strong, asymmetric hydrogen bonds [OÁ Á ÁO distances between 2.500 (3) and 2.643 (3) Å , Table 1] between each of the OH groups of the two dihydrogen arsenate tetrahedra and O atoms of adjacent tetrahedra significantly contribute to the crystal packing. These hydrogen bonds are both within a layer and towards adjacent layers (Fig. 1).
The differences between the isotypic arsenate and phosphate structures can mainly be seen in the X-O bond lengths of the anions (X = As, mean of 1.69 Å ; X = P, mean of 1.55 Å ), with Á max (X-O) of 0.15 Å between arsenate and phosphate tetrahedra. The difference with respect to the Na-O distances in the two structures is less pronounced, with Á max (Na-O) = 0.10 Å . Relevant bond lengths of the isotypic crystal structures of Na(H 2 AsO 4 ) and Na(H 2 PO 4 ) (Catti & Ferraris, 1974) are compiled in Table 2. A more quantitative comparison of the two crystal structures with the help of the COMPSTRU routine (de la Flor et al., 2016) revealed the following values: The degree of lattice distortion (S), i.e. the spontaneous strain (sum of the squared eigenvalues of the strain tensor divided by 3), is 0.0159; the maximum distance (d max. ), i.e. the maximal displacement between the atomic positions of paired atoms, is 0.1920 Å for atom pair O1; the arithmetic mean (d av ) of the distances of all atom pairs is research communications Table 1 Hydrogen-bond geometry (Å , ). (2) 1.75 (2) 2.595 (3) 175 (5) (3) 169 (4) Symmetry codes: (i) x; Ày þ 1 2 ; z þ 1 2 ; (ii) x þ 1; y; z; (iii) Àx þ 2; Ày þ 1; Àz þ 1.

Figure 1
The crystal structure of Na(H 2 AsO 4 ) in a projection along [100]. All atoms are depicted with displacement ellipsoids at the 97% probability level. Dihydrogen arsenate tetrahedra are given in polyhedral representation, Na + cations as single ellipsoids without bonds to surrounding O atoms. HÁ Á ÁO hydrogen bonds are illustrated with green lines. 0.1108 Å ; the measure of similarity (Á) (Bergerhoff et al., 1999) is a function of the differences in atomic positions (weighted by the multiplicities of the sites) and the ratios of the corresponding lattice parameters of the structures and amounts to 0.049.

Synthesis and crystallization
The title compound was prepared following a procedure by Fehé r & Morgenstern (1937). An arsenic acid solution (ca 65% wt ) was partly neutralized with diluted NaOH solution using methyl red as indicator. The resulting solution was concentrated by heating. Standing of the solution overnight on a warm plate (ca 313 K) afforded colourless crystals with a lath-like form and maximal edge lengths of 0.5 mm.

Crystal data
Na ( Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.