Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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nitric acid : HNO3
sodium hydroxide : NaOH
hydrochloric acid : HCl
hydrogen peroxide : H2O2
ammonium chloride : NH4Cl
ammonia : NH3
cobalt chloride : CoCl2.6H2O
cobalt sulfate : CoSO4.7H2O
Class:
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elem=Co
precipitation
coordination
Summary:
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Cobalt (II) salts give a blue/green precipitate when dilute ammonia is added.
This precpitate is fairly stable towards air. When more concentrated ammonia
is used, then a dark brown compound is formed, but this compound is formed
by contact with air. On addition of hydrogen peroxide also a very dark
brown compound is formed.
When the pH is increased strongly, then the blue precipitate is not stable
anymore, instead a dark bright blue compound is formed, which, however,
quickly turns pink.
When a large amount of ammonia is replaced by ammonium, then the blue
precipitate is not formed anymore, but a coordination complex is formed,
which is very easily oxidized by oxygen from the air.
Description:
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Sequence 1:
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Add a small amount of ammonia (appr. 5% by weight) to a solution of cobalt
chloride: Formation of a blue precipitate. The blue color is not a very pure
blue, it has a green/brown hue.
Add much more ammonia (5% by weight): The precipitate dissolves for the largest
part, but some solid remains. The color changes to dark brown and it becomes
darker after some time, with contact to air. Apparently some oxygen is taken
from the air and a dark cobalt (III) compound is formed.
Add some hydrochloric acid to the liquid with the blue/green precipiate: The
precipitate dissolves and the liquid becomes clear and light pink. Some smoke
can be observed above the liquid, due to formation of solid NH4Cl.
Sequence 2:
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Add a small amount of ammonia (appr. 5% by weight) to a solution of cobalt
sulfate: Formation of a blue precipitate. The blue color is not a very pure
blue, it has a green/brown hue. Exactly the same behaviour as with cobalt
chloride, hence the anion has no effect on the final result.
Add some solution of sodium hydroxide: The precipitate becomes darker and
much brighter. Now it has a very pure dark blue color. However, the color
quickly changes to fairly light pink, through grey. The final color of the
precipitate is light pink, with a brown hue.
The experiment is repeated with different amounts of hydroxide, relative to
the amount of the original liquid with the blue/green precipitate. In all
cases a change of color to bright dark blue is observed, but the speed at
which it changes to light pink becomes lower, when the concentration of
hydroxide becomes lower. In all cases, however, the final color is light
pink with a brown hue.
Let stand the liquid with the pink precipitate: The precipitate settles. At
the top of the layer with the precipitate a thin brown layer can be observed.
Below this thin layer, the precipitate still is pink with a brown hue.
Decant the clear colorless liquid above the precipitate and add some HNO3
(appr. 50% by weight): The precipitate dissolves, the liquid becomes clear
and dark red/brown. When the liquid is strongly diluted it becomes clear
and brown, with a pink hue.
Sequence 3:
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Add some ammonia (appr. 15% by weight) to a solution of cobalt sulfate:
Formation of a blue precipitate, which quickly redissolves again. The liquid
becomes fairly dark brown/pink and clear. Apparently at higher concentrations
of ammonia, cobalt (II) forms a soluble coordination complex. At the surface
of the liquid, the liquid becomes darker quickly. When the liquid is shaked,
it becomes dark brown.
Add a few drops of H2O2 (appr. 10% by weight): The liquid qickly becomes even
darker and it is very dark brown. Some gas is evolved, but only slowly.
Dilute in a large volume of water: The liquid becomes dark red/brown and
almost clear.
Add some hydrochloric acid (30% by weight): The liquid becomes completely
clear now, its final color is red/brown.
Sequence 4:
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Add some ammonia (appr. 5% by weight) to a solution of cobalt chloride and
ammonium chloride: Liquid remains pink, no precipitate is formed. At the
surface of the liquid and on the glass, a dark brown compound is formed.
Shake the liquid, in contact with air: The liquid becomes dark brown and
remains clear.
Add an excess amount of HCl (appr. 10% by weight): Liquid becomes pink;
formation of small bubbles of a gas in the liquid. This gas may be oxygen,
due to the cobalt reverting to the +2 oxidation state, releasing oxygen.