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)(2)SO4 and MgSO4 down to 50 cm(-1) and, in some cases, to extremely low concentrations (greater than or equal to0.06 mmol kg(-1)) and high temperatures (less than or equal to200degreesC). In MgSO4(aq), the well known asymmetry in the nu(1)-SO42- mode at similar to980 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)(5)MgOSO3 unit at 328 cm(-1). The CIP becomes the dominant species at higher temperatures. Alternative explanations of the broadening of the nu(1)-SO42- mode are shown to be inconsistent with this and other Raman spectral evidence such as the similarity of the nu(1)-SO42- mode for MgSO4 in H2O and D2O. After subtraction of the CIP component at 993 cm(-1), the nu(1)-SO42- band in MgSO4(aq) showed systematic differences from that in (NH4)(2)SO4(aq). This is consistent with a previously undetected nu(1)-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 nu(1)-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.