Formation of CaSO4(aq) and CaSeO4(aq) studied as a function of ionic strength and temperature by CE

Ca²⁺ complexation by both sulfate and selenate ligands was studied by CE. The species were observed to give a unique retention peak as a result of a fast equilibrium between the free ions and the complexes. The change in the corresponding retention time was interpreted with respect to the equilibrium constant of the complexation reaction. The results confirmed the formation of CaSO₄(aq) and CaSeO₄(aq) under our experimental conditions. The formation data were derived from the series of measurements carried out at about 15, 25, 35, 45 and 55°C in 0.1 mol/L NaNO₃ ionic strength solutions, and in 0.5 and 1.0 mol/L NaNO₃ ionic strength solutions at 25°C. Using a constant enthalpy of reaction enabled to fit all the experimental data in a 0.1 mol/L medium, leading to the thermodynamic parameters: Δ_rG^0.1M(25°C)=?(7.59±0.23) kJ/mol, Δ_rH^{0.1 M}=5.57±0.80 kJ/mol, and Δ_rS^{0.1 M}(25°C)=44.0±3.0 J mol^?1 K^?1 for CaSO₄(aq) and Δ_rG^{0.1 M}(25°C)=?(6.66±0.23) kJ/mol, Δ_rH^{0.1 M}=6.45±0.73 kJ/mol, and Δ_rS^{0.1 M}(25°C)=44.0±3.0 J mol^?1 K^?1 for CaSeO₄(aq). Both formation reactions were found to be endothermic and entropy driven. CaSO₄(aq) appears to be more stable than CaSeO₄(aq) by 0.93 kJ/mol under these experimental conditions, which correlates with the difference of acidity of the anions as expected for interactions between hard acids and hard bases according to the hard and soft acids and bases theory. The effect of the ionic medium on the formation constants was successfully treated using the Specific ion Interaction Theory, leading to significantly different binary coefficients mol/kg^?1 and mol/kg^?1