The volume and compressibility change for the formation of the LaSO₄⁺ ion pair at 25°C

The apparent molal volumes (φ_v) and adiabatic compressibilities [φ_K(S)] of La₂(SO₄)₃ solutions have been determined from density and sound speed data at 25°C. The large positive deviations of φ_v and φ_K(S) of La₂(SO₄)₃ from the limiting law have been attributed to the formation of the ion pair LaSO₄⁺. The observed values of φ_v and φ_K(S) have been used to estimate the change in the apparent molal volume and adiabatic compressibility for the formation of LaSO₄⁺ from {Mathematical expression} where φ{symbol}(2La³⁺, 3SO₄^2-) is the apparent molal volume or adiabatic compressibility of the free ions, and α is the degree of association. The value of {Mathematical expression} and {Mathematical expression} at infinite dilution are in reasonable agreement with the values determined from the high-pressure conductance data of Fisher and Davis. The number of hydrated water molecules (ca. 11) associated with the formation of LaSO₄⁺ determined from the volume and compressibility data are in good agreement.