Magnesium sulfate

Mg 1
S 1
O 4

Formula unit Lattice

Choose Lattice to see one unit cell. Then drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

6 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; magnesium sulfate has no molecular shape.

IUPAC name: magnesium sulfate
Molar mass: 120.361(26) g/mol
Atoms: 6
SMILES: [O-]S(=O)(=O)[O-].[Mg+2]
InChIKey: CSNNHWWHGAXBCP-UHFFFAOYSA-L
PubChem CID: 24083

Made of

Mg S O

On a US drug label, magnesium sulfate heptahydrate goes by Epsom salt, the common name that federal rule 21 CFR 201.312 gives it. That everyday form is a hydrate, a solid whose crystals hold water in a fixed proportion: MgSO₄·7H₂O, seven water molecules for every formula unit. Magnesium sulfate itself, MgSO₄ at 120.36 g/mol, is the salt with no water, built from magnesium ions, Mg²⁺, and sulfate ions, SO₄²⁻, of sulfur and oxygen.

The International Chemical Safety Cards describe the dry salt as odourless white crystals or powder that are hygroscopic, taking up moisture from the air. HSDB, the Hazardous Substances Data Bank, gives it a saline, bitter taste and a solution neutral to litmus. The heptahydrate does the reverse and effloresces: in dry air at room temperature it loses about one of its seven water molecules, by HSDB’s figures.

Structure and bonding

Nothing joins the magnesium ion to the sulfate ion in the drawing: the +2 ion and the −2 ion stand as two separate pieces. In the drawing, sulfur is bonded to four oxygen atoms, two by double bonds and two by single bonds, with −1 on each singly bonded oxygen. The sulfate ion’s own bonding is taken up on the calcium sulfate page. Together the two ions are one formula unit of the anhydrous salt, with no water, and not a particle that exists alone.

One unit cell, all that the Lattice view draws here, holds 4 magnesium ions and 4 sulfate ions. Ions on its corners, edges and faces are shared with neighbouring cells. Under Coordination, one magnesium ion is highlighted with its 6 nearest oxygen atoms, 2.01 to 2.09 Å away, each from a different sulfate ion. No sulfate ion belongs to one magnesium ion alone. The cell is the anhydrous salt, not Epsom salt.

The crystals are orthorhombic in HSDB’s description, a cell type set out on the potassium hydroxide page.

Where it turns up

Magnesium sulfate never occurs in nature as the anhydrous salt. Beds of salt and potash carry it as hydrates and as double salts. Its commonest forms, HSDB says, are Epsom salt and kieserite, MgSO₄·H₂O; the heptahydrate occurs as the mineral epsomite. On Mars, NASA/JPL reports, the Curiosity rover found magnesium sulfate, Epsom salt among its forms, in Mount Sharp’s sulfate-rich rocks.

Inside Mammoth Cave, epsomite comes and goes with the seasons. It can form in winter, when the cave air is drier, and dissolve away once more humid air moves in, according to the US National Park Service. Its crystals and those of mirabilite, another mineral of the cave’s dry passages, lie scattered across one floor, giving the Lantern Tour’s Snow Room its name.

HSDB lists fire-proofing fabrics, dyeing and printing calicoes, and making mother-of-pearl and frosted papers among its uses. It also appears in US Environmental Protection Agency (EPA) chemical reporting as a desiccant, a drying agent.

Why it matters

Seizures that begin in a woman with preeclampsia, a condition of pregnancy, mark eclampsia, which NICHD, part of the US National Institutes of Health, counts as a medical emergency. Magnesium sulfate may be given both to treat seizures under way and to prevent further ones. MeSH, the Medical Subject Headings index, files it as an anticonvulsant and an electrolyte replenisher in treating preeclampsia and eclampsia.

It is also a tocolytic, a drug that delays premature activity of the uterus. In MeSH’s account it directly inhibits action potentials, the electrical impulses of muscle cells, in the uterine wall, so contractions come less often and with less force. On isolated muscle strips, in a study HSDB cites, contractions became less frequent but kept their amplitude.

In the United States, 48 per cent of the anhydrous salt used in 2020 went into cosmetics and personal care, by the US Geological Survey’s count. Chemical intermediates took 22 per cent, pulp and paper 12 and fertiliser 10. Potash-based fertilisers carrying magnesium sulfate are gaining use on soils short of magnesium and in palm oil production.

Properties

Properties of magnesium sulfate Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 24083.
Property	Value	Unit	Source
Formula (Hill order)	MgO₄S
Molar mass	120.361(26) g/mol	g/mol	IUPAC/CIAAW 2024
Magnesium atoms	1
Sulfur atoms	1
Oxygen atoms	4
PubChem molecular weight	120.37 g/mol	g/mol	PubChem

Safety

The EPA’s Safer Choice programme marks magnesium sulfate as of low concern, yet its classification under the Globally Harmonized System (GHS) carries the signal word Warning. Of the company reports to the European Chemicals Agency that give a hazard classification, just over half call it harmful if swallowed, if inhaled and on the skin. Across all reports, 41.4% find that it meets no GHS hazard criteria. It mildly irritates the eyes and airways, and on heating it decomposes to toxic, corrosive fumes including sulfur oxides. PubChem’s Laboratory Chemical Safety Summary has the complete classification.

Questions

Is Epsom salt the same thing as magnesium sulfate?

Epsom salt is one form of it, the heptahydrate MgSO₄·7H₂O. The anhydrous compound, MgSO₄, has a molar mass of 120.36 g/mol and a density of 2.66 g/cm³; HSDB gives the heptahydrate a density of 1.67. Warmed, Epsom salt gives up its water in stages, by HSDB's account: four molecules at 70 to 80 °C, six by 120 °C and the last near 250 °C.

How does magnesium sulfate work as a laxative?

It works by holding water in the gut. HSDB explains that the salt is not absorbed from the intestinal tract, and so it keeps enough water inside the bowel to make an isotonic solution, one matching the strength of the body's own fluids. The US National Drug Code directory classes it as an osmotic laxative, and the World Health Organization's drug classification files it with the osmotically acting laxatives.

What is magnesium sulfate used for outside medicine?

Food, for one. US federal rules at 21 CFR 184.1443 affirm it as generally recognised as safe (GRAS) for direct addition to human food, a status published in the Federal Register in April 1985. In food it can act as a flavour enhancer, a nutrient supplement or a processing aid. Beyond food, it goes into dietary supplements, cosmetic lotions and ceramics, and serves as a catalyst carrier.

How does magnesium sulfate prevent seizures?

The National Cancer Institute's thesaurus describes magnesium as a doubly charged ion the body needs for nerve signalling, bone mineralisation and muscle contraction. On its account, magnesium blocks nerve-to-muscle transmission and cuts the acetylcholine, the chemical a motor nerve impulse releases where nerve meets muscle, which prevents convulsions. In HSDB's wording the exact mechanism is not fully known, although excess magnesium appears to reduce the acetylcholine released.