Magnesium Hydroxide, Mg(OH)₂

Magnesium Hydroxide, Mg(OH)₂, occurs naturally as brucite in colourless rhombohedra, similarly to calcite, though it sometimes has a bluish or greenish colour. A residue of 2MgO.3H₂O, resembling brucite, is obtained by prolonged treatment of the compact products of the action of magnesium chloride on magnesia with hot water.

Flattened, hexagonal, prismatic crystals of Mg(OH)₂ have been obtained by heating magnesium chloride with a large excess of potassium hydroxide in the presence of water to 210°-220° C., cooling and extracting the alkali. Their density is 2.36 at 15° C., and they dissolve easily in acids or in warm concentrated ammonium chloride solution.

Mg(HO)₂ precipitates when sodium or potassium hydroxide is added to a solution of a magnesium salt, or when alkali salts are electrolysed with a platinum cathode and a magnesium anode. After washing and drying at 100° C. it is a white tasteless powder. One litre of water dissolves 9 mgm. at 18° C.

Ammonium salts prevent the precipitation of magnesium hydroxide by ammonia by diminishing the dissociation of the ammonium hydroxide. The decreased concentration of hydroxyl ions decreases the formation of magnesium hydroxide below the precipitation point. Ammonium sulphate is more effective than ammonium chloride in preventing precipitation - though it is less completely dissociated. According to the equation

2MgSO₄ ⇔ Mg^••+[Mg(SO₄)₂]'',

magnesium sulphate gives fewer magnesium ions than the chloride. The presence of ammonium sulphate still further depresses the dissociation, and the formation of magnesium hydroxide is thus further kept below the precipitation point.

The moist hydroxide has an alkaline reaction, dissolves in acids, including carbonic acid, and decomposes into water and oxide at a dull red heat.