Magnesium sulfate

The heptahydrate takes its common name "Epsom salt" 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.

The heptahydrate readily loses one equivalent of water to form the hexahydrate.

The monohydrate can be prepared by heating the hexahydrate to approximately 150 °C. Further heating to approximately 300-320 °C gives anhydrous magnesium sulfate.

The undecahydrate MgSO
₄•11H
₂O, 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 an eutectic point with water at −3.9°C and 17.3% (mass) of MgSO4.[4] Large crystals can be obtained from solutions of the proper concentration kept at 0 °C for a few days.[4]

At pressures of about 0.9 GPa and at 240 K, meridianiite decomposes into a mixture of ice VI and the enneahydrate MgSO
₄•9H
₂O[6]

The enneahydrate MgSO
₄•9H
₂O was identified and characterized only recently, even though it seems easy to produce (by cooling a solution of MgSO
₄ and sodium sulfate Na
₂SO
₄ 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 MgSeO
₄•9H
₂O, but with a different crystal structure.[6]

Magnesium sulfate is used both externally (as Epsom salt) and internally.

The main external use is the formulation as bath salts, especially for foot baths to soothe sore feet. Such baths have been claimed to also soothe and hasten recovery of muscle pain, soreness, or injury. However, these claims have not been scientifically confirmed. The main benefit of the salt is cosmetic: it prevents the temporary skin wrinkling caused by prolonged immersion in plain water. [8] [1] It is also the usual component of the solution used in isolation tanks.

In the UK, a medication containing magnesium sulphate and phenol, called "drawing paste", is claimed to be useful for small boils or localised infections,[9] and removing splinters.[10]

Internally, magnesium sulfate may be administered by oral, respiratory, or intravenous routes. Internal uses include: replacement therapy for magnesium deficiency,[11] treatment of acute and severe arrhythmias,[12] as a bronchodilator in the treatment of asthma,[13] and preventing eclampsia.[14]

In agriculture, 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 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 pH neutral, compared with 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.[15]

Magnesium sulfate is used as a brewing salt in making beer.[16] It may also be used as a coagulant for making tofu.[17]

Anhydrous magnesium sulfate is commonly used as a desiccant in organic synthesis due to its affinity for water and compatibility with most organic compounds. During work-up, an organic phase is treated with anhydrous magnesium sulfate. The hydrated solid is then removed with filtration, decantation or distillation (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.

Magnesium sulfate is used to prepare specific cements by the reaction between magnesium oxide and magnesium sulfate solution, which are of good binding ability and more resistance than Portland cement. 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 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.

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.[18]