Determination of standard Gibbs free energy of formation of chukanovite and Pourbaix diagrams of iron in carbonated media

Mixtures of Fe(OH)2 and Fe2(OH)2CO3 were obtained by precipitation from FeCl2, NaOH and Na2CO3 solutions. These two-phase systems did not change with time and equilibrium conditions were supposed to be established after 6 months. The standard Gibbs free energy of formation of chukanovite could then be estimated at −1171.5 ± 3.0 kJ mol−1. In contrast, mixtures of FeCO3 and Fe2(OH)2CO3 could not been obtained by precipitation but siderite formed in some cases after 25 months of ageing. Pourbaix diagrams of iron including chukanovite were drawn showing that the Fe(II) hydroxycarbonate was metastable, at 25 °C, with respect to siderite.

► The standard Gibbs free energy of formation of chukanovite is determined.
► Potential-pH equilibrium diagrams of iron in carbonated media are drawn.
► The formation of rust layers composed of both chukanovite and siderite is explained.