Sodium sulfite heptahydrate and its relation to sodium carbonate heptahydrate

Abstract

The monoclinic crystal structure of Na2SO3(H2O)7 is characterized by an alternating stacking of (100) cationic sodium–water layers and anionic sulfite layers along [100]. The cationic layers are made up from two types of [Na(H2O)6] octahedra that form linear 1 ∞[Na(H2O)4/2(H2O)2/1] chains linked by dimeric [Na(H2O)2/2(H2O)4/1]2 units on both sides of the chains. The isolated trigonal–pyramidal sulfite anions are connected to the cationic layers through an intricate network of O—H...O hydrogen bonds, together with a remarkable O—H...S hydrogen bond, with an O...S donor–acceptor distance of 3.2582 (6) Å, which is about 0.05 Å shorter than the average for O—H...S hydrogen bonds in thiosalt hydrates and organic sulfur compounds of the type Y—S—Z (Y/Z = C, N, O or S). Structural relationships between monoclinic Na2SO3(H2O)7 and orthorhombic Na2CO3(H2O)7 are discussed in detail.