Magnesium sulfate or magnesium sulphate (in English-speaking countries other than the US) is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

, a

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

with the formula , consisting of

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...

cations (20.19% by mass) and sulfate

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s . It is a white

crystalline solid A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

, soluble in water but not in

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

. Magnesium sulfate is usually encountered in the form of a hydrate , for various values of ''n'' between 1 and 11. The most common is the heptahydrate , known as Epsom salt, which is a

household chemical Household chemicals are non-food chemicals that are commonly found and used in and around the average household. They are a type of consumer goods, designed particularly to assist cleaning, house and yard maintenance, cooking, pest control and ...

with many traditional uses, including bath salts. The main use of magnesium sulfate is in agriculture, to correct soils deficient in magnesium (an essential

plant nutrient Plant nutrition is the study of the chemical elements and compounds necessary for plant growth and reproduction, plant metabolism and their external supply. In its absence the plant is unable to complete a normal life cycle, or that the element i ...

because of the role of magnesium in chlorophyll and

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

). The monohydrate is favored for this use; by the mid 1970s, its production was 2.3 million tons per year. The anhydrous form and several hydrates occur in nature as

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

s, and the salt is a significant component of the water from some springs.

Hydrates

Magnesium sulfate can crystallize as several hydrates, including: * Anhydrous, ; unstable in nature, hydrates to form epsomite. * Monohydrate, ; kieserite,

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...

. * Monohydrate, ; triclinic. * or . * Dihydrate, ; orthorhombic. * or . * Trihydrate, . * Tetrahydrate, ; starkeyite,

. * Pentahydrate, ; pentahydrite, triclinic. * Hexahydrate, ; hexahydrite, monoclinic. * Heptahydrate, ("Epsom salt"); epsomite, orthorhombic. * Enneahydrate, , monoclinic. * Decahydrate, . * Undecahydrate, ; meridianiite, triclinic. As of 2017, the existence of the decahydrate apparently has not been confirmed. All the hydrates lose water upon heating. Above 320 °C, only the anhydrous form is stable. It decomposes without

melting Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which in ...

at 1124 °C into

magnesium oxide Magnesium oxide ( Mg O), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2� ...

(MgO) and sulfur trioxide ().

Heptahydrate

The heptahydrate takes its common name "Epsom salt" from a bitter saline spring in Epsom in

Surrey Surrey () is a ceremonial county, ceremonial and non-metropolitan county, non-metropolitan counties of England, county in South East England, bordering Greater London to the south west. Surrey has a large rural area, and several significant ur ...

, England, where the salt was produced from the springs that arise where the porous

chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. C ...

of the

North Downs The North Downs are a ridge of chalk hills in south east England that stretch from Farnham in Surrey to the White Cliffs of Dover in Kent. Much of the North Downs comprises two Areas of Outstanding Natural Beauty (AONBs): the Surrey Hills ...

meets the impervious

London clay The London Clay Formation is a marine geological formation of Ypresian (early Eocene Epoch, c. 56–49 million years ago) age which crops out in the southeast of England. The London Clay is well known for its fossil content. The fossils from ...

. The heptahydrate readily loses one equivalent of water to form the hexahydrate. It is a natural source of both magnesium and sulphur. Epsom salts are commonly used in bath salts, exfoliants, muscle relaxers and pain relievers. However, these are different from Epsom salts that are used for gardening, as they contain aromas and

perfume Perfume (, ; french: parfum) is a mixture of fragrant essential oils or aroma compounds (fragrances), fixatives and solvents, usually in liquid form, used to give the human body, animals, food, objects, and living-spaces an agreeable scent ...

s not suitable for plants.

Monohydrate

Magnesium sulfate monohydrate, or kieserite, can be prepared by heating the heptahydrate to 120 °C. Further heating to 250 °C gives anhydrous magnesium sulfate. Kieserite exhibits monoclinic symmetry at pressures lower than 2.7 GPa after which it transforms to phase of triclinic symmetry.

Undecahydrate

The undecahydrate , meridianiite, is stable at atmospheric pressure only below 2 °C. Above that temperature, it liquefies into a mix of solid heptahydrate and a saturated solution. It has a eutectic point with water at −3.9 °C and 17.3% (mass) of . Large

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...

s can be obtained from solutions of the proper concentration kept at 0 °C for a few days. At pressures of about 0.9 GPa and at 240 K, meridianiite decomposes into a mixture of ice VI and the enneahydrate .

Enneahydrate

The enneahydrate was identified and characterized only recently, even though it seems easy to produce (by cooling a solution of and sodium sulfate in suitable proportions). The structure is monoclinic, with unit-cell parameters at 250 K: ''a'' = 0.675 nm, ''b'' = 1.195 nm, ''c'' = 1.465 nm, β = 95.1°, ''V'' = 1.177 nm³ with ''Z'' = 4. The most probable space group is P21/c. Magnesium selenate also forms an enneahydrate , but with a different crystal structure.

Natural occurrence

As and ions are respectively the second cation and the second

present in

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...

after and , magnesium sulfates are common minerals in geological environments. Their occurrence is mostly connected with

supergene A supergene is a chromosomal region encompassing multiple neighboring genes that are inherited together because of close genetic linkage, i.e. much less recombination than would normally be expected. This mode of inheritance can be due to genom ...

processes. Some of them are also important constituents of evaporitic

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...

-magnesium (K-Mg) salts deposits. Bright spots observed by the Dawn Spacecraft in Occator Crater on the dwarf planet Ceres are most consistent with reflected light from magnesium sulfate hexahydrate. Almost all known mineralogical forms of are hydrates. Epsomite is the natural analogue of "Epsom salt". Meridianiite, , 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 especially sanderite – are rare. Kieserite is a monohydrate and is common among evaporitic deposits. Anhydrous magnesium sulfate was reported from some burning coal dumps.

Preparation

Magnesium sulfate is usually obtained directly from dry lake beds and other natural sources. It can also be prepared by reacting

magnesite Magnesite is a mineral with the chemical formula ( magnesium carbonate). Iron, manganese, cobalt, and nickel may occur as admixtures, but only in small amounts. Occurrence Magnesite occurs as veins in and an alteration product of ultramafic ...

( magnesium carbonate, ) or magnesia ( oxide, MgO) with

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...

(). Another possible method is to treat seawater or magnesium-containing industrial wastes so as to precipitate magnesium hydroxide and react the precipitate with

. Also, magnesium sulfate heptahydrate ( epsomite, ) is manufactured by dissolution of magnesium sulfate monohydrate ( kieserite, ) in water and subsequent crystallization of the heptahydrate.

Physical properties

Magnesium sulfate relaxation is the primary mechanism that causes the absorption of

sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by ...

at frequencies above 10 kHz ( acoustic energy is converted to thermal 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.

Uses

Medical

Magnesium sulfate is used both externally (as Epsom salt) and internally. The main external use is the formulation as bath salts, especially for

foot bath An is a Japanese public bath in which people can bathe their feet. The majority of ''ashiyu'' are free. File:Ashiyu device.jpg, An electric ''ashiyu'' foot bath sold in a department store Outline The term ''ashiyu'' is a combination of the t ...

s to soothe sore feet. Such baths have been claimed to also soothe and hasten recovery from muscle pain, soreness, or injury. Potential health effects of magnesium sulfate are reflected in medical studies on the impact of magnesium on resistant depression and as an analgesic for migraine and chronic pain. Magnesium sulfate has been studied in the treatment of

asthma Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, co ...

preeclampsia Pre-eclampsia is a disorder of pregnancy characterized by the onset of high blood pressure and often a significant amount of protein in the urine. When it arises, the condition begins after 20 weeks of pregnancy. In severe cases of the disease ...

and eclampsia. Magnesium sulfate is the usual component of the concentrated salt solution used in isolation tanks to increase its

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its dens ...

to approximately 1.25–1.26. This high density allows an individual to float effortlessly on the surface of water in the closed tank, eliminating as many of the external senses as possible. In the UK, a medication containing magnesium sulfate and

phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it r ...

, called "drawing paste", is useful for small boils or localized infections and removing splinters. Internally, magnesium sulfate may be administered by oral, respiratory, or intravenous routes. Internal uses include replacement therapy for magnesium deficiency, treatment of acute and severe arrhythmias, as a bronchodilator in the treatment of

, preventing eclampsia, a tocolytic agent, and as an anticonvulsant. It also may be used as laxative.

Agriculture

agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people ...

, magnesium sulfate is used to increase magnesium or sulfur content in

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

. It is most commonly applied to potted plants, or to magnesium-hungry crops such as

potato The potato is a starchy food, a tuber of the plant ''Solanum tuberosum'' and is a root vegetable native to the Americas. The plant is a perennial in the nightshade family Solanaceae. Wild potato species can be found from the southern Uni ...

es,

tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to the Spanish word ...

es,

carrot The carrot ('' Daucus carota'' subsp. ''sativus'') is a root vegetable, typically orange in color, though purple, black, red, white, and yellow cultivars exist, all of which are domesticated forms of the wild carrot, ''Daucus carota'', na ...

s, peppers, lemons, and

rose A rose is either a woody perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred species and tens of thousands of cultivars. They form a group of plants that can be ...

s. The advantage of magnesium sulfate over other magnesium soil amendments (such as dolomitic lime) is its high

solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...

, which also allows the option of

foliar feeding Foliar feeding is a technique of feeding plants by applying liquid fertilizer directly to the leaves. Plants are able to absorb essential elements through their leaves. The absorption takes place through their stomata and also through their epiderm ...

. Solutions of magnesium sulfate are also nearly pH neutral, compared with the slightly alkaline salts of magnesium as found in

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

; therefore, the use of magnesium sulfate as a magnesium source for soil does not significantly change the soil pH. 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. Magnesium sulfate was historically used as a treatment for lead poisoning prior to the development of

chelation therapy Chelation therapy is a medical procedure that involves the administration of chelating agents to remove heavy metals from the body. Chelation therapy has a long history of use in clinical toxicology and remains in use for some very specific me ...

, as it was hoped that any lead ingested would be precipitated out by the magnesium sulfate and subsequently purged from the digestive system. 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 inadvertently ingested lead.

Food preparation

Magnesium sulfate is used as * Brewing salt in making

beer Beer is one of the oldest and the most widely consumed type of alcoholic drink in the world, and the third most popular drink overall after water and tea. It is produced by the brewing and fermentation of starches, mainly derived from ce ...

. * coagulant for making tofu. *

Salt substitute A salt substitute, also known as low-sodium salt, is a low-sodium alternative to edible salt (table salt) marketed to circumvent the risk of high blood pressure and cardiovascular disease associated with a high intake of sodium chloride while mai ...

Chemistry

Anhydrous magnesium sulfate is commonly used as a desiccant in organic synthesis owing to its affinity for

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

and compatibility with most organic compounds. During

work-up In chemistry, work-up refers to the series of manipulations required to isolate and purify the product(s) of a chemical reaction. Typically, these manipulations may include: * quenching a reaction to deactivate any unreacted reagents. * coolin ...

, an organic phase is treated with anhydrous magnesium sulfate. The hydrated solid is then removed by filtration, decantation, or by

distillation Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the he ...

(if the boiling point is low enough). Other inorganic sulfate salts such as sodium sulfate and calcium sulfate may be used in the same way.

Construction

Magnesium sulfate is used to prepare specific

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixe ...

s by the reaction between

and magnesium sulfate solution, which are of good binding ability and more resistance than

Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19t ...

. This cement is mainly adopted in the production of lightweight insulation panels. Weakness in water resistance limits its usage. Magnesium (or sodium) sulfate is also used for testing aggregates for soundness in accordance with ASTM C88 standard, when there are no service records of the material exposed to actual

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movement ...

conditions. The test is accomplished by repeated immersion in saturated solutions followed by oven drying to dehydrate the salt precipitated in permeable pore spaces. The internal expansive force, derived from the rehydration of the salt upon re-immersion, simulates the expansion of water on

freezing Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid ...

. Magnesium sulfate is also used to test the resistance of

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wid ...

to external

sulfate attack Cement hydration and strength development mainly depend on two silicate phases: tricalcium silicate (C3S) (alite), and dicalcium silicate (C2S) (belite). Upon hydration, the main reaction products are calcium silicate hydrates (C-S-H) and calcium ...

(ESA).

Aquaria

Magnesium sulfate heptahydrate is also used to maintain the magnesium concentration in marine aquaria which contain large amounts of

stony corals Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a m ...

, as it is slowly depleted in their

calcification Calcification is the accumulation of calcium salts in a body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. D. Cardiovascular calcification: Orbicular origins. ''Nature M ...

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 In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

into calcium carbonate.

Double salts

Double salts containing magnesium sulfate exist. There are several known as sodium magnesium sulfates and potassium magnesium sulfates. A mixed

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...

-magnesium sulfate heptahydrate was recently found to occur in mine tailings and has been given the mineral name alpersite.

Research

Research on topical magnesium (for example

epsom salt Epsomite, Epsom salt, or magnesium sulfate heptahydrate, is a hydrous magnesium sulfate mineral with formula MgSO4·7H2O. Epsomite crystallizes in the orthorhombic system as rarely found acicular or fibrous crystals, the normal form is as massiv ...

baths) is very limited. The

Epsom Salt Council Epsom is the principal town of the Borough of Epsom and Ewell in Surrey, England, about south of central London. The town is first recorded as ''Ebesham'' in the 10th century and its name probably derives from that of a Saxon landowner. The ...

recommends bathing 2 or 3 times/ week, using 500-600g

Epsom salt Epsomite, Epsom salt, or magnesium sulfate heptahydrate, is a hydrous magnesium sulfate mineral with formula MgSO4·7H2O. Epsomite crystallizes in the orthorhombic system as rarely found acicular or fibrous crystals, the normal form is as massiv ...

s each time.

References

A. Dominic Fortes, Kevin S. Knight, and Ian G. Wood (2017): "Structure, thermal expansion and incompressibility of MgSO₄·9H₂O, its relationship to meridianiite (MgSO₄·11H₂O) and possible natural occurrences". ''Acta Crystallographica Section B: Structureal Science, Crystal Engineering and Materials'', volume 73, part 1, pages 47-64. Industrial Inorganic Chemistry, Karl Heinz Büchel, Hans-Heinrich Moretto, Dietmar Werner, John Wiley & Sons, 2d edition, 2000, R. C. Peterson, W. Nelson, B. Madu, and H. F. Shurvell (2007): "Meridianiite: A new mineral species observed on Earth and predicted to exist on Mars". ''American Mineralogist'', volume 92, issue 10, pages 1756–1759. . A. Dominic Fortes, Frank Browning, and Ian G. Wood (2012): "Cation substitution in synthetic meridianiite (MgSO₄·11H₂O) I: X-ray powder diffraction analysis of quenched polycrystalline aggregates". ''Physics and Chemistry of Minerals'', volume 39, issue, pages 419–441.

External links

International Chemical Safety Cards—Magnesium Sulfate
{{DEFAULTSORT:Magnesium Sulfate Desiccants Laxatives Magnesium compounds Sulfates