THE OXIDATION OF FERROUS HYDROXIDE IN CHLORIDE-CONTAINING AQUEOUS-MEDIA AND POURBAIX DIAGRAMS OF GREEN RUST ONE

The oxidation of a ferrous hydroxide precipitated by mixing solutions of ferrous chloride and caustic soda depends on the ratio R' = Cl-/OH- of the initial concentrations of the reactants. The mechanisms of oxi dation for 1 < R' less-than-or-equal-to 3 are related to the existence of the ferrous-ferric compound green rust 1 (GR1). The direct recordi ng of the pH and the electrode potential allows the trigger value of R (c)' = 8/7 to be determined, which is found to correspond to the stoic hiometric conditions of obtention of GR1. By coupling this information with that obtained from the three iron sites R1, R2, R3 detected by M ossbauer spectroscopy, the formula of GR1 is set up at 3Fe(OH)2 . Fe(O H)2Cl . nH2O. Thus GR1 contains an equal number of Cl- and Fe3+ ions. The standard chemical potential mu0 is determined to be -509,500 +/- 5 00 cal mol-1 for n = 0. E(h)/pH Pourbaix diagrams specific to the oxid ation of iron in chloride-containing aqueous media, including GR1, are drawn.