Thermal stability of the FeIIIEDTA complex in its monomeric form

Thermal degradation of the solid monomeric Na[FeEDTA(H2O)]·2H2O (EDTA=ethylenediaminetetraacetate anion) complex was studied in air by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) coupled on-line with a quadrupole mass spectrometer for evolved gas analysis (EGA-MS). The solid products were analysed by Mössbauer spectroscopy. A four-step decomposition was observed until 500°C by TG/DTA–EGA-MS, involving first the release of two molecules of crystal lattice water then one coordinated water molecule and above 200°C, the release of CO2 as a result of degradation of a carboxylate arm of the EDTA ligand. While releasing CO2, reduction of the ferric iron to ferrous iron was observed by Mössbauer spectroscopy. Two ferrous species have been found to form simultaneously which are assumed to be configurational isomers since their aerial oxidation resulted in the formation of the same μ-oxo dimeric iron(III) species. Reaction pathway for this oxidation is proposed. Finally, the organic residues are burnt out rapidly accompanied by a very sharp exothermic heat effect between 280 and 330°C; meanwhile magnetite is formed.