Magnesium sulfate

Author: Amelie Stahlbuhk
back to Sulfate

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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
].

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
.

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
].

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 a_w 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
.

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.

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.pdf
[Stark.etal:1996]	Stark, Jochen; Stürmer, Sylvia (1996): Bauschädliche Salze, Bauhaus-Univ. Weimar
[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. In: Geochimica et Cosmochimica Act, 75 (12), 3600-3626, 10.1016/j.gca.2011.03.038,

concentration given as amount of substance (mol) of the dissolved substance per kilogram solvent; unit: mol•kg-1.

the relative humidity, above which a material, e.g. a salt, absorbes humidity from the air and dissolves.

A phase is a physically distinctive form of matter and is characterized by having relatively uniform chemical and physical properties.

Hochschule für angewandte Wissenschaft und Kunst Hildesheim/Holzminden/Göttingen, University of Applied Sciences and Arts Hildesheim/Holzminden/Göttingen