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CAS Number 1309-42-8
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Linear Formula Mg(OH)2
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Molecular Weight 58.32
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EC Number 215-170-3
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MDL number MFCD00011104
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PubChem Substance ID 329759774
Magnesium hydroxide is the inorganic compound with the chemical formula Mg(OH)2. It occurs in
nature as the mineral brucite. It is a white
solid with low solubility in water (Ksp = 5.61×10−12,[4] Magnesium hydroxide is a common component of antacids, such as milk of magnesia, as well as laxatives.
Detailed description
Uses
Precursor to MgO
Most Mg(OH)2 that is produced industrially, as well
as the small amount that is mined, is converted to fused magnesia (MgO).
Magnesia is valuable because it is both a poor electrical conductor and an
excellent thermal conductor.[5]
Health
Metabolism
When the patient drinks magnesium hydroxide, the suspension enters the stomach. Depending on how
much was taken, one of two possible outcomes will occur.
As an antacid, magnesium hydroxide is dosed at
approximately 0.5–1.5 g in adults and works by simple neutralization, where the hydroxide ions from the Mg(OH)2 combine with acidic H+ ions produced in the form
of hydrochloric acid by parietal cells in the stomach to produce water.
As a laxative, magnesium hydroxide is dosed at 2–5
g, and works in a number of ways. First, Mg2+ is poorly absorbed from the intestinal
tract, so it draws water from the surrounding tissue by osmosis. Not only does this
increase in water content soften the feces, it also increases the volume of
feces in the intestine (intraluminal volume) which naturally stimulates
intestinal motility. Furthermore, Mg2+ ions cause the release of cholecystokinin (CCK), which results
in intraluminal accumulation of water, electrolytes, and increased intestinal
motility. Although it has been stated in some sources, the hydroxide ions
themselves do not play a significant role in the laxative effects of milk of
magnesia, as basic solutions (i.e., solutions of hydroxide ions) are not
strongly laxative, and non-basic Mg2+ solutions, like MgSO4, are equally strong
laxatives mole for mole.[6]
Only a small amount of the magnesium from magnesium
hydroxide is usually absorbed by the intestine (unless they are deficient in
magnesium). However, magnesium is mainly excreted by the kidneys so long-term,
daily consumption of milk of magnesia by someone suffering from renal failure
could lead in theory to hypermagnesemia.
History of Milk of Magnesia
On May 4, 1818, an American inventor named John
Callen, received a patent (No. X2952) for magnesium hydroxide.[7] In 1829, Sir James Murray used a
"condensed solution of fluid magnesia" preparation of his own design[8] to treat the Lord Lieutenant of
Ireland, the Marquis of Anglesey, of stomach pain. This was
so successful (advertised in Australia and approved by the Royal College of
Surgeons in 1838)[9] that he was
appointed resident physician to Anglesey and two subsequent Lords Lieutenants,
and knighted. His fluid magnesia product was patented two years after his death
in 1873.[10]
The term milk
of magnesia was first used by Charles Henry Phillips in 1872 for a suspension of magnesium
hydroxide formulated at about 8%w/v.[11] It was sold under
the brand name Phillips' Milk
of Magnesia for medicinal
usage.
Although the name may at some point have been owned
by GlaxoSmithKline, USPTO registrations show
"Milk of Magnesia"[12] and "Phillips'
Milk of Magnesia"[13] have both been
assigned to Bayer since 1995. In the
UK, the non-brand (generic) name of "Milk of Magnesia" and
"Phillips' Milk of Magnesia" is "Cream of Magnesia"
(Magnesium Hydroxide Mixture, BP).
Magnesium hydroxide is marketed for medical use as
chewable tablets, as capsules, and as liquid suspensions, sometimes having
various flavors added. These products are sold as antacids to neutralize
stomach acid and relieve indigestion and heartburn. It also is a
laxative to alleviate constipation. As a laxative, the osmotic force of the
magnesia acts to draw fluids from the body. High doses can lead to diarrhea, and can deplete
the body's supply of potassium, sometimes leading
to muscle cramps.[14]
Some magnesium hydroxide products sold for antacid
use (such as Maalox) are formulated to minimize unwanted laxative
effects through the inclusion of aluminum hydroxide,
which inhibits the contractions of smooth muscle cells in the
gastrointestinal tract,[15]thereby
counterbalancing the contractions induced by the osmotic effects of the
magnesium hydroxide.
Magnesium hydroxide is also a component of antiperspirant.[16] Magnesium hydroxide
is useful against canker sores (aphthous ulcer)
when used topically.[17]
Other niche uses
Waste water treatment
Magnesium hydroxide powder is used industrially as a
to neutralize acidic wastewaters.[18] It also takes part
in the Biorock method of building artificial reefs.
Fire retardant
Natural magnesium hydroxide (brucite) is used
commercially as a fire retardant.Most industrially used magnesium hydroxide is
produced synthetically.[19] Like aluminium
hydroxide, solid magnesium hydroxide has smoke suppressing and flame retardant properties. This
property is attributable to the endothermic decomposition it undergoes at
332 °C (630 °F):
Mg(OH)2 → MgO + H2O
The heat absorbed by the reaction retards the fire
by delaying ignition of the associated substance. The water released dilutes
combustible gases. Common uses of magnesium hydroxide as a flame retardant
include additives to cable insulation (i.e. cables for high quality cars, submarines, the Airbus A380, the Playstation 4, etc.), insulation
plastics, roofing (e.g. London Olympic Stadium), and various flame retardant coatings. Other mineral mixtures
that are used in similar fire retardant applications are natural mixtures of huntite and hydromagnesite.
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