Magnesium hydroxide
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2Xray.jpg) |
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| Names | |
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IUPAC name
Magnesium hydroxide
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| Other names
Milk of magnesia
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| Identifiers | |
1309-42-8 
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| 3D model (Jmol) | Interactive image |
| ChEBI |
CHEBI:6637
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| ChEMBL |
ChEMBL1200718 
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| ChemSpider |
14107
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| ECHA InfoCard | 100.013.792 |
| EC Number | 215-170-3 |
| E number | E528 (acidity regulators, ...) |
| 485572 | |
| PubChem | 14791 |
| RTECS number | OM3570000 |
| UNII |
NBZ3QY004S
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| Properties | |
| Mg(OH)2 | |
| Molar mass | 58.3197 g/mol |
| Appearance | White solid |
| Odor | odorless |
| Density | 2.3446 g/cm3 |
| Melting point | 350 °C (662 °F; 623 K) decomposes |
| 0.00064 g/100 mL (25 °C) 0.004 g/100 mL (100 °C) |
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Solubility product (Ksp)
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5.61×10−12 |
| −22.1·10−6 cm3/mol | |
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Refractive index (nD)
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1.559 |
| Structure | |
| Hexagonal, hP3 | |
| P3m1 No. 164 | |
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a = 0.312 nm, c = 0.473 nm
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| Thermochemistry | |
| 77.03 J/mol K | |
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Std molar
entropy (S |
64 J·mol−1·K−1 |
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Std enthalpy of
formation (ΔfH |
−924.7 kJ·mol−1 |
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Gibbs free energy (ΔfG˚)
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-833.7 kJ/mol |
| Pharmacology | |
| A02AA04 (WHO) G04BX01 (WHO) | |
| Hazards | |
| Safety data sheet | External MSDS |
| NFPA 704 | |
| Flash point | Non-flammable |
| Lethal dose or concentration (LD, LC): | |
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LD50 (median dose)
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8500 mg/kg (rat, oral) |
| Related compounds | |
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Other anions
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Magnesium oxide |
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Other cations
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Beryllium hydroxide Calcium hydroxide Strontium hydroxide Barium hydroxide |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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 (what is ?) |
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| Infobox references | |
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, Magnesium hydroxide is a common component of antacids, such as milk of magnesia, as well as laxatives.
Combining a solution of many magnesium salts with basic water induces precipitation of solid Mg(OH)2:
On a commercial scale, Mg(OH)2 is produced by treating seawater with lime (Ca(OH)2). 600 m3 of seawater gives about one ton of Mg(OH)2. Ca(OH)2 is far more soluble than Mg(OH)2, the latter precipitates as a solid:
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.
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.
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