Thermodynamic properties of aqueous lanthanum sulfate solutions taking into account the association

• Published in: The Journal of chemical thermodynamics 2023-10, Vol.185, p.107094, Article 107094
• Main Author: Plyasunov, Andrey V.
• Format: Article
• Language: English
• Subjects: Activity coefficients; Complex formation; Enthalpies of dilution; Lanthanum sulfate; SIT model
• ISSN: 0021-9614; 1096-3626
• DOI: 10.1016/j.jct.2023.107094

•Deviations from the Debye-Huckel theory is explained by complex formation.•Stability constants are evaluated from activity coefficients.•Enthalpy of complex formation reactions are evaluated from enthalpies of dilution. Thermodynamic properties (the mean activity coefficients, γ±, and the relative apparent molar enthalpies, φL) of lanthanum sulfate aqueous solutions, strongly deviating from the Debye-Hückel limiting law even at lowest experimentally accessible concentrations, can easily and quantitatively be reproduced assuming the formation of two sulfate complexes, LaSO4+ and LaSO42-, whose existence was established in numerous complex formation studies. The analysis was carried out using a simple SIT model for the activity coefficients of ions with only the electrostatic Debye-Hückel terms. Excellent agreement between measured values of γ± and φL and the values of equilibrium constants was obtained by using the following values of equilibrium constants and enthalpy changes of reactions of formation of LaSO4+ (La3++SO42-⇄ LaSO4+) and La(SO4)2- (La3++2SO42-⇄ La(SO4)2-): log10K1o = 3.65 ± 0.05; log10K2o = 5.63 ± 0.2, ΔrH1o = 17.6 ± 1.0 kJ mol−1; ΔrH20 = 28.1 ± 4.0 kJ mol−1. The agreement with literature data is excellent for log10K1o and ΔrH1o and satisfactory for log10K2o and ΔrH2o. With these log10K1o, log10K2o, ΔrH1o and ΔrH2o results the experimental γ± and φL values are reproduced quantitatively.