Equilibrium of Cobalt complexes

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Required Training	Required PPE
UC Lab Safety Fundamentals	Lab coat, safety glasses/goggles, nitrile gloves

Equipment	Chemicals
Large test tube	~10 mL 0.2 M CoCl₂· 6H₂O (MW: ~238)
DI water squirt bottle	Concentrated HCl in a drip top bottle
Hot water bath
Ice/salt bath

Procedure:

Putting drops of conc. HCl will change the pink solution to blue
Adding water changes the solution back to pink
This reaction can have several color changes if given a large enough test tube.
Heat will turn the solution blue, and the ice/salt bath will turn it pink.
It is possible to turn only half of the test tube a color by carefully immersing it in a hot/ice bath with a steady hand.

Discussion:

This demonstration operates on the following reaction:

Co(H₂O)₆²⁺_aq + 4 Cl^-_aq (+ heat) → CoCl₄^2-_aq + 3 H₂O_l

The blue form is anhydrous and tetrahedral coordinated, the pink form is hydrated and is octahedral coordinated. ΔH = ~42-54kJ/mol. Conc. HCl is not only a source of Cl^- but also has a dehydrating effect. This demonstration projects well on an overhead, with a petri dish.

Hazards:

Be careful when using conc. HCl and make sure to wear all PPE. If you get any solution on your skin, be sure to wash for 15 minutes.

SOP:

Corrosive – Hydrochloric Acid

Disposal (by Storeroom)

Return the demonstration to the storeroom, where the cobalt waste will be submitted to EH&S.

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