Magnesium hydroxide
Template:Short description Template:Chembox
Magnesium hydroxide is an 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 (Template:Nowrap).<ref>Template:Cite book</ref> Magnesium hydroxide is a common component of antacids, such as milk of magnesia.
Preparation
Treating the solution of different soluble magnesium salts with alkaline water induces the precipitation of the solid hydroxide Mg(OH)2: Template:Block indent
As Template:Chem is the second most abundant cation present in seawater after Template:Chem, it can be economically extracted directly from seawater by alkalinisation as described above. On an industrial scale, Mg(OH)2 is produced by treating seawater with lime (Ca(OH)2). A volume of Template:Cvt of seawater gives about Template:Convert of Mg(OH)2. Ca(OH)2 Template:Nowrap)<ref name="CRC">Template:Cite book</ref> is far more soluble than Mg(OH)2 Template:Nowrap) and dramatically increases the pH value of seawater from 8.2 to 12.5. The less soluble Template:Chem precipitates because of the common ion effect due to the Template:Chem added by the dissolution of Template:Chem:<ref name="ullmann">Template:Ullmann</ref> Template:Block indent
For seawater brines, precipitating agents other than Template:Chem2 can be utilized, each with their own nuances:
- Use of Template:Chem2 can yield CaSO4 or CaCO3, which reduces the final purity of Template:Chem2.
- NH4OH can produce explosive nitrogen trichloride when the brine is used for chlorine production.
- NaOH as the precipitating agent has longer settling times and is difficult to filter.
It has been demonstrated that sodium hydroxide, Template:Chem2, is the better precipitating agent compared to Ca(OH)2 and Template:Chem2 due to higher recovery and purity rates, and the settling and filtration time can be improved at low temperatures and higher concentration of precipitates. Methods involving the use of precipitating agents are typically batch processes.<ref>Template:Cite journal</ref>
It is also possible to obtain Template:Chem2 from seawater using electrolysis chambers separated with a cation exchange membrane. This process is continuous, lower-cost, and produces oxygen gas, hydrogen gas, sulfuric acid (if Template:Chem2 is used; Template:Chem2 can alternatively be used to yield Template:Chem2), and Template:Chem2 of 98% or higher purity. It is crucial to deaerate the seawater to mitigate co-precipitation of calcium precipitates.<ref name="sano2018">Template:Cite journal</ref>
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.<ref name=ullmann/>
Medical
Only a small amount of the magnesium from magnesium hydroxide is usually absorbed by the intestine (unless one 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 kidney failure could lead in theory to hypermagnesemia. Unabsorbed magnesium is excreted in feces; absorbed magnesium is rapidly excreted in urine.<ref>Template:Cite web</ref>
Applications
Antacid
As an antacid, magnesium hydroxide is dosed at approximately 0.5–1.5Template:Nbspg in adults and works by simple neutralization, in which the hydroxide ions from the Mg(OH)2 combine with acidic H+ ions (or hydronium ions) produced in the form of hydrochloric acid by parietal cells in the stomach, to produce water.
Laxative
As a laxative, magnesium hydroxide is dosed at Template:Convert, 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 to 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 and electrolytes, and increased intestinal motility. Some sources claim that the hydroxide ions themselves do not play a significant role in the laxative effects of milk of magnesia, as alkaline solutions (i.e., solutions of hydroxide ions) are not strongly laxative, and non-alkaline Mg2+ solutions, like MgSO4, are equally strong laxatives, mole for mole.<ref>Template:Cite journal</ref>
History of milk of magnesia
On May 4, 1818, American inventor Koen Burrows received a patent (No. X2952) for magnesium hydroxide.<ref>Template:US patent</ref> In 1829, Sir James Murray used a "condensed solution of fluid magnesia" preparation of his own design<ref>Michael Hordern. A World Elsewhere (1993), p. 2.</ref> to treat the Lord Lieutenant of Ireland, the Marquess of Anglesey, for stomach pain. This was so successful (advertised in Australia and approved by the Royal College of Surgeons in 1838)<ref>Template:Cite news</ref> that he was appointed resident physician to Anglesey and two subsequent Lords Lieutenant, and knighted. His fluid magnesia product was patented two years after his death, in 1873.<ref>Ulster History. Sir James Murray – Inventor of Milk of Magnesia. 1788 to 1871 Template:Webarchive, 24 February 2005</ref>
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.<ref>When was Phillips' Milk of Magnesia introduced? Template:Webarchive FAQ, phillipsrelief.com, accessed 4 July 2016</ref> It was sold under the brand name Phillips' Milk of Magnesia for medicinal usage.
USPTO registrations show that the terms "Milk of Magnesia"<ref>results from the TARR web server: Milk of Magnesia</ref> and "Phillips' Milk of Magnesia"<ref>results from the TARR web server: Phillips' Milk of Magnesia</ref> 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).
As food additive
It is added directly to human food, and is affirmed as generally recognized as safe by the FDA.<ref>Template:Cite web</ref> It is known as E number E528.
Magnesium hydroxide is marketed for medical use in the form of chewable tablets, capsules, powder, and as liquid suspensions, which are sometimes flavored. These products are sold as antacids to neutralize stomach acid and relieve indigestion and heartburn.
It is also a laxative used 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.<ref>Magnesium Hydroxide – Revolution Health</ref> 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,<ref name="Washington1991">Template:Cite book</ref> thereby counterbalancing the contractions induced by the osmotic effects of the magnesium hydroxide.
Other niche uses
Magnesium hydroxide is also a component of antiperspirant.<ref>[needs citation]</ref>
Waste water treatment
Magnesium hydroxide powder is used industrially to neutralize acidic wastewaters.<ref>Aileen Gibson and Michael Maniocha. White Paper: The Use Of Magnesium Hydroxide Slurry For Biological Treatment Of Municipal and Industrial Wastewater, August 12, 2004</ref> It is also a component of the Biorock method of building artificial reefs. The main advantage of Template:Chem over Template:Chem, is to impose a lower pH better compatible with that of seawater and sea life: pH 10.5 for Template:Chem in place of pH 12.5 with Template:Chem.
Fire retardant
Natural magnesium hydroxide (brucite) is used commercially as a fire retardant. Most industrially used magnesium hydroxide is produced synthetically.<ref>Template:Cite book</ref> Like aluminum hydroxide, solid magnesium hydroxide has smoke suppressing and flame retardant properties. This property is attributable to the endothermic decomposition it undergoes at Template:Cvt:
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, insulation plastics, roofing, and various flame retardant coatings.<ref name="Rev1">Template:Cite journal</ref><ref name="Rev2">Template:Cite journal</ref><ref name="Fire1">Template:Cite journal</ref><ref name="Therm1">Template:Cite journal</ref><ref name="Fire2">Template:Cite journal</ref>
Mineralogy
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 usually 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 or shrink.
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.
Concrete degradation
Brucite may also crystallize in cement and concrete in contact with seawater. Indeed, the Template:Chem2 cation is the second-most-abundant cation in seawater, just behind Template:Chem2 and before Template:Chem2.
When cement or concrete are exposed to Template:Chem2 and Template:Chem2 ions simultaneously present in seawater, the precipitation of the poorly soluble brucite contributes to enhance the formation of gypsum in the sulfate attack :
The precipitation of insoluble Template:Chem2 helps to considerably drive the chemical equilibrium of the reaction to the right. It exacerbates the sulfate attack resulting in the formation of gypsum and ettringite (an expansive phase) responsible for the mechanical stress in the hardened cement paste. However, brucite, a phase with a small molar volume (Template:Nowrap),<ref name="Molar_Volume">Template:Cite web</ref> may contribute to clogging the porous network in the hardened cement paste, hindering the diffusion of these harmful reactive species in the cement matrix. This can delay the decalcification of the C-S-H phase (the "glue" phase in the hardened cement paste responsible for the cohesion in concrete) and its transformation into an M-S-H phase.
Prolonged contact between seawater or Mg-rich brines and concrete may induce durability issues for regularly immersed concrete components or structures.
The exact mechanism of brucite degradation of hardened cement paste remains a matter of debate.<ref name="Lee2002">Template:Cite journal</ref> If brucite had a high molar volume, it could be de facto considered a swelling phase (like ettringite, or highly hydrated minerals), but this does not appear to be the case. It is unclear if it causes expansion or not, and how. If it replaces another phase locally (topo chemical replacement), in cases where its molar volume is smaller than that of the phase it replaces, no expansion is expected; rather, a decrease in porosity is anticipated. However, if it crystallizes in a large number of tiny crystals growing between existing ones, even with a small molar volume, it could exert a considerable crystallization pressure in the cement matrix, resulting in tensile stress, expansion and cracking.
For the same reason, dolomite 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 brucite, a mineral phase with a low molar volume, but often accompanied by other expansive reaction products (with a higher molar volume than brucite compensating for its shrinkage).
This reaction, one of the two prominent alkali–aggregate reaction (AAR), is also known as alkali–carbonate reaction.
See also
- Portlandite – calcium hydroxide: Template:Chem
References
Template:Magnesium compounds Template:Hydroxides Template:Urologicals Template:Antacids Template:Laxatives