E740: Equilibrium – Complex Ions – Metal + Ammonia Complexes

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E740: Equilibrium – Complex Ions – Metal + Ammonia ComplexesE740: Equilibrium – Complex Ions – Metal + Ammonia Complexes

Introduction

 Ammonia is added to solutions of cobalt, nickel, and copper 2+ cations.  First formation of the hydroxide precipitate is seen, then of the ammonia complex.

Co2+ + NH4OH   →  Co(OH)2(s)	blue ppt. (gives green mixture)
Cu2+ + NH4OH   →  Cu(OH)2(s)	blue ppt.
Ni2+ + NH4OH   →   Ni(OH)2(s)	green gel
Co(OH)2(s) + 6NH3   →   Co(NH3)62+(aq)	brown complex
Cu(OH)2(s) + 4NH3   →   Cu(NH3)42+(aq)	dark blue complex
Ni(OH)2(s) + 6NH3   →   Ni(NH3)62+(aq)	blue complex

To Conduct Demonstration

1. Pour 0.1M solutions of Cu2+, Co2+, and Ni2+ into pairs of labeled beakers (to a depth of 1/2") on an overhead projector or document projector, or into 200mm test tubes (to a depth of 1") in a lighted rack.
2. Add 3M or 6M NH4OH drop wise while stirring with the stir rod to one beaker or test tube of each pair.  The solutions will become cloudy and change color due to the precipitation of metal hydroxides.
3. Add 15M NH4OH with stirring to the same beaker or test tube.  The precipitates will dissolve and darkly colored ammonia complexes will form.  (Cobalt requires 6 times the cation volume to form the complex.)  Use the other beakers or test tubes as references.  Use watch glasses over the beakers or crystallizing dishes to contain the ammonia fumes.

Safety 

Ammonia is caustic and irritating to skin, eyes, and the respiratory system.

Acknowledgement:

Stewart Strickler, Spring 1987; Margaret Asirvatham, Fall 1990.

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