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Raman spectroscopic investigation of speciation in MgSO4(aq)

Rudolph, W.W., Irmer, G. and Hefter, G. (2003) Raman spectroscopic investigation of speciation in MgSO4(aq). Physical Chemistry Chemical Physics, 5 (23). pp. 5253-5261.

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Careful measurements have been made of the Raman spectra of aqueous solutions of Mg(ClO4)2, MgCl2, (NH 4)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 MgSO 4(aq), the well known asymmetry in the ν1SO 42- 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-SO 42- mode for MgSO4 in H2O and D 2O. After subtraction of the CIP component at 993 cm-1, the ν1-SO42- band in MgSO4(aq) showed systematic differences from that in (NH4)2SO 4(aq). This is consistent with a previously undetected ν 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 ν1-SO42- mode was observed at 1005 cm-1, which has been tentatively assigned to a Mg2SO 42+(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 Mg 2+/ 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.

Publication Type: Journal Article
Murdoch Affiliation: School of Chemical and Mathematical Science
Publisher: Royal Society of Chemistry
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