The mutual replacement of SO32- and SO42- anions was studied in Na2SO3.7 H2O crystals where sulphite is slowly oxidized to sulphate by air oxygen. In a series of Na2(SO3, SO4).7 H2O samples, the C1 or Cs site symmetry appears for the SO32- anion and the site symmetry of the SO42- anion is lowered gradually from Td through D2d or C3v to C1. The replacement was also studied for anhydrous sodium sulphite obtained by dehydration or by evaporation from aqueous solution. Despite the different structures of Na2SO3 and Na2SO4 crystals (C3i1 and D2h24, respectively), the favourable dimensions of the sterically different anions allow the SO42- ions, with their own symmetry Td, to be located in the C3 sites of the SO32- ions in Na2SO3.
