Why is sodium sulfate insoluble in water

Sodium sulfate (Glauber's salt)

Author: H. Lohninger

CAS no.7757-82-6
formulaN / A2SO4
molar mass142,043 g / mol
Melting point884 ° C
boiling point1429 ° C
density2.7 g / cm3
Solubility [25 ° C]281 g / L

Sodium sulfate is a crystalline, white compound of sodium, either available as anhydrous Na2SO4 or as a decahydrate, Na2SO410H2O, appear. The decahydrate is also under the name Glauber's salt (after J.R. Glauber, who described it as early as the 17th century) or Carlsbad salt (after the Karlovy Vary mineral water, the residues of which contain large amounts of sodium sulfate). Sodium sulfate was originally used as a laxative.

physical and chemical properties

Sodium sulphate is chemically stable and does not or hardly react at room temperature with oxidizing or reducing reagents. At high temperatures it can be reduced to sodium sulfide. A sodium sulfate solution has a neutral pH of 7 because it is the salt of a strong base with a strong acid. In aqueous solution, sodium sulfate reacts with sulfuric acid in an equilibrium reaction to form sodium hydrogen sulfate:

N / A2SO4(aq) + H2SO4(aq) 2 NaHSO4(aq)

If you mix an aqueous solution of Na2SO4 With aqueous solutions of barium or lead salts, a precipitate is formed, as both barium and lead form insoluble sulfates:

N / A2SO4(aq) + BaCl2(aq) 2 NaCl (aq) + BaSO4(s)

The solubility of sodium sulfate first rises sharply with temperature up to 32.4 ° C, and then changes only slightly above this.
Sodium sulfate shows unusual solubility properties in water (see diagram on the right). Its solubility increases between 0 ° C and 32.4 ° C by a factor greater than ten, where the solubility has a maximum of 49.7 g Na2SO4 achieved per 100 g of water. From this point on, the solubility changes only a little. If sodium chloride is added, the solubility drops significantly. This special solubility behavior is used on the one hand in passive solar heating and on the other hand to purify sodium sulphate.

The unusual change in the dissolution behavior can be explained by the hydration, since sodium sulfate crystallizes as decahydrate (Glauber's salt) below 32.4 ° C, but above it as anhydrate (thenardite). Since the heat of dissolution of the two forms differ massively (thenardite dissolves with the development of heat, Glauber's salt with consumption of heat), this peculiar solubility curve is the result.

Occurrence and manufacture

About half of the world's consumption of sodium sulfate is obtained from mirabilite and thenardite, which occurs in large deposits in southern Saskatchewan (Canada), the Caspian Sea (in Turkmenistan) and in Nevada (USA).

Sodium sulfate is also a by-product of many industrial processes, the most important of which is the production of hydrochloric acid from sodium chloride and sulfuric acid:

2 NaCl + H2SO4 N / A2SO4 + 2 HCl

This reaction is also the preliminary stage to the Leblanc process for the production of soda (sodium carbonate). As an alternative, sodium sulfate can also be produced using the Hargreaves process:

4 NaCl + 2 SO2 + O2 + 2 H.2O 2 Well2SO4 + 4 HCl

In addition, sodium sulfate is a by-product of all processes in which sulfuric acid is neutralized with sodium hydroxide. This method is also the most common method for representing Na2SO4 in the laboratory:

2 NaOH (aq) + H2SO4(aq) N / A2SO4(aq) + 2H2Oil)

N / A2SO4 is normally purified by recrystallization in the form of decahydrate, as the anhydrate tends to adsorb iron and organic compounds. The anhydrate can be obtained from the decahydrate simply by heating.


Sodium sulfate is used in the glass, paper, pulp and detergent industries. Na is used in glass production2SO4 as a flux, so that the molten glass becomes thinner and the formation of bubbles and foam is avoided. Bulk consumers of Na2SO4 are the detergent manufacturers who use it as a filler for washing powder. In the paper industry, hot alkaline solutions are used to dissolve lignin from wood, thus releasing the cellulose.

Glauber's salt served and is used as a laxative; There are also proposals to use the heat of crystallization (78.2 kJ / mol) to store solar energy, but the process has not yet caught on. Sodium sulfate anhydrous is a mild desiccant for organic solvents.