Ammonia forms the complex CuNH342+ with copper ions in alkaline solution but not in acidic solution. The reason for this is that :

Cu2+(aq)+4NH3⇌OH−CuNH342+Cu2+(aq)+4NH3⇌OH+Cu2++4NH4+  (No complex formation)] As NH3 combines with H+ of acid and changes to NH4+ which have no donor site.