A KINETIC AND MECHANISTIC STUDY OF THE ISOTHERMAL DECOMPOSITION OF FERRIC OXALATE HYDRATE

We report a kinetic and mechanistic study of the isothermal decomposition of ferric oxalate hydrate [Fe2(C2O4)3.4.6H2O]. The principal products of reaction between 410 and 450 K are carbon dioxide and ferrous oxalate which retains a small proportion (approx. 10%) of the ferric salt. From microscopic studies, it was concluded that reaction was not accompanied by melting. Yield-time curves were sigmoid shaped providing evidence that this is a nucleation and growth process occurring in the solid state. Data did not fit a single-rate equation but was satisfactorily expressed in two stages: the acceleratory first half fitted the Avrami-Erofe'ev equation (n = 2) and the second half fitted the contracting cube equation. The activation energies were relatively low for this type of reaction, 107 +/- 8 and 120 +/- 10 kJ mol-1 respectively. The reaction mechanism is discussed and it is concluded that the rate is controlled either by electron transfer or rupture of the C-C bond in the anion. The participation of water retained in the solid through intermediate formation of oxalic acid is discussed.