Magnetite stability in aqueous sodium phosphate solutions at elevated temperatures

Magnetite (Fe3O4) is shown to transform to a sodium ferric hydroxyphosphate compound in concentrated, alkaline sodium phosphate solutions at elevated temperatures via 1/3 Fe3O4(s) + 133 Na+ + 2 HPO+4 left-reversible Na4Fe(OH)(PO4)2·31NaOH(s) + 13H+ + 16H2(g). The thermodynamic equilibrium for this reaction was defined in the system Na2OP2O5-Fe3O4-H2O for Na/P molar ratios between 2.1 and 3. Quantitative chemical, infrared and Moessbauer spectroscopic, and X-ray diffraction analysis are employed to verify that the precipitated solid was a single phase having the non-whole number stoichiometric Na/P ratio = 2.15 ± 0.02. On the basis of observed reaction threshold values for sodium phosphate concentration and temperature, the standard entropy (S0) and free energy of formation (ΔG01) for sodium ferric hydroxyphosphate were calculated to be 729.2 J/(mol-K) and -3550.3 Kh/mol, respectively