On the stoichiometry and Pourbaix diagram of Fe(II)-Fe(III) hydroxy-sulphate or sulphate-containing green rust .2. An electrochemical and Mossbauer spectroscopy study

A new preparation of sulphate containing green rust 2, GR2(SO4--), is shown to reach its chemical formula at stoichiometry. It consists of mixing FeCl2 . 4H(2)O and NaOH to precipitate Fe(OH)(2) before adding immediately Na2SO4. E(h) and pH vs time curves are recorded during oxidation: the formula is established to be [Fe(II)(4) Fe(III)(2) (OH)(12)](++) . [SO4 . 2H(2)O](--). The chemical potential is determined to be mu(0)[GR2(SO4--)] = -1014500 +/- 2500 cal mol(-1) and the Pourbaix diagram of iron in sulphated aqueous media is drawn. Mossbauer spectra display two quadrupole doublets in the 2:1 intensity ratio attributed to ferrous and ferric states, respectively. The unique ferrous environment which is detected indicates that all Fe++ ions are in the first neighbourhood of one oxygen atom which belongs to a SO2-- anion or to a water molecule. Copyright (C) 1996 Elsevier Science Ltd