Why Ca2+ does not precipitate in NH3, but precipitates in NaOH?

Question

In our chemistry laboratory experiment, it was observed that the reaction of Ca(NO3)2 and NaOH yielded a precipitate based on the net ionic equation:

Ca2+(aq) + 2OH-(aq) --> Ca(OH)2(s)

Now, it makes me wonder why it did not precipitate in NH3. NH3 in water yields NH4+ and OH- based on this chemical equation:

NH3 + H2O --> NH4+(aq) + OH-(aq)

In such occurrences in the other part of the laboratory work, Fe3+ precipitated to Fe(OH)3 just the same way as it precipitated when it was allowed to react with NaOH. Cu2+ did precipitate in NaOH, but not in NH3 because it forms the blue complex [Cu(NH3)4]2+.

I searched the internet to find if a similar formation of complex ion could be observed when Ca2+ reacts with NH3, resulting to its non-precipitation to Ca(OH)2, but I found none. Further, I read that only the transition metals form complex ions.

Could you help me with this? Do you know why Ca2+ did not precipitate in NH3?

Answer 1

Since ammonia solution is a weak base it does not produce the same hydroxide ion concentration as sodium hydroxide even in very concentrated solution. Calcium hydroxide will not precipitate until its solubility product is exceeded and the concentration of hydroxide ions does not reach a high enough level in ammonia.  

Answer 2

Mg2+ will ppt in NaOH (solid in Milk of Magnesia) but not Ca2+ - so I don't know how you get it to ppt. Therefore no supprise Ca(OH)2(s) doesn't form in NH3. NaOH is a strong base; NH3 is a weak base.