Magnesium sulfate

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Author: Amelie Stahlbuhk
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Abstract

The different hydrated forms of magnesium sulfate and the behaviour concerning solubility and hygroscopicity will be presented.

Hydrated forms

Kieserite MgSO4•H2O
Sanderite MgSO4•2H2O
Starkeyite MgSO4•4H2O
Pentahydrite MgSO4•5H2O
Hexahydrite MgSO4•6H2O
Epsomite MgSO4•7H2O
Meridianiite MgSO4•11H2O

Solubility

As it is shown in table 1, the different hydrated forms of magnesium sulfate are easily soluble salts, which leads to a high mobility of the salts in porous materials.


Table 1: Solubilities in mol/kg of different hydrated forms of magnesium sulfate at 20°C [according to [Steiger.etal:2011a]Title: Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars
Author: Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy
Link to Google Scholar
].
Hydrated form Solubility [mol/kg] at 20°C
Kieserite 5.60
Starkeyite 5.04
Pentahydrite 4.40
Hexahydrite 3.61
Epsomite 2.84


Due to the different hydrated forms of magnesium sulfate with stable and meta stable equilibria, the solubility diagram of the system MgSO4-H2O contains more information than diagrams of salts with less or also without any hydrated forms. With the temperature dependence of the solubility it is possible that temperature changes are accompanied by a hydration or a dehydration of a considered phase.


Figure 1: Solubilities in the system MgSO4-H2O and its temperature dependence. The molality m [n(MgSO4•xH2O)•kg(H2O)-1] is plotted versus the temperature. Equilibria of the different hydrated forms can be distiguished by the different colours of the curves. Dotted lines represent the metastable equilibria. In addition to the solubilities, freezing and boiling points are given. According to [Steiger.etal:2011a]Title: Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars
Author: Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy
Link to Google Scholar
.


Hygroscopicity

In the system MgSO4-H2O changes in temperature or relative humidity may lead to hydration/dehydration or deliquescence/crystallization processes. At 20 °C epsomite is the present crystalline phase when the relative humidity is below its deliquescence humidity of 91.3 %. When the relative humidity reaches values below 47 % the dehydration to lower hydrated levels sets in, as it is represented by the curves of the equilibrium humidities in figure 2.


Table 2: Deliquescence and equilibrium humidities at 20 °C [according to [Steiger.etal:2011a]Title: Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars
Author: Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy
Link to Google Scholar
].
Phase transition Deliquescence or equilibrium humidity at 20°C
Epsomite-solution 91.3 %
Epsomite-Hexahydrite 46.6 %
Epsomite-Kieserite 46.7 %
Hexahydrite-Starkeyite 39.1 %


accompanied by a hydration or a dehydration of a considered phase.


Figure 2: Deliquescence behaviour in the system MgSO4-H2O. The water activity aw is plotted versus the temperature. Different colours label equilibrium and deliquescence humidities of the different phases of the system. Dotted lines represent metastable equilibria. According to [Steiger.etal:2011a]Title: Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars
Author: Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy
Link to Google Scholar
.


In the temperature range of -10 to 100 °C the deliquescence humidities of the present hydrated forms (depending on the temperature) lie always above 80 % r.h., so the salts do not belong to the hygroscopic salts.


Weblinks


Literature

[Mainusch:2001]Mainusch, Nils (2001): Erstellung einer Materialsammlung zur qualitativen Bestimmung bauschädlicher Salze für Fachleute der Restaurierung, Diplomarbeit, HAWK Hochschule für angewandte Wissenschaft und Kunst Hildesheim/Holzminden/Göttingen, file:Diplomarbeit Nils Mainusch.pdfLink to Google ScholarFulltext link
[Stark.etal:1996] Stark, Jochen; Stürmer, Sylvia (1996): Bauschädliche Salze, Bauhaus-Univ. WeimarLink to Google Scholar
[Steiger.etal:2011a]Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy (2011): Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars. Geochimica et Cosmochimica Act, 75 (12), 3600-3626, 10.1016/j.gca.2011.03.038,Link to Google Scholar