Dehydration of iron(II) sulfate heptahydrate

We studied the dehydration behavior of iron(II) sulfate heptahydrate (FeSO4·7H2O) as part of NASA's in situ resource utilization program for the recovery of water on Mars. We examined the effect of the variables important in using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The temperatures at which waters of hydration are removed and the enthalpy values for the various dehydration steps were measured using DSC. For the dehydration of FeSO4·7H2O, a particle size <45 μm, a heating rate of 2.5 °C/min, an open DSC pan, and a horizontal baseline correction produced experimental values for the enthalpies of each dehydration step and overall dehydration of 145.7, 180.4, 64.8 and 390.9 kJ/mol, respectively, which were in −8.8, 1.2, 3.35 and −2.47%, respectively, of those estimated from standard heats of formation and literature heat capacities. Generally, lower heating rates and smaller particle sizes are better for resolving the dehydration steps. The horizontal baseline corrections give better results when there is little or no peak overlap.As discussed above, proper selection of DSC and TGA operating parameters and baseline analysis technique is required to obtain accurate thermal results.