Raman spectroscopic investigation of speciation in MgSO4(aq)

Careful measurements have been made of the Raman spectra of aqueous solutions of Mg(ClO4)2, MgCl2, (NH4)2SO4 and MgSO4 down to 50 cm−1 and, in some cases, to extremely low concentrations (≥0.06 mmol kg−1) and high temperatures (≤200 °C). In MgSO4(aq), the well known asymmetry in the ν1-SO42− mode at ∼980 cm−1 that develops with increasing concentration has been assigned to a mode at 993 cm−1 associated with the formation of an MgOSO3 contact ion pair (CIP). Confirmation of this assignment is provided by the simultaneous and quantitative appearance of stretching modes for the Mg–OSO3 bond of the ligated SO42− at 245 cm−1 and for the (H2O)5MgOSO3 unit at 328 cm−1. The CIP becomes the dominant species at higher temperatures. Alternative explanations of the broadening of the ν1-SO42− mode are shown to be inconsistent with this and other Raman spectral evidence such as the similarity of the ν1-SO42− mode for MgSO4 in H2O and D2O. After subtraction of the CIP component at 993 cm−1, the ν1-SO42− band in MgSO4(aq) showed systematic differences from that in (NH4)2SO4(aq). This is consistent with a previously undetected ν1-SO42− mode at 982.2 cm−1 that can be assigned to the presence of solvent-shared ion pairs (SIPs). In solutions with high Mg2+/SO42− concentration ratios, a further ν1-SO42− mode was observed at 1005 cm−1, which has been tentatively assigned to a Mg2SO42+(aq) triple ion. All of these observations are shown to be in excellent agreement with recent dielectric relaxation spectroscopy measurements. In addition, the correct relationship between the Mg2+/SO42− association constant determined by Raman spectroscopic measurements and those obtained by other techniques is derived. It is shown that thermodynamic data measured by Raman spectroscopy for systems involving other (Raman-undetected) ion-pair types in addition to CIPs, cannot and should not be compared directly with those obtained by traditional techniques.