Physical properties of iodate solutions and the deliquescence of crystalline I₂O₅ and HIO₃

Abstract. Secondary aerosol produced from marine biogenic sources in algal-rich coastal locations will initially be composed of iodine oxide species, most likely I₂O₅, or its hydrated form HIO₃, formed as a result of iodine gas-phase chemistry. At present, there is no quantitative hygroscopic data for these compounds and very little data available for iodate solutions (HIO₃ and I₂O₅ share a common aqueous phase). With increased interest in the role of such aerosol in the marine atmosphere, we have conducted studies of (i) the deliquescence behaviour of crystalline HIO₃ and I₂O₅ at 273–303 K, (ii) the efflorescence behaviour of aqueous iodate solution droplets, and (iii) properties (water activity, density, and viscosity) of subsaturated and saturated iodate solutions.

The deliquescence of I₂O₅ crystals at 293 K was observed to occur at a relative humidity (DRH) of 80.8±1.0%, whereas for HIO₃, a DRH of 85.0±1.0% was measured. These values are consistent with measured water activity values for saturated I₂O₅ and HIO₃ solutions at 293 K of 0.80±0.01 and 0.84±0.01 respectively. At all temperatures, DRH values for HIO₃ crystals were observed to be higher than for those of I₂O₅. The temperature-dependent DRH data, along with solubility and water activity data were used to evaluate the enthalpy of solution (ΔH_sol) for HIO₃ and I₂O₅. A ΔH_sol value of 8.3±0.7 kJ mol⁻¹ was determined for HIO₃ which is consistent with a literature value of 8.8 kJ mol⁻¹. For I₂O₅, we report for the first time its solubility at various temperatures and ΔH_sol = 12.4±0.6 kJ mol⁻¹. The measured water activity values confirm that aqueous iodate solutions are strongly non-ideal, consistent with previous reports of complex ion formation and molecular aggregation.