Magnesium Nitride, Mg₃N₂

When magnesium is heated in air it begins to combine with oxygen at 600° C., and a volume of air can be deprived of its oxygen by keeping the temperature well below 670° C. At the latter temperature the metal combines with nitrogen to form magnesium nitride. Magnesium nitride is an amorphous powder, greenish grey or greenish yellow in colour, and corresponding to the formula Mg₃N₂. It is stable in dry air, but water converts it into ammonia and magnesium hydroxide -

Mg₃N₂+6H₂O = 2NH₄+3Mg(OH)₂.

Dilute acids decompose it similarly. It is said to dissociate completely at 1500° C., and to react when heated in carbon dioxide according to the equation -

Mg₃N₂+3CO₂ = 3MgO+3CO+N₂.

Carbon monoxide acts less readily and a much higher temperature is needed -

Mg₃N₂+3CO = 3MgO + N₂+3C.

When it is warmed with anhydrous nickel chloride, nickel nitride is formed, and other metallic chlorides react similarly.

It was first prepared by heating magnesia in nitrogen, and finally, much purer and more readily, by heating the metal in ammonia. In some methods of preparation the metal was heated in mass with a slight exposure of surface: the outer rind was oxidised and the inner core converted into nitride.

Borchers and Beck electrolysed with an anode liquid of magnesium oxide and alkali fluoride, and a cathode liquid of fused zinc. The liberated magnesium alloyed with the zinc and combined with the nitrogen of air which was blown into it.

Its heat of formation has been given as +119.7 Cal.