Activity coefficients of lanthanum salts at 298.15 K

Abstract

Activity coefficients of La(NO₃)₃, La(ClO₄)₃, and La₂(SO₄)₃ are determined using the method of liquid membrane cells, which allows us to study high dilution levels previously unattainable (10⁻⁴ to 10⁻⁵ mol kg⁻¹ depending on the salt). Literature values are found to be correct for La(ClO₄)₃, moderately biased for La(NO₃)₃ and completely incorrect for La₂(SO₄)₃. This last salt in the diluted regions displays dramatic negative deviations—not recognised earlier because the measurements terminated at ca. 10⁻³ mol kg⁻¹, where a Debye–Hückel-like misleading trend occurs. Numerical integration of the Poisson–Boltzmann equation and Bjerrum's theory of ion pairing show an astounding ability to reproduce the trend of La₂(SO₄)₃; however, there is evidence of short-range interactions that add to long-range interactions for lanthanum and bivalent metal sulfates, as if sulfate ions were displacing water from the hydration shells of the cations. Pitzer's equation parameters that reproduce the activity and osmotic coefficients of the three salts and those (recalculated) of LaCl₃ are reported.