Milk of magnesia
|3D model (Jmol)||Interactive image|
|E number||E528 (acidity regulators, ...)|
|Molar mass||58.3197 g/mol|
|Melting point||350 °C (662 °F; 623 K) decomposes|
|0.00064 g/100 mL (25 °C)
0.004 g/100 mL (100 °C)
Solubility product (Ksp)
Refractive index (nD)
|P3m1 No. 164|
a = 0.312 nm, c = 0.473 nm
|77.03 J/mol K|
Std enthalpy of
Gibbs free energy (ΔfG˚)
|A02AA04 (WHO) G04BX01 (WHO)|
|Safety data sheet||External MSDS|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|8500 mg/kg (rat, oral)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Magnesium hydroxide is an inorganic compound with the chemical formula of hydrated Mg(OH)2. It is often known as milk of magnesia, because of its milk-like appearance as a suspension. While magnesium hydroxide has a low solubility in water, with a Ksp of 5.61×10−12, it is large enough that it will partially dissolve to produce ions in the solution, forming the suspension. A relatively high concentration of magnesium or hydroxide ions would be required to revert the suspension to the solid precipitate by reversing the equilibrium.
In this suspended form, magnesium hydroxide is a common component of antacids and laxatives; it interferes with the absorption of folic acid and iron. The antacid properties come from the hydroxide ions which are responsible for neutralising the acid. The solid mineral form of magnesium hydroxide is known as brucite.
On May 4, 1818, an American inventor named John Callen, received a patent (No. X2952) for magnesium hydroxide.
In 1829, Sir James Murray used a "condensed solution of fluid magnesia" preparation of his own design 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) 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.
The term milk of magnesia was first used for a white-colored, aqueous, mildly alkaline suspension of magnesium hydroxide formulated at about 8%w/v by Charles Henry Phillips in 1872. and 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"  and "Phillips' Milk of Magnesia" 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 composed of magnesium ions and hydroxide ions and it will precipitate whenever the two are present together - i.e., combine in a metathesis reaction. Due to the availability of alkaline hydroxide salts, this is commonly between magnesium salts and sodium, potassium, or ammonium hydroxide, but this may happen in any basic environment where the hydroxide species is present. The ionic reaction is as follows:
- Mg2+ (aq) + 2 OH− (aq) → Mg(OH)2 (s)
Natural magnesium hydroxide exists in the form of brucite, which is used commercially as a fire retardant. However, most industrially used magnesium hydroxide is chemically produced from sea water or brine. Magnesium chloride in the sea water is reacted with lime or dolomitic lime to form a precipitated magnesium hydroxide.
Magnesium hydroxide is sold 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) or as laxatives to alleviate constipation. As a laxative, the osmotic force of the magnesia acts to draw fluids from the body into the lumen of the intestine and to retain those already within it, which distends the bowel; this distension stimulates nerves within the colon wall, inducing rhythmic contractions (peristalsis) and resulting in evacuation of colonic contents. Excessive doses can lead to diarrhea, and can deplete the body's supply of potassium, sometimes leading to muscle cramps.
Some magnesium hydroxide products sold for antacid use (such as Maalox) are formulated to minimize unwanted laxative effects through the inclusion of equal concentrations of aluminum hydroxide, which inhibits the contractions of smooth muscle cells in the gastrointestinal tract,  thereby counterbalancing the contractions induced by the osmotic effects of the magnesium hydroxide.
Solid magnesium hydroxide also has smoke suppressing and fire retarding properties. This is due to the endothermic decomposition it undergoes at 332 °C (630 °F):
- Mg(OH)2 (s) → MgO (s) + H2O (g)
The heat absorbed by the reaction acts as a retardant by delaying ignition of the associated substance. The water released dilutes any combustible gases and inhibits oxygen from aiding the combustion. Common uses of magnesium hydroxide as a fire retardant include plastics, roofing, and coatings. Other mineral mixtures that are used in similar fire retardant applications are natural mixtures of huntite and hydromagnesite.
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.
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.
Brucite, the mineral form of Mg(OH)2 commonly found in nature also occurs in the 1:2:1 clay minerals amongst others, in chlorite, in which it occupies the interlayer position normally filled by monovalent and divalent cations such as Na+, K+, Mg2+ and Ca2+. As a consequence, chlorite interlayers are cemented by brucite and cannot swell nor shrink anymore.
Brucite, in which some of the Mg2+ cations have been substituted by Al3+ cations, becomes positively charged and constitutes the main basis of layered double hydroxide (LDH). LDH minerals as hydrotalcite are powerful anion sorbents but are relatively rare in nature.
Brucite may also crystallise in cement and concrete in contact with seawater. Indeed, the Mg2+ cation is the second most abundant cation in seawater, just behind Na+ and before Ca2+. Because brucite is a swelling mineral, it causes a local volumetric expansion responsible for tensile stress in concrete. This leads to the formation of cracks and fissures in concrete, accelerating its degradation in seawater.
For the same reason, dolostone cannot be used as construction aggregate for making concrete. The reaction of magnesium carbonate with the free alkali hydroxides present in the cement porewater also leads to the formation of expansive brucite.
- MgCO3 + 2 NaOH → Mg(OH)2 + Na2CO3
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