Magnesium sulfate

Magnesium sulfate (or magnesium sulphate) is a chemical compound containing magnesium, sulfur and oxygen, with the formula MgSO₄. It is often encountered as the heptahydrate epsomite (MgSO₄·7H₂O), commonly called Epsom salt, from the town of Epsom in Surrey, England, where the salt was distilled from the springs that arise where the porous chalk of the North Downs meets non-porous London clay. Another hydrate form is kieserite.

Anhydrous magnesium sulfate is used as a drying agent. Since the anhydrous form is hygroscopic (readily absorbs water from the air) and is therefore difficult to weigh accurately, the hydrate is often preferred when preparing solutions, for example in medical preparations. Epsom salt has been traditionally used as a component of bath salts.

Properties

Magnesium sulfate is the primary substance that causes the absorption of sound in seawater.^[1] Absorption, in this case, means the conversion of acoustic energy to heat energy. The conversion is a strong function of frequency. Lower frequencies are less affected by the salt, so that the acoustic energy travels much farther in the ocean. Boric acid also contributes to absorption, but the most abundant salt in seawater, sodium chloride, has no known effect on sound absorption.

Hydrates

Almost all known mineralogical forms of MgSO₄ occur as hydrates. Epsomite is the natural analogue of "Epsom salt". Another heptahydrate, the copper-containing mineral alpersite (Mg,Cu)SO₄·7H₂O,^[2] was also recently recognized. Both are however not the highest known hydrates of MgSO₄, due to the recent terrestrial find of meridianiite, MgSO₄·11H₂O, which is thought to also occur on Mars. Hexahydrite is the next lower (6) hydrate. Three next lower hydrates - pentahydrite (5), starkeyite (4) and especially sanderite (2) are more rarely found. Kieserite is a monohydrate and is common among evaporitic deposits. Anhydrous magnesium sulfate was reported from some burning coal dumps, but never treated as a mineral.

The pH of hydrates is average 6.0 (5.5 to 6.5). Magnesium hydrates have, like Copper(II) sulfate, coordinated water.^[3]

Occurrence

Magnesium sulfates are common minerals in geological environments. Their occurrence is mostly connected with supergene processes. Some of them are also important constituents of evaporitic potassium-magnesium (K-Mg) salts deposits.

Applications

In agriculture and gardening, magnesium sulfate is used to correct magnesium deficiency in soil, since magnesium is an essential element in the chlorophyll molecule. It is most commonly applied to potted plants, or to magnesium-hungry crops, such as potatoes, roses, tomatoes, peppers and cannabis. The advantage of magnesium sulfate over other magnesium soil amendments (such as dolomitic lime) is its high solubility.

Anhydrous magnesium sulfate is commonly used as a desiccant in organic synthesis due to its affinity for water. During work-up, an organic phase is saturated with magnesium sulfate until it no longer forms clumps. The hydrated solid is then removed with filtration or decantation. Other inorganic sulfate salts such as sodium sulfate and calcium sulfate may also be used in the same way.

Magnesium sulfate is used in bath salts, particularly in flotation therapy where high concentrations raise the bath water's specific gravity, effectively making the body more buoyant. Traditionally, it is also used to prepare foot baths, intended to soothe sore feet. The reason for the inclusion of the salt is partially cosmetic: the increase in ionic strength prevents some of the temporary skin wrinkling ("pruning" – partial maceration) which is caused by prolonged immersion of extremities in pure water. However, magnesium sulfate can also be absorbed into the skin, reducing inflammation. It is also sometimes found in bottled mineral water, and accordingly is sometimes listed in the contents thereof.

It may also be used as a coagulant for making tofu.^[4]

Magnesium sulfate heptahydrate is also used to maintain the magnesium concentration in marine aquaria which contain large amounts of stony corals as it is slowly depleted in their calcification process. In a magnesium-deficient marine aquarium calcium and alkalinity concentrations are very difficult to control because not enough magnesium is present to stabilize these ions in the saltwater and prevent their spontaneous precipitation into calcium carbonate.^[5]

Magnesium sulfate is used as a reactive compound in the manufacture of black powder where it contributes to a violent burn rate thus reducing material costs.

Magnesium sulfate is used as the electrolyte to prepare copper sulfate. A magnesium sulfate solution is electrolyzed with a copper anode to form copper sulfate, magnesium hydroxide, and hydrogen:

Cu + MgSO₄ + 2 H₂O → H₂ + CuSO₄ + Mg(OH)₂.

Magnesium sulfate is used as a brewing salt in beer production to adjust the ion content of the brewing water and enhance enzyme action in the mash or promote a desired flavor profile in the beer.

Medical use

Oral magnesium sulfate is commonly used as a saline laxative or osmotic purgatives. Epsom salt is also available in a gel form for topical application in treating aches and pains.

Indications for its internal use are:

• Replacement therapy for hypomagnesemia.^[6]
• Magnesium sulfate is the first-line antiarrhythmic agent for torsades de pointes in cardiac arrest under the 2005 ECC guidelines and for managing quinidine-induced arrhythmias.^[7]
• As a bronchodilator after beta-agonist and anticholinergic agents have been tried, e.g. in severe exacerbations of asthma.^[8] Recent studies have revealed that magnesium sulfate can be nebulized to reduce the symptoms of acute asthma.^[8] It is commonly administered via the intravenous route for the management of severe asthma attacks.
• Magnesium sulfate can be used to treat eclampsia in pregnant women.^[9]
• Magnesium sulfate can also delay labor in the case of premature labor, to delay preterm birth.^[10][11]
• Intravenous magnesium sulfate may be able to prevent cerebral palsy in preterm babies.^[12]
• Solutions of sulfate salts such as Epsom salt may be given as first aid for barium chloride poisoning.^[13]

Indications for topical use are

• Soaking in a warm bath containing Epsom salt (magnesium sulfate) can be beneficial to soothe, relax, and relieve herpes outbreak symptoms, such as itching and lesions relating to genital herpes and shingles.^[14][15]
• A 2004 research study showed that both magnesium and sulfate are absorbed through the skin when bathing in 1% w/v solution.^[16]

See also

• A06AD04
• A12CC02
• B05XA05
• D11AX05
• V04CC02

References