Magnesium sulfate
A versatile salt used in agriculture, medicine, and industry.
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Magnesium sulfate is a chemical compound and salt with the formula MgSO₄, composed of magnesium cations and sulfate anions, with magnesium making up about 20.19% of its mass. It appears as a white crystalline solid that readily dissolves in water. The compound is most commonly encountered as a hydrate, with the heptahydrate form—widely known as Epsom salt—being a familiar household substance used traditionally as bath salts. Various hydrates exist, with between one and eleven water molecules per formula unit.
Natural occurrence
In nature, the anhydrous form and several hydrates occur as minerals; for instance, epsomite is the natural counterpart of Epsom salt, while meridianiite has been observed on frozen lakes and is thought to exist on Mars. Hexahydrite is a lower hydrate, and kieserite, a monohydrate, is common in evaporitic deposits. Magnesium and sulfate ions are respectively the second most abundant cation and anion in seawater, making magnesium sulfates widespread in geological settings, particularly in supergene environments and evaporitic potassium-magnesium salt deposits. Bright spots on the dwarf planet Ceres, imaged by the Dawn spacecraft, are most consistent with magnesium sulfate hexahydrate.
Preparation
The compound is typically obtained from dry lake beds or other natural sources, but can also be prepared by reacting magnesite or magnesia with sulfuric acid, or by treating seawater or industrial wastes to precipitate magnesium hydroxide and then reacting it with sulfuric acid. The heptahydrate can be manufactured by dissolving the monohydrate in water and recrystallizing. A key physical property is that magnesium sulfate relaxation is the primary mechanism absorbing sound in seawater at higher frequencies, converting acoustic energy to thermal energy, while lower frequencies travel farther.
Agriculture
Its main use is in agriculture to correct magnesium-deficient soils, as magnesium is essential for chlorophyll and photosynthesis. The monohydrate is favored for this purpose; by the mid-1970s, production was 2.3 million tons per year, rising to about 2.6 million tons by 2024. Unlike other magnesium soil amendments, magnesium sulfate is highly soluble and nearly pH neutral, allowing foliar feeding without significantly altering soil pH, though excessive use can pollute water. It does not control pests, aid germination, or improve nutrient uptake beyond correcting magnesium deficiency.
Quick Facts
- Chemical formula
- MgSO4
- Common name
- Epsom salt (heptahydrate)
- Field
- Chemistry
- Agriculture
- Medicine
- Key use
- Soil amendment for magnesium deficiency
- Natural occurrence
- Seawater
- evaporitic deposits
- minerals such as epsomite and kieserite
- Notable hydrate
- Heptahydrate (MgSO4, 7H2O)
Facts from the source article.
Lore & Background
Magnesium sulfate is a white crystalline solid that dissolves readily in water, composed of magnesium and sulfate ions. It naturally occurs in several hydrated mineral forms, with the heptahydrate known as Epsom salt being the most common household variant. The anhydrous form and various hydrates are found in geological environments, particularly associated with supergene processes and evaporitic potassium-magnesium salt deposits. Epsomite is the natural mineral equivalent of Epsom salt, while kieserite is the monohydrate common in evaporitic deposits.
Other hydrates include hexahydrite, pentahydrite, starkeyite, sanderite, and meridianiite, the last of which has been observed on frozen lakes and is thought to occur on Mars. Magnesium sulfate is also a significant component of water from certain springs. On the dwarf planet Ceres, bright spots in Occator Crater are most consistent with reflected light from magnesium sulfate hexahydrate.
The salt is obtained directly from dry lake beds and other natural sources, or can be prepared by reacting magnesite or magnesia with sulfuric acid. It can also be produced by treating seawater or magnesium-containing industrial wastes to precipitate magnesium hydroxide, then reacting that with sulfuric acid. In seawater, magnesium sulfate relaxation is the primary mechanism absorbing sound at frequencies above a certain threshold, with lower frequencies traveling farther. Its main agricultural use is correcting magnesium-deficient soil, with the monohydrate favored for this purpose.
Reader's Guide
Magnesium sulfate is significant primarily for its role in agriculture, where it corrects magnesium deficiency in soil—magnesium being essential for chlorophyll and photosynthesis. Its high solubility and nearly neutral pH make it preferable over other magnesium sources. In medicine, it is used externally as bath salts and internally for magnesium deficiency, arrhythmias, asthma, eclampsia, and as a laxative, though its effectiveness for some conditions (e.g., acute bronchiolitis in children under 2) is not well understood. Historically, it was used to treat lead poisoning in animals before chelation therapy.
The compound also absorbs sound in seawater above 10 kHz, and its anhydrous form serves as a desiccant in organic synthesis. Bright spots on the dwarf planet Ceres are consistent with magnesium sulfate hexahydrate, and the undecahydrate meridianiite is thought to occur on Mars. Its legacy spans from household baths to planetary science.
Did You Know?
- Magnesium sulfate heptahydrate is commonly known as Epsom salt, named after a bitter saline spring in Epsom, Surrey, England.
- The undecahydrate MgSO4·11H2O, meridianiite, is stable at atmospheric pressure only below 2 °C and is thought to occur on Mars.
Frequently Asked Questions
Who is Magnesium sulfate?
Magnesium sulfate is an inorganic salt made up of a magnesium cation bonded to a sulfate anion, carrying the formula MgSO4. It typically shows up as a white crystalline solid that dissolves easily in water, and its heptahydrate form (MgSO4·7H2O) is the version most people recognize as Epsom salt.
What is Magnesium sulfate known for?
Its headline role is correcting magnesium-deficient agricultural soils so crops can photosynthesize efficiently. It also serves in medicine as a magnesium supplement or anticonvulsant, appears in various industrial processes, and shows up in household products.
Where does Magnesium sulfate appear in the natural world?
It is present dissolved in seawater and concentrated in evaporitic mineral deposits. It also crystallizes naturally as the minerals epsomite (the heptahydrate) and kieserite (the monohydrate), both of which have long been mined for practical use.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Magnesium sulfate (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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