Subject: Reaction of the magnesium ion with ammonium hydroxide

We use the difference in reactions with 3 drops and then an 
excess of sodium hydroxide and ammonium hydroxide to show  
differences in the way certain metal ions react like Al, Zn, Mg.
With three drops of ammonium hydroxide all three ions 
precipitate.  With an excess of ammonium hydroxide Al and Mg are 
precipitated and Zn clears up.  We noticed that if nitric acid is 
used it will clear up all of those that precipitate.  If ammonium 
hydroxide is again added in excess the Al reprecipitates, the Zn 
first precipitates and then clears, but the "Mg never again forms 
a precipitate".  We would like to use this idea to have our 
students see the presence of both Zn and Mg in an unknown 
solution, but we are looking for and explanation why the Mg never 
reprecipitates.  Any help would be appreciated.

Re: Reaction of the magnesium ion with ammonium hydroxide