We separated maghemite, a ferrimagnetic oxide (γ-Fe2O3), from soil and identified it by x-ray diffraction. The solubility product (pK) of this mineral was determined in 0.1 MCa(NO3)2 solution. The pH of the magnetitic soil fraction-Ca(NO3)2 suspensions was adjusted between 2 and 3.5. The pH and pe of each suspension were measured after 0.12, 0.5, 1, 5, and 15 d of shaking at 25 ± 1°C. Ten milliliters of the solution phase was drawn and filtered at each interval. The concentrations of total Fe in the aliquots were determined with an atomic absorption spectrometer.
From the analytical data of this study, the activity of Fe3+ and the activity of the solubility product (pK± of (Fe3+)((OH-)3) for γ-Fe2O3 (maghemite) were calculated. The mean pK° value was 40.36 + 0.07. Free energy of formation for soil maghemite, as calculated from the pK° value, corresponds to AG, ° = −173.8 ± 0.01 kcal mole−1.
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