Physical properties
Magnesium Hydroxide decomposes when heated to 360°C and forms
the oxide, MgO. It is very slightly soluble in water at
0.00122 g/100 ml and has a solubility product of
5.61×10-12. Crystals of Mg(OH)2 have a refractive
index of 1.559. Its heat of formation is –925 kJ/mol
with an entropy of 63 J/mol·K.
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”.
It can be prepared by the metathesis reaction
between any soluble magnesium salts and an alkali
hydroxide such as sodium or even ammonium:
Mg2+(aq)＋2OH-(aq)→Mg(OH)2(solid)
Magnesium hydroxide is a common component of
antacids and laxatives. It interferes with the absorption
of folic acid and iron. Its low solubility makes it a weak base.
Another method of commercial manufacture of
magnesium hydroxide involves the chloride obtained from seawater. After the chloride is recrystallized
several times to obtain a level of purity, it is calcined at
about 1000°C:
MgCl2·6H2O → MgCl2＋6H2O
→ Mg(OH)-xCl2-x＋xHCl
→ MgO＋(2－x)HCl
Occurrence
Magnesium Hydroxide occurs naturally as the mineral brucite. Brucite, usually found as a low temperature, hydrothermal vein mineral associated with calcite, aragonite, talc, or magnesite, appears as a decomposition product of magnesium silicates associated with serpentine, dolomite, magnesite, and chromite. Brucite also occurs as a hydrated form of periclase, and is found in serpentine, marble, chlorite schists, and in crystalline limestone.
Definition
ChEBI: A magnesium hydroxide in which the magnesium atom is bound to two hydroxide groups.
Brand name
Oxaine M (Wyeth-Ayerst); Phillips Magnesia
Tablets (Sterling Health U.S.A.); Phillips Milk of Magnesia
Liquid (Sterling Health U.S.A.).
Flammability and Explosibility
Notclassified
Pharmaceutical Applications
The synthesis of magnesium hydroxide [Mg(OH)2]
is based on a metathesis reaction in which magnesium salts are reacted with sodium or potassium hydroxide.
Magnesium hydroxide (Mg(OH)2) has only limited solubility
in water and the resulting suspension is called milk of magnesia, which is commonly used as an antacid and is
known to be a mild base.
Magnesium hydroxide [Mg(OH)2] is present in antacids because of its laxative properties and is also the
main ingredient of the ‘milk of magnesia’. The ‘milk of magnesia’ is a suspension of Mg(OH)2 in water,
which has a milk-like appearance because of the low aqueous solubility of Mg(OH)2. It is considered as a
strong electrolyte and a weak base and is given to the patient for indigestion and heartburn. The alkaline
suspension neutralises any excess stomach acid and therefore works as an antacid. It also stimulates intestinal
movement, as the magnesium ions increases the water content in the intestines through its osmotic effect and
as a result softens any faeces present.
Biochem/physiol Actions
Magnesium hydroxide is a flame retardant. Magnesium hydroxide is commonly used as an antacid which has a long-lasting effect. The liquid formulation of magnesium hydroxide is called milk of magnesia. It is used as a laxative. It is used in the treatment of diarrhea along with aluminium hydroxide. The naturally occurring form of magnesium hydroxide is brucite.
Veterinary Drugs and Treatments
Magnesium hydroxide in combination with aluminum salts have
been used in veterinary medicine for the adjunctive treatment of
esophagitis, gastric hyperacidity, peptic ulcer and gastritis. In foals
and small animals, because of difficulty in administration, the frequent
dosing that is often required, and availability of the histamine-
2 blocking agents (cimetidine, ranitidine, etc.), proton-pump
inhibitors (e.g., omeprazole) and sucralfate, antacids have largely
been relegated to adjunctive roles in therapy for these indications.
Magnesium hydroxide alone (milk of magnesia) is sometimes used
as an oral laxative in small animals.
In ruminants, magnesium hydroxide is used to increase rumen
pH and as a laxative in the treatment of rumen overload syndrome
(aka acute rumen engorgement, rumen acidosis, grain overload,
engorgement
toxemia, rumen impaction).
