Magnesium hydroxide

Magnesium hydroxide
Magnesium hydroxide
CAS number 1309-42-8 YesY
PubChem 14791
ChemSpider 14107 YesY
RTECS number OM3570000
Jmol-3D images Image 1
Molecular formula Mg(OH)2
Molar mass 58.3197 g/mol
Appearance White solid
Density 2.3446 g/cm3
Melting point

350 °C (decomp.)

Solubility in water 0.012 g/L
Solubility product, Ksp 1.5×10−11
Refractive index (nD) 1.559[1]
Std enthalpy of
–925 kJ/mol
Standard molar
63 J K−1 mol−1
MSDS External MSDS
EU Index Not listed
NFPA 704
NFPA 704.svg
Flash point Non-flammable
Related compounds
Other anions Magnesium oxide
Other cations Beryllium hydroxide
Calcium hydroxide
Strontium hydroxide
Barium hydroxide
 N (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Magnesium hydroxide is an inorganic compound with the chemical formula Mg(OH)2. As a suspension in water, it is often called milk of magnesia because of its milk-like appearance. The solid mineral form of magnesium hydroxide is known as brucite.

Magnesium hydroxide is a common component of antacids and laxatives; it interferes with the absorption of folic acid and iron.[2] Magnesium hydroxide has low solubility in water, with a Ksp of 1.5×10−11; all of magnesium hydroxide that does dissolve does dissociate. Since the dissociation of this small amount of dissolved magnesium hydroxide is complete, magnesium hydroxide is considered a strong electrolyte.



In 1829, Sir James Murray used a fluid magnesia preparation of his own design to treat the Lord Lieutenant of Ireland, the Marquis of Anglesey. This was so successful (advertised in Australia and approved by the Royal College of Surgeons in 1838)[3] 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.[4]

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 1880 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" to be registered to Bayer,[5] and "Phillips' Milk of Magnesia" to Sterling Drug.[6] 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 can be precipitated by the metathesis reaction between magnesium salts and sodium, potassium, or ammonium hydroxide:

Mg2+ (aq) + 2 OH (aq) → Mg(OH)2 (s)


Suspensions of magnesium hydroxide in water, often called Milk of Magnesia, are used as an antacid to neutralize stomach acid, and as a laxative. The diarrhoea caused by magnesium hydroxide carries away much of the body's supply of potassium, and failure to take extra potassium may lead to muscle cramps.[7] Magnesium hydroxide is also used as an antiperspirant underarm deodorant.[8] Milk of magnesia is useful against canker sores (aphthous ulcer) when used topically.[9]

Milk of magnesia is sold for medical use as chewable tablets, capsules, and as liquids having various added flavors. It is primarily used to alleviate constipation, but also to relieve indigestion and heartburn. When taken orally as a laxative, the osmotic force of the magnesia suspension acts to draw fluids from the body and to retain those already within the lumen of the intestine, serving to distend the bowel, thus stimulating nerves within the colon wall, inducing peristalsis and resulting in evacuation of colonic contents. It is also used as an antacid, though more modern formulations such as Maalox combine the antimotility effects of equal concentrations of aluminum hydroxide to avoid unwanted laxative effects.

Milk of magnesia is also used as a folk remedy, applied and massaged into the scalp a few minutes before washing, to relieve symptoms of seborrhea and dandruff. The mechanism for its effectiveness in this application, like the causes of seborrhea itself, are unknown. An additional folk use is for the treatment of acne or oily skin by applying topically, allowing to dry, and then washing it off the face (or other body part). It is also said to be used for seborrheic dermatitis, which is a drying and flaking of the skin similar to dandruff but often occurring on the face.[10]

Magnesium hydroxide powder is used industrially as a non-hazardous alkali to neutralise acidic wastewaters.[11] It also takes part in the Biorock method of building artificial reefs.

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 → MgO + H2O

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. Another mineral mixture that is used in similar fire retardant applications are natural mixtures of huntite and hydromagnesite.[12][13]

Biological metabolism

When the patient drinks the milk of magnesia, the suspension enters the stomach. Depending on how much was taken, one of two possible outcomes will occur.

As an antacid, milk of magnesia is dosed at approximately 0.5–1.5g 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, milk of magnesia 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.[14]

Only a small amount of the magnesium from milk of magnesia is usually absorbed from a person's intestine (unless the person is 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.

As with any other medication, some people may have adverse reactions to milk of magnesia. These can include weakness, nausea, vomiting, and diarrhea. High doses increase the likelihood of these reactions. Patients with severe chronic kidney disease are advised to avoid overconsumption of milk of magnesia. Because the kidney functions to excrete magnesium, taking too much would wear out the kidney and lead to toxic levels of magnesium in the blood. Healthy individuals should not use this type of medication continuously for longer than one week, or an excessively harsh laxative effect may result. Any use should be done in consultation with a doctor.


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 more 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

This reaction, one of the two main alkali-aggregate reaction (AAR) is also known as alkali-carbonate reaction.


  1. ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0070494398
  2. ^ "Magnesium Hydroxide". University of Michigan. 09-01-2007. 
  3. ^ Sir James Murray's condensed solution of fluid magnesia, The Sydney Morning Herald – October 7, 1846
  4. ^ Ulster History. Sir James Murray – Inventor of Milk of Magnesia. 1788 to 1871, 24 February 2005
  5. ^ results from the TARR web server: Milk of Magnesia
  6. ^ results from the TARR web server: Phillips' Milk of Magnesia
  7. ^ Magnesium Hydroxide – Revolution Health
  8. ^ Milk of Magnesia Makes Good Antiperspirant
  9. ^ Canker sores, 2/1/2009
  10. ^ Try milk of magnesia for pesky acne
  11. ^ Aileen Gibson and Michael Maniocha White Paper: The Use Of Magnesium Hydroxide Slurry For Biological Treatment Of Municipal And Industrial Wastewater, August 12, 2004
  12. ^ Hollingbery, L.A., Hull, T.R. (2010). "The Fire Retardant Behaviour of Huntite and Hydromagnesite – A Review". Polymer Degradation and Stability 95 (12): 2213–2225. doi:10.1016/j.polymdegradstab.2010.08.019. 
  13. ^ Hollingbery, L.A., Hull, T.R. (2010). "The Thermal Decomposition of Huntite and Hydromagnesite – A Review". Thermochimica Acta 509 (12): 1-11. doi:10.1016/j.polymdegradstab.2010.08.019. 
  14. ^ Tedesco FJ, DiPiro JT (1985). "Laxative use in constipation". Am. J. Gastroenterol 80 (4): 303–9. PMID 2984923. 

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Look at other dictionaries:

  • magnesium hydroxide — n. (Chem.) A slightly alkaline chemical substance, {Mg(OH)2}, the active principle in the antacid milk of magnesia, also used as a laxative. [WordNet 1.5] …   The Collaborative International Dictionary of English

  • magnesium hydroxide — n a slightly alkaline crystalline compound Mg(OH)2 that is obtained by hydration of magnesia or by precipitation (as from seawater by lime in the production of magnesium chloride) as a white nearly insoluble powder and is used chiefly in medicine …   Medical dictionary

  • magnesium hydroxide — noun a white crystalline powder used chiefly in medicines • Hypernyms: ↑hydroxide, ↑hydrated oxide * * * noun : a slightly alkaline crystalline compound Mg(OH)2 that occurs naturally as brucite, is obtained by hydration of magnesia or by… …   Useful english dictionary

  • magnesium hydroxide — a magnesium salt used as an osmotic laxative to treat constipation. It is also combined with aluminium hydroxide in antacid preparations …   The new mediacal dictionary

  • magnesium hydroxide — noun Date: circa 1909 a slightly alkaline crystalline compound Mg(OH)2 used especially as a laxative and gastric antacid …   New Collegiate Dictionary

  • magnesium hydroxide — Chem. a white, crystalline, slightly water soluble powder, Mg(OH)2, used chiefly in medicine as an antacid and as a laxative. [1905 10] * * * …   Universalium

  • magnesium hydroxide — noun The base Mg(OH) that occurs naturally as brucite; it is used as an antacid and in many industrial processes …   Wiktionary

  • magnesium hydroxide — magne′sium hydrox′ide n. chem. a white, crystalline, slightly water soluble powder, Mg(OH)2, used as an antacid and laxative • Etymology: 1905–10 …   From formal English to slang

  • Magnesium carbonate — Magnesium carbonate …   Wikipedia

  • Magnesium oxide — IU …   Wikipedia

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