Raman and Infrared Spectroscopic Investigation of Speciation in BeSO4(aq)

Measurements have been made of the Raman spectra of aqueous solutions of Be(ClO4)(2), BeCl2, (NH4)(2)SO4 and BeSO4 to 50 cm(-1). In some cases low concentrations (0.000770 mol.kg(-1)) have been used and two temperatures (23 and 40 degrees C) were studied. In BeSO4(aq), the nu(1)-SO42- mode at 980 cm(-1) broadens with increasing concentration and shifts to higher wavenumbers. At the same time, a band at 1014 cm(-1) is detectable with this mode being assigned to [BeOSO3], an inner-sphere complex (ISC). Confirmation of this assignment is provided by the simultaneous appearance of stretching bands for the Be2+-OSO32- bond of the complex at 240 cm(-1) and for the BeO4 skeleton mode of the [(H2O)(3)BeOSO3] unit at 498 cm(-1). The ISC concentration increases with higher temperatures. The similarity of the nu(1)-SO42- Raman bands for BeSO4 in H2O and D2O is further strong evidence for formation of an ISC. After subtraction of the ISC component at 1014 cm(-1), the nu(1)-SO42- band in BeSO4(aq) showed systematic differences from that in (NH4)(2)SO4(aq). This is consistent with a nu(1)-SO42- mode at 982.7 cm(-1) that can be assigned to the occurrence of an outer-sphere complex ion (OSCs). These observations are shown to be in agreement with results derived from previous relaxation measurements. Infrared spectroscopic data show features that are also consistent with a beryllium sulfato complex such as the appearance of a broad and weak nu(1)-SO42- mode at similar to 1014 cm(-1), normally infrared forbidden, and a broad and asymmetric nu(3)-SO42- band contour which could be fitted with four band components (including nu(3)-SO42- (aq)). The formation of ISCs in BeSO4(aq) is much more pronounced than in the similar MgSO4(aq) system studied recently.