Entropy titrations: a reassessment of data for the reaction of the sulphate ion with trivalent lanthanoid ions

Abstract

The simultaneous determination of log K° and ΔH° for anion–cation interactions is examined with reference to the reactions M³⁺+ SO₄^2–⇌ MSO₄⁺ and MSO₄⁺+ SO₄^2–⇌ M(SO₄)₂–(M = lanthanoid). Limitations of the ‘entropy titration’ approach are outlined. From published enthalpimetric data ΔH° values for the formation of the monosulphate ion pairs have been recalculated using published K° data. For the formation of the bis(sulphate) ion pair ΔH° and K° have been recalculated from enthalpimetric data by graphical methods and by general non-linear least-squares analysis.