Scandium Sulfate Complexation in Aqueous Solution by Dielectric Relaxation Spectroscopy

Ion association in aqueous solutions of scandium sulfate has been investigated at 25 °C and at concentrations from 0.01 to 0.8 M by broadband dielectric spectroscopy over the frequency range 0.2 ≤ ν/GHz ≤ 89. Detailed analysis of the spectra reveals the presence of both inner- and outer-sphere 1:1 [ScSO4]+(aq) complexes, similar to solutions of other high-valent metal sulfates. Outer−outer-sphere 1:1 complexes are probably also formed, but their contribution is swamped by the presence of higher-order inner-sphere complexes. The latter predominate in the more concentrated solutions, causing major changes to the low-frequency end of the spectrum. The data, while not definitive, are consistent with fac-[Sc(SO4)3(OH2)3]3− as the major species present. The speciation is strikingly different from that recently reported for aluminum sulfate solutions and indicates that the often-postulated similarity between the aqueous chemistry of Al(III) and Sc(III) has to be treated with caution.