Sodium sulfate

Authors: Hans-Jürgen Schwarz , Michael Steiger, Tim Müller, Amelie Stahlbuhk
back to Sulfate

Template:UnderConstuction

Abstract

In this article the different phases of sodium sulfate and their properties will be presented.

Phases and hydrate phases

There are four different phases of sodium sulfate whereof only two are stable. The others are metastable but were also detected.
Thenardite Na₂SO₄
Sodium sulfate phase III Na₂SO₄ metastable
Sodium sulfate heptahydrate Na₂SO₄•7H₂O metastable
Mirabilite Na₂SO₄•10H₂O

Solubility

Figure 1: Solubilities in the system Na₂SO₄-H₂O as a function of temperature. The molalitym as [n(Na₂SO₄•xH₂O)•kg(H₂O)^-1] is plotted versus the temperature. The equilibria of different phases are distinguished by different colours. Dashed lines mark metastable equilibria. According to [Steiger.etal:2008]Title: Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4–H2O and the generation of stress
Author: Steiger, Michael; Asmussen, Sönke
.

The phases of sodium sulfate are easily soluble in water (figure 1), so they have a high mobility in porous materials as well. The solubility of the phases depends on the temperature.
The decahydrate mirabilite is stable below 32.4 °C. Above this temperature the anhydrous thenardite is the stable crystalline phase, which in turn is metastable below this transition temperature. In the case of a temperature dropin a solution that is saturated with respect to thenardite, high supersaturations with respect to mirabilite and with that its precipitation are possible, which involves a certain damage potential.

Table 1: Solubilities in mol/kg of different phases of sodium sulfate at 20 °C [according to [Steiger.etal:2008]Title: Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4–H2O and the generation of stress
Author: Steiger, Michael; Asmussen, Sönke
].

phase	solubility [mol/kg] at 20°C
thenardite	3.706
sodium sulfate phase III	4.428
sodium sulfate heptahydrate	3.143
mirabilite	1.353

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