Standard free enthalpy of formation of Fe(II)Fe(III) hydroxysulphite green rust one and its oxidation into hydroxysulphate green rust two

• Published in: Corrosion science Vol. 39; no. 9; pp. 1673 - 1685
• Main Authors: Simon, L., Génin, J.-M.R., Refait, Ph
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 1997; Elsevier Science
• Subjects: A. Green Rusts; Applied sciences; B. Mössbauer spectroscopy; B. XRD; C. oxidation; C. Pourbaix diagram; Corrosion; Corrosion environments; Exact sciences and technology; Metals. Metallurgy; B. XRD; C. oxidation; B. Mössbauer spectroscopy; A. Green Rusts; C. Pourbaix diagram; Sulfites; Rust; Corrosion; Potential pH diagram; Free enthalpy; Oxidation; Thermodynamic properties; Heat of formation; Corrosion product; Pourbaix diagram; Electrode potential
• ISSN: 0010-938X; 1879-0496
• DOI: 10.1016/S0010-938X(97)00074-7

The oxidation of Fe(OH) 2 in sulphite-containing water solution gives rise in a first stage to a new Green Rust one, GR1(SO 3 2−), as identified by X-ray diffraction. Mössbauer spectroscopy reveals that this Fe(II)Fe(III) hydroxysulphite is characterised by a Fe(III) Fe(II) ratio equal to 1 3 , therefore corresponding to the chemical formula [Fe 6 IIFe 2 III(OH) 16] 2+[SO 3 · 4H 2O] 2−. The Mössbauer spectrum of GR1(SO 3 2−) is found similar to that of the chloride-containing compound GR1(Cl −), which has the same Fe(III) Fe(II) ratio of 1 3 . A second oxidation stage leads from GR1(SO 3 2−) to the Fe(II)Fe(III) hydroxysulphate known as Green Rust two, GR2(SO 4 2−). This process is due to the sulphate ions present in solution, which result from the oxidation of the initial sulphite ions. In a third and final stage, GR2(SO 4 2−) oxidises in turn and ferric oxyhydroxides are obtained. From the measurements of the electrode potential E h and pH of the solution, the standard chemical potential of GR1(SO 3 2−) can be estimated at μ∘[GR1(SO 3 2−)]= −5465 ± 14 kJ mol −1 or μ∘[GR1(SO 3 2−)] anhydrous = −4516 ± 14 kJ mol −1 when intercalated water molecules are not considered. Eh-pH equilibrium diagrams of iron in sulphite-containing aqueous media are drawn.