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-SO ₄ ^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 v_1-SO ₄ ^2- 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⁺OSO^3/2-, as is the splitting of the v_3-SO ₄ ^2- antisymmetric stretching mode which is observed in the FeSO₄ solution. The bending modes v_2-SO ₄ ^2- and v_4-SO ₄ ^2- , 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 HSO4 ₄ ^- could be detected at temperatures up to 303‡C in 1 molal solutions of FeSO₄ with an excess of 2 molal H₂SO₄.