Raman spectroscopic investigation of aqueous FeSO4 in neutral and acidic solutions from 25‡C to 303‡C: inner- and outer-sphere complexes

Raman spectra of aqueous FeSO4 and (NH4)2SO4 solutions have been recorded over broad concentration and temperature ranges. Whereas the v1-SO42- 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 v1-SO42- band is the same for all forms of sulfate in (NH4)2SO4 and FeSO4 solutions and is independent of temperature up to 150‡C, the highest temperature studied. The high-frequency shoulder is attributed to the formation of a contact ion pair, Fe2+OSO3/2-, as is the splitting of the v3-SO42- antisymmetric stretching mode which is observed in the FeSO4 solution. The bending modes v2-SO42- and v4-SO42-, 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 HSO44- could be detected at temperatures up to 303‡C in 1 molal solutions of FeSO4 with an excess of 2 molal H2SO4.

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