P. Refait et al., Identification, composition, thermodynamic and structural properties of a pyroaurite-like iron(II)-iron(III) hydroxy-oxalate Green Rust, EUR J SOL S, 35(10-11), 1998, pp. 655-666
Citations number
49
Categorie Soggetti
Inorganic & Nuclear Chemistry
Journal title
EUROPEAN JOURNAL OF SOLID STATE AND INORGANIC CHEMISTRY
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).