The only cations present in a slightly acidic solution areFe3+,Zn2+ and Cu2+. The reagent, added in excess to this solution, that would identify and separate Fe3+ in one step is:
6M NH3
Fe3+dil.NH3−−−−−→excessFe(OH)3 ↓ pptZn2+dil.NH3−−−−−→excess[Zn(NH3)4]2+Cu2+dil.NH3−−−−−→excess[Cu(NH3)4]2+