A Raman spectroscopic investigation of speciation in La2(SO4)3(aq)

Abstract

Raman spectroscopic measurements have been made of aqueous solutions of La(ClO₄)₃, La₂(SO₄)₃, and Na₂SO₄ in water and heavy water, in the terahertz frequency region (40–1400 cm⁻¹) and down to low concentrations (0.000263 mol L⁻¹). Temperature dependent measurements of a 0.0098 mol L⁻¹ La₂(SO₄)₃ solution have been carried out from 23–98 °C. In solutions of La(ClO₄)₃ with water and heavy water, the [La(OH₂)₉]³⁺ and [La(OD₂)₉]³⁺ have been characterized and a weak, strongly polarized band observed at 343 cm⁻¹ and 326 cm⁻¹ respectively assigned to the ν₁ LaO₉ mode, the breathing mode of the clusters. In La₂(SO₄)₃(aq), in addition to the ν₁-SO₄²⁻ mode at 980 cm⁻¹, a pronounced band component at 991 cm⁻¹ has been assigned to an inner-sphere complex (ISC) and a similar ν₁-SO₄²⁻ band contour has been observed in La₂(SO₄)₃ solutions in D₂O. Sulfate may act as a monodentate ligand. Conformation of this assignment is provided by the component at 312 cm⁻¹ of the [La(OH₂)₈OSO₃]⁺ species in addition to the band at 343 cm⁻¹ for the fully hydrated cluster, [La(OH₂)₉]³⁺. After subtraction of the component of the ISC at 991 cm⁻¹, the ν₁-SO₄²⁻ band in La₂(SO₄)₃(aq) showed systematic differences from that in Na₂SO₄(aq). This is consistent with a ν₁-SO₄²⁻ band at 983.3 cm⁻¹ that can be assigned to the existence of an outer-sphere complex (OSCs). The observed change of the degree of sulfato-complex formation with dilution reflects the stepwise sulfato-complex formation. A K₃-value has been determined at 0.9 of the equilibrium between OSC and ISC. Temperature dependent measurements on a dilute La₂(SO₄)₃ solution has shown that the concentration of the La³⁺ sulfato-complex rises with increasing temperature while at the same time the concentration of the “free” sulfate diminished. The sulfato-complex formation is an endothermic process absorbing heat with increasing temperature. The following thermodynamic parameters for the rate determining equilibrium, [La(OH₂)SO₄]⁺ ↔ [LaOSO₃]⁺ has been determined: ΔH⁰ = 18.6 kJ mol⁻¹ and ΔS⁰ = 62.1 J mol⁻¹ K⁻¹.