Determination of standard Gibbs free energy of formation of green rusts and its application to the Fe(II–III) hydroxy-oxalate

The oxalate green rust GR(C2O42−) is obtained by oxidation of aerated aqueous suspensions of Fe(OH)2 precipitated from Fe(II) oxalate and NaOH solutions. The standard Gibbs free energy of formation of GR(C2O42−) is determined from redox potential measurements. The influence of O2/H2O redox couple is carefully studied and its impact on previous determinations of Gibbs free energy of various green rust compounds, namely GR(SO42−), GR(Cl−) and GR(CO32−), is discussed. Moreover, the ΔG°f values of GRs are given for hydrated compounds. The competitive formation of GR(C2O42−) and GR(SO42−) is studied similarly via the oxidation of Fe(OH)2 in aqueous media containing both C2O42− and SO42− anions. In the experimental conditions considered here, GR(C2O42−) forms preferentially, even when the sulphate ions are predominating in the solution. This indicates that oxalate ions may be preferred to sulphate for the building of a GR structure.