Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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potassium dichromate : K2Cr2O7
sulphuric acid : H2SO4
hydrogen peroxide : H2O2
ammonia : NH3
hydrochloric acid : HCl
cobalt carbonate : CoCO3
Class:
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elem=Co
coordination
redox
Summary:
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Cobalt (II) is stable in acidic environments, but in alkaline environments
in the presence of ammonia it reacts with oxygen from the air, resulting
in the formation of a dark red/brown coordination complex.
Description:
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Add some cobalt carbonate to an excess amount of HCl (appr. 10% by weight):
The solid partly dissolves. The liquid starts foaming (production of CO2
from the carbonate). A small amount of the solid does not dissolve. The
liquid turns pink.
Heat the liquid: The remaining solid now also dissolves. The liquid becomes
bright pink, however, with increasing temperature the color quickly shifts
from bright pink to intense dark blue. When the liquid cools down, it becomes
pink again. This change of color can be repeated and the steady state color
depends on the temperature.
Add the cooled down (and hence pink) liquid to an excess amount of NH3 (appr.
5% by weight): The liquid becomes turbid and brown. When the liquid is
shaken with air-contact, it becomes darker and darker brown.
Add two drops of H2O2 (6% by weight): Liquid turns even darker brown, no
evolution of oxygen.
Boil the liquid: A pungent smell of NH3 is produced (this is driven off the
liquid by the heat). The colour slowly shifts from dark brown to ruby red.
Let liquid cool down: The liquid remains ruby red and still has a fairly
pungent odour of ammonia. The intensity of the ruby red colour is much larger
than the intensity of the original acidic cobalt (II) solution. This liquid
is called RUBY_RED.
Add an excess amount of dilute H2SO4 (appr. 2 mol/l): The liquid becomes
brown/pink.
Add some K2CrO7 in order to check the presence of H2O2: No more H2O2 was
left anymore, the liquid does not turn blue/indigo.
When part of RUBY_RED is left in contact with air, then it slowly turns
brown again and the liquid becomes darker. When the liquid is shaken with
air, then it remains dark brown. When an excess amount of HCl (appr. 10%)
is added, then it becomes brown/pink and some white smoke is observed (due
to formation of NH4Cl from NH3-fumes and HCl-fumes).