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, pa-metry and Mossbauer spectroscopy are applied to follow ail 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 (Fe4Fe2III)-Fe-II(OH)(12)SO4.nH(2)O 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 mu degrees[GR2(SO42-)((s))] in 'anhydrous form' (n = 0) is determined at -3790 +/- 10 kJ mol(-1). A consistent value of mu degrees[Fe(OH)(2(s))] at -490 +/- 1 kJ mol(-1) is obtained.