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

Raman spectroscopic measurements have been made of aqueous solutions of La(ClO4)(3), La-2(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 La-2(SO4)(3) solution have been carried out from 23-98 degrees 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 v(1) LaO9 mode, the breathing mode of the clusters. In La-2(SO4)(3)(aq), in addition to the v(1)-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 v(1)-SO42- band contour has been observed in La-2(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)(8)OSO3](+) 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 v(1)-SO42- band in La-2(SO4)(3)(aq) showed systematic differences from that in Na2SO4(aq). This is consistent with a v(1)-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 K-3-value has been determined at 0.9 of the equilibrium between OSC and ISC. Temperature dependent measurements on a dilute La-2(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: Delta H-0 = 18.6 kJ mol(-1) and Delta S-0 = 62.1 J mol(-1) K-1.