Sodium sulfite hepta­hydrate and its relation to sodium carbonate hepta­hydrate

The monoclinic crystal structure of Na(2)SO(3)(H(2)O)(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(H(2)O)(6)] octa­hedra that form linear (1) (∞)[Na(H(2)O)(4/2)(H(2)O)(2/1)] chains linked by dimeric [Na(H(2)O)(2/2)(H(2)O)(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 Na(2)SO(3)(H(2)O)(7) and ortho­rhom­bic Na(2)CO(3)(H(2)O)(7) are discussed in detail.