1. Summary

Magnesium sulfate is commercially available as heptahydrate, monohydrate, anhydrous or dried form containing the equivalent of 2 – 3 waters of hydration. Magnesium sulfate heptahydrate is manufactured by dissolution of kieserite in water and subsequent crystallization of the heptahydrate. Magnesium sulfate is also prepared by sulfation of magnesium oxide.

Magnesium sulfate is available as brilliant colourless crystals, granular crystalline powder or white powder with a bitter salty cooling taste. It is freely soluble in water, very soluble in boiling water, and sparingly soluble in alcohol.

Magnesium sulfate is used as a nutrient, firming agent and flavour enhancer. It is also used as a fermentation aid in the processing of beer and malt beverages.

2. Description

Magnesium sulfate occurs with one (monohydrate), seven (heptahydrate), or no (anhydrous) molecules of water of hydration. The C.A.S. numbers of magnesium sulfates are 7487-88-9 (anhydrous), 14168-73-1(monohydrate), 10034-99-8 (heptahydrate), and 15244-36-7 (dried). The chemical formulae for magnesium sulfate are MgSO4 (anhydrous) and MgSO4.xH2O (hydrated forms), and the molecular weights are 120.36 (anhydrous), 138.38 (monohydrate), and 246.47 (heptahydrate). The monohydrate and heptahydrate forms occur in nature as the minerals kieserite and epsomite, respectively. Dried magnesium sulphate, which contains 62 to 70% of MgSO4 and the equivalent of approximately 2 to 3 waters of hydration, is prepared by heating the heptahydrate until approximately 25% of its weight is lost. Anhydrous magnesium sulphate (obtained by ignition) contains not less than 99.0% and not more than 100.5% of MgSO4.

Synonyms of magnesium sulphate include Epsom salts, Magnesii Sulfas, Magnesium Sulfuricum Heptahydricum, Magnesium Sulphate, Sal Amarum, Sel Anglais and Sel de Seditz.

Magnesium sulfate heptahydrate is primarily intended for use in bottled water products as a flavour enhancer however, it is also used as a nutrient in salt replacers, carbonated diet soft drink beverages, sports drinks and enhanced (fortified) water beverages and as a fermentation aid in the production of beer and malt beverages. Magnesium sulphate heptahydrate has a bitter, saline, cooling taste.

3. Manufacturing

The raw material used in the manufacture of magnesium sulfate heptahydrate is the mineral kieserite. Kieserite, an almost white salt that is slowly soluble in cold water and more easily soluble in hot water, forms naturally in marine evaporate deposits where seawater has been concentrated and exposed to prolonged evaporation. A typical analysis of kieserite reveals 15.0% magnesium (25% as magnesium oxide, 75% as magnesium sulfate), 20.0% sulfur (as sulphate), up to 2.5% chlorides, and 12% water.

When not stored in closed containers, specimens of kieserite will absorb water from the air and be converted to magnesium sulfate heptahydrate.

Magnesium sulfate is produced from sea water, mineral spring and minerals such as kieserite and epsomite, or by reacting magnesium oxide with sulphuric acid.

Manufacturing Process

Magnesium oxide is prepared by igniting magnesium hydroxide (obtained from sea water) or ignition of magnesite ore (consists of MgCO3). The magnesium oxide formed is then reacted with sulfuric acid to produce magnesium sulfate. The target product is recrystallized for higher purity.

4. Chemical Characterization

Magnesium sulfate heptahydrate is isolated via crystallization in the heptahydrate form with a minimum chemical purity of 99.5% (w/w) following ignition. It also contains trace levels of other mineral components. Magnesium sulfate heptahydrate is stable in moist air under 48°. While, in dry air, it is effloresce to lose water of crystallization and become hexahydrate at room temperature, monohydrate over 68° and anhydrous at 200 – 300°. Magnesium sulfate is partially decomposed to magnesium oxide over 450°.