Anhydrous magnesium sulfate
|CAS number||, (monohydrate), (tetrahydrate), (pentahydrate), (hexahydrate), (heptahydrate)|
|ATC code||A06,A12 B05 D11 V04|
|Jmol-3D images||Image 1|
|Molar mass||120.366 g/mol (anhydrous)
246.47 g/mol (heptahydrate)
|Appearance||white crystalline solid|
|Density||2.66 g/cm3 (anhydrous)
2.445 g/cm3 (monohydrate)
1.68 g/cm3 (heptahydrate)
1.512 g/cm3 (11-hydrate)
anhydrous decomposes at 1124 °C
|Solubility in water||anhydrous
26.9 g/100 mL (0 °C)
25.5 g/100 mL (20 °C)
71 g/100 mL (20 °C)
|Solubility||1.16 g/100 mL (18 °C, ether)
slightly soluble in alcohol, glycerol
insoluble in acetone
|Refractive index (nD)||1.523 (monohydrate)
|Crystal structure||monoclinic (hydrate)|
|EU Index||Not listed|
|Other cations||Beryllium sulfate
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Magnesium sulfate (or magnesium sulphate) is an inorganic salt (chemical compound) containing magnesium, sulfur and oxygen, with the formula MgSO4. It is often encountered as the heptahydrate sulfate mineral epsomite (MgSO4·7H2O), commonly called Epsom salt, taking its name from a bitter saline spring in Epsom in Surrey, England, where the salt was produced from the springs that arise where the porous chalk of the North Downs meets non-porous London clay. Epsom salt occurs naturally as a pure mineral. Another hydrate form is kieserite.
Anhydrous magnesium sulfate is used as a drying agent. 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. Epsom salt can also be used as a beauty product. Athletes use it to soothe sore muscles, while gardeners use it to improve crops. It has a variety of other uses.
Magnesium sulfate is a common pharmaceutical preparation of magnesium, commonly known as Epsom salts, used both externally and internally. Epsom salts are used as bath salts. The sulfate is supplied in a gel preparation for topical application in treating aches and pains. Oral magnesium sulfate is commonly used as a saline laxative or osmotic purgative. Magnesium sulfate is the main preparation of intravenous magnesium.
Bathing in a 1% solution of Epsom salts (about 500g of Epsom salts for a standard size bathtub of 60 litres) is a "a safe and easy way to increase sulfate and magnesium levels in the body". 
Indications for internal use are:
- Replacement therapy for hypomagnesemia.
- 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.
- As a bronchodilator after beta-agonist and anticholinergic agents have been tried, e.g. in severe exacerbations of asthma. Studies conducted have revealed that magnesium sulfate can be nebulized to reduce the symptoms of acute asthma. 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.
- Magnesium sulfate can also delay labor by inhibiting uterine muscle contraction in the case of premature labor, to delay preterm birth. However, meta-analyses have failed to support it as a tocolytic agent. And its use for prolonged periods (more than 5 to 7 days) may result in health problems for the baby.
- Intravenous magnesium sulfate has been shown to prevent cerebral palsy in preterm babies. A recent systematic review suggests that antenatal intravenous magnesium sulphate can reduce the risk of cerebral palsy and gross motor dysfunction in preterm infants by on average 30%.
- Magnesium sulfate has been used as an experimental treatment of Irukandji syndrome caused by envenomation by certain species of Irukandji jellyfish, however the efficacy of this treatment remains unproven.
- Solutions of sulfate salts such as Epsom salt may be given as first aid for barium chloride poisoning.
An overdose of magnesium causes hypermagnesemia.
In gardening and other agriculture, magnesium sulfate is used to correct a magnesium or sulfur deficiency in soil; magnesium is an essential element in the chlorophyll molecule, and sulfur is another important Macronutrient. It is most commonly applied to potted plants, or to magnesium-hungry crops, such as potatoes, roses, tomatoes, lemon trees and peppers The advantage of magnesium sulfate over other magnesium soil amendments (such as dolomitic lime) is its high solubility, which also allows the option of foliar feeding. Solutions of magnesium sulfate are also nearly neutral, as compared to alkaline salts of magnesium, as found in limestone; therefore the use of magnesium sulfate as a magnesium source for soil does not significantly change the soil pH.
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 (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 naturally present in some mineral waters.
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.
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.
Magnesium sulfate is the primary substance that causes the absorption of sound in seawater (acoustic energy is converted to thermal energy). Absorption is strongly dependent on frequency: lower frequencies are less absorbed by the salt, so that the sound travels much farther in the ocean. Boric acid also contributes to absorption, but the most abundant salt in seawater, sodium chloride, has negligible sound absorption.
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, was 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.
Anhydrous magnesium sulfate is prepared only by the dehydration of a hydrate.
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.
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