Identification, composition, thermodynamic and structural properties of a pyroaurite-like iron(II)-iron(III) hydroxy-oxalate Green Rust

The aerial oxidation of aqueous suspensions of ferrous hydroxide precipitat ed from ferrous oxalate and caustic soda can lead to an iron (II)-iron (III ) hydroxy-oxalate of the pyroaurite group, a GR(C2O42-) Green Rust. As othe r GR compounds, it is unstable with respect to the action of oxygen and oxi dises later on. Its chemical composition was determined to be [Fe-6(II) Fe- 2(III) (OH)(16)](2+)[C2O42- . nH(2)O], with n more likely equal to 3 on the basis of structural considerations. The composition does not vary and the Fe (II) / Fe (III) ratio in the compound is measured by means of transmissi on Mossbauer spectroscopy at 78 K and 20 K in the range from 2.8 to 3.2 for various samples at various stages of the reaction. GR(C2O42-) is paramagne tic at both temperatures and is unambiguously distinguished from ferrous hy droxide, the initial reactant, and magnetite, the main final product, which are magnetically ordered at 20 K. The spectrum of the GR compound is compo sed of three quadrupole doublets, one due to the Fe(III) cations characteri sed by a small quadrupole splitting Delta E-Q of 0.40 mm s(-1), and two due to the Fe(II) cations, characterised by larger Delta E-Q values of about 2 .55 and 2.85 mm s(-1). Finally, from the observed equilibrium conditions be tween ferrous hydroxide and GR(C2O42-), the standard free enthalpy of forma tion of GR(C2O42-) was computed to be : Delta G(f)(o) {[Fe-6(II) Fe-2(III) (OH)(16)](2+) [C2O42- . 3H(2)O]} = -5383 +/- 3 kJ mol(-1).