 Magnesium sulfate hexahydrate | |
 Anhydrous magnesium sulfate | |
 Epsomite (Magnesium sulfate heptahydrate) | |
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| IUPAC name Magnesium sulfate | |
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| ECHA InfoCard | 100.028.453 |
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| Properties | |
| MgSO4 | |
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| Appearance | white crystalline solid |
| Odor | odorless |
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Solubility product (Ksp) | 738 (502 g/L) |
| Solubility indiethyl ether | 1.16 g/100 mL @ 18 °C (64 °F) |
| Solubility inethanol | slight |
| Solubility inglycerol | slight |
| −50×10−6 cm3/mol | |
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| Structure | |
| monoclinic (hydrate) | |
| Pharmacology | |
| A06AD04 (WHO) A12CC02 (WHO)B05XA05 (WHO)D11AX05 (WHO)V04CC02 (WHO) | |
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Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Magnesium sulfate ormagnesium sulphate is achemical compound, asalt with the formulaMgSO4, consisting ofmagnesiumcationsMg2+ (20.19% by mass) andsulfateanionsSO2−4. It is a whitecrystalline solid,soluble in water.
Magnesium sulfate is usually encountered in the form of ahydrateMgSO4·nH2O, for various values ofn between 1 and 11. The most common is the heptahydrateMgSO4·7H2O,[1] known asEpsom salt, which is ahousehold chemical with many traditional uses, includingbath salts.[2]
The main use of magnesium sulfate is in agriculture, to correct soils deficient in magnesium (an essentialplant nutrient because of the role of magnesium inchlorophyll andphotosynthesis). The monohydrate is favored for this use; by the mid 1970s, its production was 2.3 million tons per year with production increasing to approximately 2.6 million tons in 2024.[3][4] Theanhydrous form and several hydrates occur in nature asminerals, and the salt is a significant component of the water from somesprings.
Magnesium sulfate cancrystallize as severalhydrates, including:
As of 2017, the existence of the decahydrate apparently has not been confirmed.[10]
All the hydrates lose water upon heating. Above 320 °C (608 °F), only the anhydrous form is stable. It decomposes withoutmelting at 1,124 °C (2,055 °F) intomagnesium oxide (MgO) andsulfur trioxide (SO3).
The heptahydrate takes its common name "Epsom salt" from a bitter saline spring inEpsom inSurrey, England, where the salt was produced from the springs that arise where theporouschalk of theNorth Downs meets theimperviousLondon clay.
The heptahydrate readily loses one equivalent of water to form the hexahydrate.
It is a natural source of both magnesium andsulphur. Epsom salts are commonly used inbath salts,exfoliants, muscle relaxers and pain relievers.
Magnesium sulfate monohydrate, or kieserite, can be prepared by heating the heptahydrate to 120 °C (248 °F).[11] Further heating to 250 °C (482 °F) gives anhydrous magnesium sulfate.[11] Kieserite exhibits monoclinic symmetry at pressures lower than 2.7 gigapascals (27,000 atm) after which it transforms to phase of triclinic symmetry.[7]
The undecahydrateMgSO4·11H2O,meridianiite, is stable atatmospheric pressure only below 2 °C (36 °F). Above that temperature, it liquefies into a mix of solid heptahydrate and asaturated solution. It has aeutectic point with water at −3.9 °C (25.0 °F) and 17.3% (mass) ofMgSO4.[8] Largecrystals can be obtained from solutions of the proper concentration kept at 0 °C (32 °F) for a few days.[8]
At pressures of about 0.9 gigapascals (8,900 atm) and at 240 K (−33 °C; −28 °F), meridianiite decomposes into a mixture ofice VI and the enneahydrateMgSO4·9H2O.[10]
The enneahydrateMgSO4·9H2O was identified and characterized only recently, even though it seems easy to produce (by cooling a solution ofMgSO4 andsodium sulfate (Na2SO4) in suitable proportions).
The structure is monoclinic, with unit-cell parameters at 250 K (−23 °C; −10 °F):a=0.675 nm,b=1.195 nm,c=1.465 nm,β=95.1°,V=1.177 nm3 withZ=4. The most probable space group is P21/c.Magnesium selenate also forms an enneahydrateMgSeO4·9H2O, but with a different crystal structure.[10]
AsMg2+ andSO2−4 ions are respectively the second most abundant cation and anion present in seawater afterNa+ andCl−, magnesium sulfates are common minerals in geological environments. Their occurrence is mostly connected withsupergene processes. Some of them are also important constituents ofevaporiticpotassium-magnesium (K-Mg) salts deposits.[citation needed]
Bright spots observed by theDawn Spacecraft inOccator Crater on thedwarf planetCeres are most consistent with reflected light from magnesium sulfate hexahydrate.[12]
Almost all known mineralogical forms ofMgSO4 are hydrates.Epsomite is the natural analogue of "Epsom salt".Meridianiite,MgSO4·11H2O, has been observed on the surface of frozen lakes and is thought to also occur on Mars.Hexahydrite is the next lower hydrate. Three next lower hydrates –pentahydrite,starkeyite, and especiallysanderite – are rare.Kieserite is a monohydrate and is common among evaporitic deposits.
Magnesium sulfate is usually obtained directly from dry lake beds and other natural sources. It can also be prepared by reactingmagnesite (magnesium carbonate,MgCO3) ormagnesia (oxide, MgO) withsulfuric acid (H2SO4):
Another possible method is to treat seawater or magnesium-containing industrial wastes so as to precipitatemagnesium hydroxide and react theprecipitate withsulfuric acid.
Also, magnesium sulfate heptahydrate (epsomite,MgSO4·7H2O) can be manufactured by dissolution of magnesium sulfate monohydrate (kieserite,MgSO4·H2O) in water and subsequent crystallization of the heptahydrate.
Magnesium sulfaterelaxation is the primary mechanism that causes the absorption ofsound in seawater atfrequencies above10 kHz[13] (acoustic energy is converted tothermal energy). Lower frequencies are less absorbed by the salt, so that low frequency sound travels farther in the ocean.Boric acid and magnesium carbonate also contribute to absorption.[14]
Magnesium sulfate is used both externally (as epsom salt) and internally.
The main external use is the formulation asbath salts, especially forfoot baths to soothe sore feet. Such baths have been claimed to also soothe and hasten recovery from muscle pain, soreness, or injury.[15]Health effects of magnesium sulfate that have been proposed include improvement of treatment resistant depression[16] and as ananalgesic formigraine andchronic pain.[17]
Magnesium sulfate is usually the main component of the concentrated salt solution used inisolation tanks to increase itsspecific gravity to approximately1.25–1.26. This high density allows an individual to float effortlessly on the surface of water in the closed tank, eliminating stimulation of as many of the external senses as possible.[18]
In theUK, a medication containing magnesium sulfate andphenol, called "drawing paste", is useful for small boils or localized infections[19] and removing splinters.[20]
Internally, magnesium sulfate may be administered by oral, respiratory, orintravenous routes. Internal uses include replacement therapy formagnesium deficiency,[21][22] treatment of acute and severearrhythmias,[22] as abronchodilator in the treatment ofasthma,[23] preventingeclampsia[24] andcerebral palsy,[25][26] atocolytic agent,[27] and as ananticonvulsant.[27] The effectiveness and safety of magnesium sulfate for treating acute bronchiolitis in children under the age of 2 years old is not well understood.[28]
It also may be used aslaxative.[29]
Inagriculture, magnesium sulfate is used to increase magnesium or sulfur content in soil. It is most commonly applied to potted plants, or to magnesium-hungry crops such as potatoes, tomatoes, carrots, peppers, lemons, and roses. The advantage of magnesium sulfate over other magnesiumsoil amendments (such asdolomitic lime) is its high solubility, which also allows the option offoliar feeding. Solutions of magnesium sulfate are also nearly pH neutral, compared with theslightly alkaline salts of magnesium as found inlimestone; therefore, the use of magnesium sulfate as a magnesium source for soil does not significantly change thesoil pH.[27] Contrary to the popular belief that magnesium sulfate is able to control pests and slugs, helps seeds germination, produce more flowers, improve nutrient uptake, and is environmentally friendly, it does none of the purported claims except for correcting magnesium deficiency in soils. Magnesium sulfate can even pollute water if used in excessive amounts.[30]
Magnesium sulfate was historically used as a treatment forlead poisoning prior to the development ofchelation therapy, as it was hoped that any lead ingested would be precipitated out by the magnesium sulfate and subsequently purged from thedigestive system.[31] This application saw particularly widespread use among veterinarians during the early-to-mid 20th century; Epsom salt was already available on many farms for agricultural use, and it was often prescribed in the treatment of farm animals that had inadvertently ingested lead.[32][33]
Magnesium sulfate is used as:
Anhydrous magnesium sulfate is commonly used as adesiccant inorganic synthesis owing to its affinity for water and compatibility with most organic compounds.[38] Duringwork-up, an organic phase is treated with anhydrous magnesium sulfate. The hydrated solid is then removed byfiltration,decantation, or bydistillation (if the boiling point is low enough). Other inorganic sulfate salts such as sodium sulfate andcalcium sulfate may be used in the same way.
Magnesium sulfate is used to prepare specific cements by the reaction betweenmagnesium oxide and magnesium sulfate solution.[39] This cement is mainly used in the production of lightweight insulation panels, although its poor water resistance limits its usage.
Magnesium (or sodium) sulfate is also used for testingaggregates for soundness in accordance withASTM C88 standard, when there are no service records of the material exposed to actualweathering conditions. The test is accomplished by repeated immersion in saturated solutions followed by oven drying to dehydrate the salt precipitated in permeablepore spaces. The internal expansive force, derived from the rehydration of the salt upon re-immersion, simulates the expansion of water on freezing.
Magnesium sulfate is also used to test the resistance of concrete to externalsulfate attack (ESA).[40]
Magnesium sulfate heptahydrate is also used to maintain the magnesium concentration in marine aquaria which contain large amounts ofstony corals, as it is slowly depleted in theircalcification process. In a magnesium-deficient marine aquarium, calcium andalkalinity concentrations are difficult to control because not enough magnesium is present to stabilize these ions in the saltwater and prevent their spontaneousprecipitation intocalcium carbonate.[41]
Double salts containing magnesium sulfate exist. There are several known assodium magnesium sulfates andpotassium magnesium sulfates. A mixedcopper-magnesium sulfate heptahydrate(Mg,Cu)SO4·7H2O was found to occur inmine tailings and was given the mineral namealpersite.[42]
Previous ACLS guidelines addressed the use of magnesium in cardiac arrest with polymorphic ventricular tachycardia (ie, torsades de pointes) or suspected hypomagnesemia, and this has not been reevaluated in the 2015 Guidelines Update. These previous guidelines recommended defibrillation for termination of polymorphic VT (ie, torsades de pointes), followed by consideration of intravenous magnesium sulfate when secondary to a long QT interval.
The treatment of acute lead-poisoning consists in the evacuation of the stomach, if necessary, the exhibition of the sulphate of sodium or of magnesium, and the meeting of the indications as they arrive. The Epsom and Glauber's salts act as chemical antidotes, by precipitating the insoluble sulphate of lead, and also, if in excess, empty the bowel of the compound formed.
Udall (1) suggests sodium citrate as of some value together with Epsom salts which will bring about a precipitation of the lead in the form of an insoluble compound. Nelson (3) reported a case that survived after using a 20% magnesium sulphate solution intravenously, subcutaneously and orally. McIntosh (5) has suggested that purgative doses of Epsom salts may effectively combine with the lead and overcome the toxicity.
The specific antidotes to metal poisoning had not been discovered, and the only thing which sometimes did a bit of good was magnesium sulphate which caused the precipitation of insoluble lead sulphate. The homely term for magnesium sulphate is, of course, epsom salts.
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