Thermoanalytical investigation of iron phosphate obtained by spontaneous precipitation from aqueous solutions

Iron(III) phosphate has been precipitated under supersaturation conditions from equimolecular aqueous solutions of 0.025 M Fe(NO3)3·9H2O and K2HPO4, at pH=2.00 and at ambient temperature. The precipitate has been characterised by TG/DTG/DTA and DSC techniques, chemical analysis, IR-spectroscopy and X-ray powder diffraction. A yellowish-white amorphous solid of formula Fe2(HPO4)3·xH2O has been obtained. Alternatively, a pinkish-white amorphous precipitate of formula FePO4·2H2O is obtained under the same conditions from Fe(NH4)2(SO4)2·6H2O using hydrogen peroxide as the oxidising agent. Although the IR spectra and the thermal behaviour of both compounds are quite different, they crystallise as FePO4 when annealed at 650 °C in air.