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Chloro Complexes of Cobalt(II)


Crystalizing Dish

0.1M Cobalt Chloride

Concentrated Hydrochloric Acid (12M)

Rinse Bottle with DI water

Gloves and Goggles



Pour enough cobalt solution to cover the bottom of a crystallizing dish to a depth of about 3-5 mm.  Pour concentrated Hydrochloric acid into the Cobalt Chloride solution.  The Hydrochloric Acid turns  the Cobalt solution blue. After the whole solution turns blue, add distilled water.  Where the water touches the Cobalt Chloride solution, it turns back to a pink color. This procedure may be repeated.  



The reaction responsible for the color change is:

CoCl(H2O)5+(aq) + Cl- <---> CoCl2(H2O)2 (aq) + 3H2O
Octahedral (pink)
Tetrahedral (blue)


Evidence exists that the shift to tetrahedral coordination and the blue color occurs when the second chloride ion enters the coordination sphere.

In the totally aqueous system where the color change is induced by the addition of concentrated HCl, the relationship has been reported to be:

Octahedral (pink)
CoCl(H2O)5+   CoCl3(H2O)-




Waste Management:

The used solutions from this demonstration should be given to EH&S as Hazardous Waste (Toxic and Corrosive)