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The solubility of carefully characterized magnetite, Fe3O4, in dilute aqueous solutions saturated with H2 has been measured at temperatures from 100 to 300°C in a flow apparatus. Solution compositions included either HCl or NaOH molalities of up to 1 and 40 mmole-kg-1, respectively, and H2 molalities of 0.0779, 0.779, and 8.57 mmole-kg-1. The dependence of the equilibrium solubility on the pH and reduction potential were fitted to a scheme of soluble ferrous and ferric species consisting of Fe2+, FeOH+, Fe(OH)2, Fe(OH)3-, Fe(OH)3, and Fe(OH)4-. Solubility products from the fit, corresponding to the reactions $$\tfrac{1}{3}Fe_3 O_4 + (2 - b)H^ + + \tfrac{1}{3}H_2 \rightleftharpoons Fe(OH)_b^{2 - b} + (4/3 - b)H_2 O$$ and $$\tfrac{1}{3}Fe_3 O_4 + (3 - b)H^ + \rightleftharpoons Fe(OH)_b^{3 - b} + \tfrac{1}{6}H_2 + (4/3 - b)H_2 O$$ were used to derive thermodynamic constants for each species. The extrapolared value for the Gibbs energy of formation of Fe2+ at 25°C is -88.92±2.0 kJ-mole-1, consistent with standard reduction potentials in the range Eo(Fe2+)=-0.47±0.01 V. The temperature coefficient of the equilibrium Fe molality, (?m(Fe, sat.)/?T)m(H2).m(NaOH), changes from negative to positive as the NaOH molality is increased to the point where Fe(OH)3- and Fe(OH)4- predominate.

Raman spectra of aqueous FeSO4and (NH4)2SO4solutions have been recorded over broad concentration and temperature ranges. Whereas the v1-SO42-band profile is symmetrical in noncomplexing (NH4)2SO4solutions, in FeSO4solutions 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)2SO4and FeSO4solutions 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 FeSO4solution. 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 FeSO4with an excess of 2 molal H2SO4.