Absorption and diffusion of sulphur dioxide into aqueous sodium chloride solutions

Abstract

Multicomponent diffusion equations have been used to describe the absorption of an ionizing gas into a solution that contains an insert supporting electrolyte. The example of absorption of sulphur dioxide into aqueous sodium chloride solutions has been treated in detail. In this case the electric field generated by diffusion of partially ionized sulphur dioxide (SO₂+ H₂O = HSO^–₃+ H⁺) produces large coupled ionic flows that respectively increase and decrease the interfacial concentrations of Na⁺ and Cl^– ions relative to bulk values. The added NaCl enables highly mobile H⁺ ions to diffuse into the bulk absorbent several times more rapidly than the sulphur-containing species. Loss of H⁺ ions from the interfacial solution is shown to increase the interfacial solubility and the rate of absorption of sulphur dioxide by up to 60% relative to the corresponding values for absorption into pure water.