Chemical composition and Gibbs standard free energy of formation of Fe(II)-Fe(III) hydroxysulphate green rust and Fe(II) hydroxide
The redox potential and pH of aerated suspensions of iron (II) hydroxide in sulphate - containing aqueous solutions are measured during the oxidation process. Plateaux corresponding to the equilibrium conditions between Fe(OH)2(s) and Fe(II)-Fe(III) hydroxysulphate GR2(SO42-)(s) on the one hand, and between GR2(SO42-)(s) and FeOOH(s) on the other hand, are displayed. Potentiometry, voltammetry, pH-metry and Mössbauer spectroscopy are applied to follow all reactions. The thermodynamic meaning of the measured potential of the first plateau which corresponds to the GR2(SO42-)(s) /Fe(OH)2(s) equilibrium is demonstrated. The chemical composition of GR2(SO42-)(s) is found to be Fe4IIFe2III(OH)12SO4.nH2O all along the oxidation process, implying that this compound must be considered as a pure phase with a well-defined composition. The Gibbs standard free energy of formation or chemical potential °[GR2(SO4-)(s)] in "anhydrous form" (n = 0) is determined at -3790 ± 10 kJ mol-1. A consistent value of °[Fe(OH)2(s)] at -490 ± 1 kJ mol-1 is obtained.
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