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

Thermodynamic properties (the mean activity coefficients, & gamma;& PLUSMN;, and the relative apparent molar enthalpies, & phi;L) of lanthanum sulfate aqueous solutions, strongly deviating from the Debye-Huckel limiting law even at lowest experimentally accessible concentrations, can easily and quantitatively be reproduced assuming the formation of two sulfate complexes, LaSO+4 and La(SO4)2 , 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-Huckel terms. Excellent agreement between measured values of & gamma;& PLUSMN; and & phi;L 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++SOa & REVARR; LaSO4+) and La(SO4)2- (La3++2SOa & REVARR; La(SO4)2-): log10Ko1 = 3.65 & PLUSMN; 0.05; log10Ko2 = 5.63 & PLUSMN; 0.2, & UDelta;rHo1 = 17.6 & PLUSMN; 1.0 kJ mol  1; & UDelta;rH02 = 28.1 & PLUSMN; 4.0 kJ mol  1. The agreement with literature data is excellent for log10Ko1 and & UDelta;rHo1 and satisfactory for log10Ko2 and & UDelta;rHo2. With these log10Ko1, log10Ko2, & UDelta;rHo1 and & UDelta;rHo2 results the experimental & gamma;& PLUSMN; and & phi;L values are reproduced quantitatively.