Entropy titrations: a reassessment of data for the reaction of the sulphate ion with protons and with bivalent metal ions

Abstract

The method of simultaneous determination of log K° and ΔH° for anion-cation interactions is critically discussed by reference to the reactions H⁺+ SO₄^2–⇌ HSO₄^– and M²⁺+ SO₄^2–⇌ MSO₄. For these reactions the ‘entropy titration’ approach is less reliable than that employing potentiometrically determined K° data to calculate ΔH° from calorimetric measurements. The effects of the choice of activity coefficient expression are discussed. A general non-linear least-squares procedure has been used for the simultaneous determination of ΔH° and K°. The procedure is contrasted with the iterative method used by other workers. ΔH° for M²⁺+ SO₄^2–⇌ MSO₄ have been recalculated by use of potentiometric K° data and published calorimetric data.