Studies on alkaline earth sulfites — III [1]. Transient solubilities and phase changes of calcium sulfite in seawater

Calcium sulfite was found to be sufficiently soluble in seawater of various compositions to allow the use of SO2 for pretreatment of seawater (to prevent corrosion and alkaline scale formation) in desalting plants. The solubility of calcium sulfite, prepared by mixing 0.4M solutions of Na2SO3 and CaCl2, in natural or synthetic seawater decreases with time and does not reach equilibrium for several weeks. Transient mmolal solubilities, at 25°C after 20 min and 24 h respectively, are 2.9 and 2.2 in natural seawater, 3.9 and 3.4 in 0.7M NaCl solution. This decrease is due to a gradual transformation, by a slow dissolution-recrystallization process, of a meta-stable phase of crystalline calcium sulfite into a more stable, less soluble, modification observed by X-ray crystallography, scanning electron microscopy and thermal analysis. The addition of Na2SO3 to natural seawater produced solutions strongly supersaturated with respect to calcium sulfite. These remained stable for extended periods, and were only slowly affected by solid calcium sulfite added later.