Sodium sulfite heptahydrate and its relation to sodium carbonate heptahydrate.

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.