Thermodynamic properties of iron oxides and hydroxides. III. Surface and bulk thermodynamic properties of lepidocrocite (γ-FeOOH) to 500 K
Diakonov, Igor I.
published: Jan 26, 1998
manuscript accepted: Sep 1, 1997
manuscript received: Sep 30, 1996
ArtNo. ESP147051001004, Price: 29.00 €
Abstract A consistent set of bulk thermodynamic properties up to 500 K for lepidocrocite (γ-FeOOH) was generated on the basis of a critical analysis of literature data with the account for the surface thermodynamic properties. Experimental measurements of the heat capacity of lepidocrocite are consistent with the equation: Cp° = 65.205 + 0.067665 T-0.81564 × 106/T2 (298.15-500 K) which yields a value of the standard heat capacity at 298.15 K: Cp° = 76.2 ± 1.5 J/mol K. Measured enthalpies of dehydration of this mineral to disordered maghemite and of its transition reaction to goethite result in a standard enthalpy of formation of lepidocrocite at 298.15 K: ΔfH° = -556.4±2.0 kJ/mol. Standard entropy at 298.15 K was estimated as S° = 62.5 + 5.0 J/mol K from those of goethite, diaspore and boehmite. Standard Gibbs energy of formation at 298.15 K and the solubility product of lepidocrocite were calculated as: ΔfG° = -486.3 ±2.5 kJ/mol and pKsp = 41.4 ±0.6. Solubility curves of lepidocrocite were calculated at 298.15 K as a function of pH and grain size.