RAMAN-SPECTROSCOPIC INVESTIGATION OF AQUEOUS FESO4 IN NEUTRAL AND ACIDIC SOLUTIONS FROM 25-DEGREES-C TO 303-DEGREES-C - INNER-SPHERE AND OUTER-SPHERE COMPLEXES

Raman spectra of aqueous FeSO4 and (NH4)(2)SO4 solutions have been rec orded over broad concentration and temperature ranges. Whereas the nu( 1)-SO42- band profile is symmetrical in noncomplexing (NH4)(2)SO4 solu tions, in FeSO4 solutions a shoulder appears on the high-frequency sid e, which increases in intensity with increasing concentration and temp erature. The molar scattering coefficient of the nu(1)-SO42- band is t he same for all forms of sulfate in (NH4)(2)SO4 and FeSO4 solutions an d is independent of temperature up to 150 degrees C, the highest tempe rature studied. The high-frequency shoulder is attributed to the forma tion of a contact ion pair, Fe2+OSO32-, as is the splitting of the nu( 3)-SO42- antisymmetric stretching mode which is observed in the FeSO4 solution. The bending modes nu(2)-SO42- and nu(4)-SO42-, normally forb idden in the isotropic spectrum, show a gain in intensity with increas ing ion-pair formation. A polarized band has been assigned to the Fe2-O ligand vibration. No higher associates or anionic complexes are req uired to interpret the spectroscopic data. No evidence of contact ion pairing between Fe2+ and HSO4- could be detected at temperatures up to 303 degrees C in 1 molal solutions of FeSO4 with an excess of 2 molal H2SO4.