Magnesium Bromide, MgBr₂

Anhydrous MgBr₂ has been prepared by the action of bromine on a heated mixture of carbon and magnesium oxide, or on the heated metal, by melting the anhydrous chloride with ammonium bromide, and by heating a mixture of ammonium bromide and hydrated magnesium bromide. It is a white crystalline mass, resembling magnesium chloride, which is hygroscopic, easily oxidised in air, and not volatile. It melts at 695° C.

It dissolves freely in water with the evolution of heat, and crystallises, under ordinary conditions, from its aqueous solutions as MgBr₂.6H₂O. This hexahydrate occurs naturally in the upper layers of the Stassfurt deposits, and can be prepared by dissolving magnesium in hydrobromic acid and crystallising over sulphuric acid, or by the action of bromine on magnesium dust and water. The pure product from the decomposition of the dietherate melts at 164° C. MgBr₂.10H₂O crystallises at lower temperatures: it melts and loses water at 12° C.

The hexahydrate loses 2 molecules of water on heating, and at higher temperatures both hydrogen bromide and water are liberated. An atmosphere of hydrogen bromide inhibits the decomposition.

The heat of combination of MgBr₂ is 21.700 Cal., and its heat of solution is 43.300 Cal.

Magnesium bromide occurs in sea-water and in saline springs.

MgBr₂.3MgO.12H₂O has been prepared in small acicular crystals by dissolving calcined magnesia in magnesium bromide solution. It loses 6 molecules of water at 120° C., and is decomposed by water or alcohol.

MgBr(OH).MgBr₂.2(C₂H₅)₂O has been prepared as colourless hygroscopic crystals which are decomposed by water.

The following have been described: KBr.MgBr₂.6H₂O, NH₄Br.MgBr₂.6H₂O, MgBr₂.SnBr₄.10H₂O, 2MgBr₂.PbBr₂.16H₂O, MgBr₂.HgBr₂, MgBr₂.2HgBr₂, MgBr₂.2MnBr₂.12H₂O.