Description of experiment
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Needed compounds:
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sodium dithionite : Na2S2O4 . 2H2O
ammonia : NH3
sodium hydroxide : NaOH
potassium hydroxide : KOH
potassium nitrite : KNO2
sodium nitrite : NaNO2
acetic acid : CH3COOH
sulphuric acid : H2SO4
cobalt carbonate : CoCO3
Class:
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elem=Co
coordination
redox
Summary:
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Cobalt carbonate does not dissolve completely in dilute mineral acids. With
some heating, however, it does dissolve completely.
With nitrite a yellow/orange complex is formed, which forms a precipitate
when treated with alkalies. The precipitate is resistant to treatment with
acid and bases.
Description:
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Add some CoCO3 to dilute sulphuric acid (appr. 1 mol/l): The solid dissolves,
while fizzling. Carbon dioxide gas is evolved. The liquid becomes red/brown
and turbid. Not all of the cobalt carbonate dissolves.
Add a little bit more of dilute sulphuric acid (1 mol/l): No visible changes.
The final amount of cobalt carbonate does not dissolve.
Boil gently for a while: Now the liquid becomes clear and bright pink/red.
Mix the liquid with some dilute acetic acid (appr. 5% by weight) and add some
solid sodium nitrite: The solid dissolves, a small amount of NO/NO2 is formed
(gas mixture above liquid becomes light brown), the liquid becomes clear and
yellow/orange/brown.
Add some solid potassium nitrite: This dissolves and a little more NO/NO2
escapes from the liquid. The liquid itself does not change visibly.
Add one scale of potassium hydroxide (the scale has a size of appr. 5 mm
diameter and a thickness of appr. 3 mm): The solid quickly dissolves. A
yellow/ochre solid precipitates from the liquid. A very weak odour of acetic
acid can be observed, but there also is another smell (NO/NO2?).
Let stand for a few days: A bright yellow solid has settled on the bottom and
the liquid above the precipitate is clear and light pink.
Decant the pink liquid and add a fairly large amount of water to the yellow
precipitate: The yellow compound does not dissolve. After again a day of
standing, a bright yellow compound has settled on the bottom and a colorless
clear liquid is above the precipitate.
Decant the water and add new water. Add a fairly large amount of solid NaOH:
The solid dissolves and the precipitate changes color from bright yellow to
dirty yellow/green. After a few days of standing, the precipitate has become
very dark brown, almost black.
Decant the liquid and add an excess amount of dilute sulphuric acid (appr.
2 mol/l): The color of the precipitate changes, it becomes much lighter,
its color becomes yellow/brown. It does not dissolve.
Decant the dilute sulphuric acid and add an excess amount of ammonia (appr.
5% by weight): No visible changes.
Add some solid sodium dithionite: The solid dissolves, no further visible
changes (the precipitate does not dissolve).
Wait for appr. 10 minutes: The precipitate did dissolve in the 10 minute
period. The liquid is clear and dark brown. Smell of ammonia, mixed with
an unpleasant smell, due to the addition of the sodium dithionite.