Group 2: Alkaline earth metals
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When aqueous ammonia is added to a cation in solution, e.g. magnesium, is it the hydroxide, magnesium hydroxide that is produced, or is it the amine, magnesium amine that is produced which is responsible for the white precipitate formed? Basically, what I generally want to know is what makes the precipitate form when aqueous ammonia is added to a cation?
230310
Corrie writes.........
As you know, ammonia solutions are alkaline and therefore they contain hydroxide ions:
NH3(aq) + H2O(l) <=> NH4+(aq) + OH-(aq)
However, this reaction is an equilibrium, so the concentration of hydroxide ions is lower than would be the case in a solution of sodium hydroxide of the same concentration.
Nevertheless, adding ammonia solution to a solution containing magnesium ions results in the formation of a precipitate of magnesium hydroxide. I'm not sure what you mean by 'magnesium ammine' - some metal ions do form complex ions containing ammonia as a ligand, in place of water molecules, but not magnesium to any significant extent here. For instance, adding ammonia solution to a solution of zinc ions first produces a precipitate of zinc hydroxide, which then dissolves on addition of excess ammonia due to the formation of the [Zn(NH3)4]2+ complex ion.
Interestingly, if an ammonium salt is added to a solution of a magnesium salt before adding ammonia, the presence of the ammonium ions suppresses the above ammonia equilibrium to such an extent that the concentration of hydroxide ions does not reach the value needed for the magnesium hydroxide to precipitate.
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updated: 29 April 2010
