Abstract
Single crystals of the title compound, Na(H2AsO4), were obtained by partial neutralization of arsenic acid with sodium hydroxide in aqueous solution. The crystal structure of Na(H2AsO4) is isotypic with the phosphate analogue and the asymmetric unit consists of two sodium cations and two tetra-hedral H2AsO4- anions. Each of the sodium cations is surrounded by six O atoms of five H2AsO4- groups, defining distorted octa-hedral coordination spheres. In the extended structure, the sodium cations and di-hydrogen arsenate anions are arranged in the form of layers lying parallel to (010). Strong hydrogen bonds [range of O⋯O distances 2.500 (3)-2.643 (3) Å] between adjacent H2AsO4- anions are observed within and perpendicular to the layers. The isotypic structure of Na(H2PO4) is comparatively discussed.
Highlights
Single crystals of the title compound, Na(H2AsO4), were obtained by partial neutralization of arsenic acid with sodium hydroxide in aqueous solution
The crystal structure of Na(H2AsO4) is isotypic with the phosphate analogue and the asymmetric unit consists of two sodium cations and two tetrahedral H2AsO4
The sodium cations and dihydrogen arsenate anions are arranged in the form of layers lying parallel to (010)
Summary
Arsenic acid is triprotic and can form various salts, depending on the degree of deprotonation (H2AsO4, HAsO42, AsO43), the condensation grade of the anion (mono-, di-, tri-, polyarsenate, etc) and the amount of water incorporated in the crystal. With respect to sodium arsenates, numerous crystal structures have been determined so far, including arsenic in tetrahedral and/or in octahedral coordination by oxygen atoms. Arsenate structures with arsenic exclusively in tetrahedral coordination resemble those of the related phosphates and in some cases show isotypism with them (marked by an asterisk): Na3.25(AsO4)(OH)0.25(H2O)12*. Arsenate structures with arsenic in (complete or partial) octahedral coordination include Na(H2As3O9). A detailed discussion of the structural principles and crystal chemical characteristics of arsenates with arsenic in octahedral coordination was given some time ago by Schwendtner & Kolitsch (2007). Dihydrogen arsenate tetrahedra are given in polyhedral representation, Na+ cations as single ellipsoids without bonds to surrounding O atoms.
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