Journal of Solution Chemistry, Vol. 26, No. 8, 1997 Raman Spectroscopic Investigation of Aqueous FeSO4 in Neutral and Acidic Solutions from 25~ to 303~ Inner-and Outer-Sphere Complexes W. Rudolph) Murray H. Brooker, l and Peter R. Tremaine 1,* Received October 23, 1996; revised June 19, 1997 Raman spectra of aqueous FeSO4 and (NH4)2SO4 solutions have been recorded over broad concentration and temperature ranges. Whereas the v~-SO 2- band profile is symmetrical in noncomplexing (NH4)2SO4 solutions, in FeSO4 solutions a shoulder appears on the high-frequency side, which increases in intensity with increasing concentration and temperature. The molar scattering coefficient of the vi-SO~- band is the same for all forms of sulfate in (NH4)2SO4 and FeSO4 solutions and is independent of temperature up to 150~ the highest temperature studied. The high-frequency shoulder is attributed to the formation of a contact ion pair, Fe2+OSO32-, as is the splitting of the v3-SO]- antisymmetric stretching mode wtiich is observed in the FeSO4 solution. The bending modes v2-SO]- and v4-SO2-, normally forbidden in the isotropic spectrum, show a gain in intensity with increasing ion-pair formation. A polarized band has been assigned to the Fe2+-O ligand vibration. No higher associates or anionic complexes are required to interpret the spectroscopic data. No evidence of contact ion pairing between Fe2§ and HSO~ could be detected at temperatures up to 303~ in 1 molal solutions of FeSO4 with an excess of 2 molal H2SO4. KEY WORDS: Raman spectra; ion pair formation; temperature dependence; equilibrium constant; enthalpy of formation; aqueous FeSO4; aqueous (NH4)2SO4. 1. INTRODUCTION A knowledge of the properties of aqueous iron sulfate solutions at elevated temperatures and pressures is required to model speciation and mass Department of Chemistry, Memorial University of Newfoundland, St. John's, Newfoundland, Canada AIB 3X7. 2present address: Department of Earth Science, Memorial University of Newfoundland, St. John's Newfoundland, Canada AI B 3X5. 757 0095-9782/97/0800-0757512.50/0 1997 Plenum Publishing Corporation