Research Article: Sodium sulfite hepta­hydrate and its relation to sodium carbonate hepta­hydrate

Date Published: May 01, 2020

Publisher: International Union of Crystallography

Author(s): Matthias Weil, Kurt Mereiter.


The crystal structure of Na2SO3(H2O)7 shows close structural similarities with Na2CO3(H2O)7, though the two hepta­hydrates belong to different crystal systems (monoclinic and ortho­rhom­bic, respectively) and contain anions with different shapes.

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Sodium sulfite is used extensively in industrial processes, for example, as an anti­oxidant and preservative in food industries (E number for food additives E221), as a corrosion inhibitor in aqueous media, as a bleaching agent, as a solubilizing agent for cellulose, straw and wood in the pulp and paper industry, or as an additive in dying processes. In the USA alone, the production of sodium sulfite reached 150 000 tons in 2002 (Weil et al., 2007 ▸). Solid sodium sulfite is stable in its anhydrous form and as the hepta­hydrate. Despite its use at industrial scales, structural details are known only for anhydrous Na2SO3 that crystallizes with two formula units in the trigonal system in the space group P (Larsson & Kierkegaard, 1969 ▸). Bond lengths and near-neighbour distances of sodium sulfite in aqueous solution have been calculated by ab initio quantum mechanical charge field mol­ecular dynamics (QMCF MD) studies and determined experimentally by large-angle X-ray scattering (LAXS) by Eklund et al. (2012 ▸). For crystalline Na2SO3(H2O)7, lattice parameters and the space group (P21/n) have previously been determined from Weissenberg photographs without providing further structural details, except for a close metrical resemblance with ortho­rhom­bic Na2CO3(H2O)7 (Dunsmore & Speakman, 1963 ▸). To obtain a more detailed picture of the relationship between the hepta­hydrates of Na2SO3 and Na2CO3, we grew single crystals of Na2SO3(H2O)7 and determined its crystal structure. Indeed, the two hepta­hydrates show not only a close metrical relationship (Table 1 ▸), but also structural similarities, though they belong to different crystal systems and contain differently shaped divalent anions, viz. trigonal–pyramidal SO32− and trigonal–planar CO32−.




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