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Chloro Complexes of Cobalt(II) (part 2)


Large Test Tube (at least 25 x 200)

Solution of 0.1M Cobalt Chloride

Concentrated Hydrochloric Acid (12M)

Boiling Chips

Bunsen Burner and Striker

2 Pronged Clamp (to hold test tube)

Ice and Water Bath

Gloves and Goggles



Before class starts, add 12M HCl to cobalt solution until it just turns blue. Add just enough water until the cobalt solution becomes pink again then pour the solution into a big fat test tube, and add a boiling chip.

For the demonstration itself, heat the tube with a torch or burner near the top of the tube. Heating at the bottom makes the solution jump out of the tube. When the solution starts boiling, it will turn blue. Submerge the bottom half of the tube in an ice water solution, this causes the bottom half to turn pink, while the top half remains blue.



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 labeled for pickup by EH&S (Toxic, Corrosive).