SOLUBILITY OF HEMATITE, FE2O3(κ, α) AT 200°C AND THE STANDARD ENTROPY OF FE3+ ION IN AQUEOUS SOLUTION

Abstract

The solubility of hematite, Fe2O3 (cr, α) in acidic (0.5 < pH < 2) and alkalic (12.7 < pH < 13.4) aqueous perchlorate solutions with ionic strengths of I = 0.1 and 0.316 was measured at 200°C and vapor saturation pressure. The measurements were carried out in solutions of perchloric acid and sodium hydroxide at concentrations of 0.01 < CHClO4 < 0.32 M and 0.112 < CNaOH < 0.301 M. The equilibrium constants of hematite dissolution reactions were calculated from the measured values at 200°C in acidic environments: 0.5Fe2O3(Cr, α + 2H+(sol) = FeOH2+(sol) + 0.5H2O(liq) logAr0(473.15 K) = -3.88 ± 0.36; 0.5Fe2O3(Cr, α) + 3H+(sol) = Fe3+(sol) + 1.5H2O(liq) logK0 (473.15 K) = -4.54 ± 0.36; and in alkalic environments: 0.5Fe2O3(Cr, α) + OH-(sol) + 1.5H2O(liq) = Fe(OH)-4 (sol) logK0(473.15 K) = -4.56 ± 0.10. Proceeding from the measured values and published data, thermodynamic properties were estimated for some iron-bearing compounds in the system Fe(III)-H2O at temperatures from 25-200°C. In particular, on the basis of hematite solubility measurements, we determined for the first time the standard entropy of Fe3+ ion in aqueous solution, S0 (298.15 K) Fe3+(Sol) = -275.0 ± 8.0 J/mol K and its standard free energy of formation, ΔfG0(298.15) Fe3+(Sol) = -16.5 ± 3.4 kJ/mol. The equilibrium constant of the hematite dissolution reaction (and Fe3+ ion formation) at 25°C in acidic environments was calculated: logK0(298.15 K) = 0.2 ± 0.4. The direct determination of this value by hematite dissolution at 25°C is hampered by kinetic problems.