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]+ 虠 [LaoSO3]+ has been determined: ΔH0 = 18.6 kJ mol-1 and ΔS0 = 62.1 J mol-1 K-1.