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Sodium sulfate, anhydrous

(salt cake)

CAS no: 7757-82-6   Formula: Na2SO4   Molecular weight: 142.044

Physical Data

Appearance: White orthorhombic bipyramidal crystalline powder

Melting point: 884C
Boiling point: -

Density: 2.70 g/cm3
Solubility: 28.1 g/100 g H2Ograph

Thermodynamic Data

Enthalpy of formation: -1387.1 kJ/mol
Gibbs energy of formation: -1270.2 kJ/mol

Entropy: 149.6 J/mol K
Heat capacity: 128.2 J/mol K

Production and preparation

Occurs in nature as mirabilite and thernadite. The latter is usually deposited as an evaporation product near playas and salt lakes. It is often found in arid regions of nothern Africa and western part of the United Sates. It appears as an efflorescence on arid soil and as a crust in volcanic fumarole deposits.

Sodium sulfate can be obtained directly from the minerals or from the treatment of sodium chloride with sulfuric acid and evaporating to crystallization:

Na2SO4 production

It is also produce industrially by the reaction of magnesium sulfate and sodium chloride in solution followed by crystallization. In both cases, Sodium sulfate is subsequently crystallised as the decahydrate product which can easily convert to anhydrous form upon heating.

In the laboratory it can be prepared by the neutralization reaction by mixing sodium hydroxide and sulfuric acid.

neutralization reaction

Behavior and Chemical Properties

Sodium sulfate is neutral and does not hydrolyse in water. The solubility of sodium sulfate increases sharply with temperature and achieve maximum solubility at around 33C and decreases slowly beyond this temperature. The anhydrous salt readily adsorbs moisture in air to give the efflorescence decahydrate crystals. If, say a solution of barium chloride, is added to a solution of sodium sulfate, an insoluble white precipitate of barium sulfate is formed.

barium sulfate precipitate

This is an important reaction which is used both as a qualitative and quantitative test for the barium and sulfate ions.

History and Uses

The preparation of sodium sulfate from sodium chloride (common salt) and sulfuric acid was discovered by Johann R Glauber (1604-1668), a German-Dutch chemist. The white efflorescense residue is called sal mirabile, or Glauber's salt.

Because of its moiture-adsorbing ability, the anhydrous salt is used for drying certain organic liquids. The loose, granular anhydrous solids changed to the hydrated from which is lumped together. It is also used in the Kjeldahl method of nitrogen determination.

It is also used in the manufacture of glass, paper pulp, paper board, and as a raw material for the production of various chemicals. For example, in the kraft process for paper making, non-cellulose parts of the wood (such as lignin) are removed by heating the wood pulp with an alkaline solution of sodium sulfide at 170C for three hours. The latter chemical is produced from the reduction of sodium sulfate:

sodium sulfate reduction

Sodium sulfate is also used as a filler and extender in dry powder, laundary and dishwashing detergent products.

In textile industry, the anhydrous salt is added to the dyebath to drive the dye from the solution onto the fibres and to promote an even finish.

Hazard, Storage and Handling

Keep in a tightly closed container and isolate from incompatible substances such as aluminium, magnesium and strong acids and bases.

Not expected to be a health hazard when sodium sulfate is inhaled or come into skin contact. It may be mildly toxic by ingestion. Because of its osmosis activity, it will draw water into the lumen of the bowel and may cause purging, fluid loss, fall of blood pressure and blood in stools.

No adverse effects is expected upon eye contact, although this should be avoided as it may couse mild irritation.

Use usual emergency measures upon skin contacts: wash with soap and water. Upon eye contact splash with running water. Get medical attention if irritation develops. If swallowed, give several glasses of water to drink.

(Last update: February 2006)

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