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.