Experimentally Determined Standard Thermodynamic Properties of Synthetic MgSO₄·4H₂O (Starkeyite) and MgSO₄·3H₂O: A Revised Internally Consistent Thermodynamic Data Set for Magnesium Sulfate Hydrates

Abstract

The enthalpies of formation of synthetic MgSO 4 $4H 2 O (starkeyite) and MgSO 4 $3H 2 O were obtained by solution calorimetry at T = 298.15 K. The resulting enthalpies of formation from the elements areThe standard entropy of starkeyite was derived from low-temperature heat capacity measurements acquired with a physical property measurement system (PPMS) in the temperature range 5 K < T < 300 K:Additionally, differential scanning calorimetry (DSC) measurements with a Perkin Elmer Diamond DSC in the temperature range 270 K < T < 300 K were performed to check the reproducibility of the PPMS measurements around ambient temperature.The experimental C p data of starkeyite between 229 and 303 K were fitted with a Maier-Kelley polynomial, yielding C p (T) = 107.925 + 0.5532$T -1048894$T -2 . The hydration state of all Mg sulfate hydrates changes in response to local temperature and humidity conditions. Based on recently reported equilibrium relative humidities and the new standard properties described above, the internally consistent thermodynamic database for the MgSO 4 $nH 2 O system was refined by a mathematical programming (MAP) analysis.As can be seen from the resulting phase diagrams, starkeyite is metastable in the entire T-%RH range. Due to kinetic limitations of kieserite formation, metastable occurrence of starkeyite might be possible under martian conditions.

Thermodynamic study of MgSO4 – H2O system dehydration at low pressure in view of heat storage

“…The O, where n=11, 5, 4, 3, 2, 5/4) can be produced synthetically under several conditions or by evaporation of aqueous solution of MgSO 4 [6][7][8][9]. Next investigation of synthetic MgSO 4 •5H 2 O is not possible because the compound decompose immediately in contact with the ambient atmosphere [10].…”

“…All the results obtained by means of thermogravimetric analysis allow to plot the phase diagram of MgSO 4 -H 2 O system by representing various isosteres ( = constant) in a 2diagram ( Figure 7). Solid lines indicate a phase transition between crystalline phases [9,25] and dash lines represent equilibrium curves for five chosen non-stoichiometric solids obtained in this work. The solid phase is considered as a solid solution of water molecules in the magnesium sulfate skeleton in equilibrium with the gaseous atmosphere.…”

New kinetic model of the dehydration reaction of magnesium sulfate hexahydrate: Application for heat storage

“…Pentahydrate, tetrahydrate, and dihydrate of magnesium sulfate have been observed as efflorescent salts in some mines where sulfides are located under specific humid conditions [31]. Other authors obtained various hydrates synthetically under several conditions by using the humidity buffer technique or by evaporation of an aqueous solution of MgSO4 [31][32][33][34][35][36]. The interaction with water or water vapor plays an important role in the formation of hydrates.…”

Majzlanite, K2Na(ZnNa)Ca(SO4)4, a new anhydrous sulfate mineral with complex cation substitutions from Tolbachik volcano

“…In addition to other many achievements and contributions in the fields of mineralogy and crystallography, Juraj Majzlan has made significant contribution to the mineralogy of sulfates (see e.g. Majzlan et al ., 2017; Zittlau et al ., 2013; Grevel et al , 2012).…”