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Formation of CaSO 4 (aq) and CaSeO 4 (aq) studied as a function of ionic strength and temperature by CE
Author(s) -
Philippini Violaine,
Aupiais Jean,
Vercouter Thomas,
Moulin Christophe
Publication year - 2009
Publication title -
electrophoresis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.666
H-Index - 158
eISSN - 1522-2683
pISSN - 0173-0835
DOI - 10.1002/elps.200900261
Subject(s) - ionic strength , chemistry , enthalpy , endothermic process , equilibrium constant , ionic bonding , ion , standard enthalpy of formation , analytical chemistry (journal) , thermodynamics , aqueous solution , adsorption , chromatography , organic chemistry , physics
Ca 2+ 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 CaSO 4 (aq) and CaSeO 4 (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 NaNO 3 ionic strength solutions, and in 0.5 and 1.0 mol/L NaNO 3 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 r G 0.1M (25°C)=−(7.59±0.23) kJ/mol, Δ r H 0.1 M =5.57±0.80 kJ/mol, and Δ r S 0.1 M (25°C)=44.0±3.0 J mol −1 K −1 for CaSO 4 (aq) and Δ r G 0.1 M (25°C)=−(6.66±0.23) kJ/mol, Δ r H 0.1 M =6.45±0.73 kJ/mol, and Δ r S 0.1 M (25°C)=44.0±3.0 J mol −1 K −1 for CaSeO 4 (aq). Both formation reactions were found to be endothermic and entropy driven. CaSO 4 (aq) appears to be more stable than CaSeO 4 (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 ${\rm {\varepsilon}}_{{{{\rm Na}^+}, {{\rm SO}_{4}^{2-}}}}= -{(0.15 \pm 0.06)}\,$ mol/kg −1 and ${\rm {\varepsilon}}_{{{{\rm Na}^+}, {{\rm SO}_{4}^{2-}}}}= -{(0.26 \pm 0.10)}\,$ mol/kg −1