Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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potassium nitrite : KNO2
sodium nitrite : NaNO2
sulphuric acid : H2SO4
hydrochloric acid : HCl
hydrogen peroxide : H2O2
sodium bicarbonate : NaHCO3
ammonia : NH3
cobalt sulfate : CoSO4.7H2O
Class:
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elem=Co
coordination
Summary:
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In this experiment, some of Werner's experiments are repeated. Here it is
shown that cobalt forms beautifully colored carbonato complexes. The exact
complex formed, depends on the experimental conditions. It is remarkable
what kinds of reactions are shown by cobalt in its complexes and it is
really difficult to precisely determine what is happening and it even is
difficult already to get the same results without precisely specifying the
exact experimental conditions (e.g. using HCl instead of H2SO4 already
results in a different outcome).
Description:
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Sequence 1 (control sequence without carbonate):
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Add a solution of cobalt sulfate to a liquid, containing approximately 5%
NH3 and 2% H2O2: The liquid becomes turbid and dark yellow/brown.
Inside the liquid, dark brown/black particles are formed. There also is
some evolution of a gas (most likely this is O2 from decomposing H2O2).
Wait for a few hours: Liquid is very dark brown. On strong dilution the
liquid is yellow/brown.
Add a large excess amount of dilute HCl (appr. 10% by weight): Liquid remains
dark yellow/brown.
This sequence shows that without carbonate no deep red/purple complexes are
formed.
Sequence 2:
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Add a solution of cobalt sulfate to a solution of NaHCO3 in 5% NH3: Formation
of a purple/pink precipitate, that on shaking dissolves quickly again. The
liquid becomes clear and deep purple/red (like aqueous Co2+, but much
stronger). After some shaking, the liquid shifts from purple/red to brown/red.
This is due to oxidation of the cobalt (II) complex by oxygen from the air.
When the liquid is shaken for a longer time with air-contact, then it still is
clear and it has a deep red color, like red wine.
Add a few drops of H2O2 (10% by weight): The liquid becomes a little darker
for a short while, but on shaking it becomes deep purple/red (almost like a
solution of KMnO4, but a little more reddish).
Heat liquid, until it starts boiling: The boiling does not lead to changes
of the color of the liquid. It remains beautifully deep purple/red. Most
likely, this deep purple/red color is the color of [Co(NH3)4(CO3)2]- or
[Co(NH3)4(CO3)]+. This is the famous Werner experiment, with which the
chemistry of coordination complexes really started.
Add an excess amount of dilute sulphuric acid (appr. 2 mol/l): The liquid
starts foaming strongly (formation of CO2 from excess carbonate) and the
liquid becomes deep red with a purple tinge.
Add a solution of NaNO2: Evolution of some gas, gas mixture above liquid
becomes light brown. The liquid turns red/orange.
Add some KNO2 to the deep red acidic liquid with H2SO4: Same result as with
NaNO2, no precipitate is formed of K3Co(NO2)6. The red/orange compound might
be a really complex compound, containg NH3, [NO2]- and [CO3]2-.
Sequence 3 (as sequence 2, but now H2O2-oxidation instead of O2-oxidation):
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Add a solution of cobalt sulfate to a solution of NaHCO3 in 5% NH3, which
also contains approximately 2% H2O2 by weight: The liquid becomes turbid and
dark green. A little later, it becomes almost black and it looks as if it
becomes clear again. The color has shifted from green to blue. On strong
dilution of a little part of this black solution, it appears to be blue.
It is hard to decide, whether the liquid really is clear or that it is a
colloidal suspension.
Add an excess amount of dilute hydrochloric acid (10% by weight): Violent
foaming, due to formation of CO2. The liquid becomes beautifully deep purple.
The liquid obtained here, is more bluish, than the acidic liquid obtained
in sequence 2. This may be due to the presence of chloride, it may also be
due to the presence of hydrogen peroxide from the start of the experiment.
Remark:
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It is quite surprising that even in strongly acidic media the deep red/purple
carbonate complex is not destroyed. This shows how remarkably inert cobalt
complexes are for cobalt in its +3 oxidation state.