Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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acetone : CH3 CO CH3
sulphuric acid : H2SO4
potassium metabisulfite : K2S2O5
potassium dichromate : K2Cr2O7
Class:
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redox
Summary:
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When dichromate is reduced with metabisulfite in acidic environment, then
a chromium (III) compound is formed, which does not easily crystallize
from the liquid as chrome alum. The chromium (III) compound is green,
instead of violet.
Description:
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Prepare a very concentrated solution of potassium dichromate in dilute H2SO4
(appr. 2 mol/l) by heating the liquid to almost boiling and dissolving as
much K2Cr2O7 as possible: The liquid becomes intense red/orange.
In small portions solid K2S2O5 is added: Every time, when the solid touches
the liquid, it fizzles for a few seconds. Finally a dark green syrup remains
with a faint odour of SO2.
Drive off the remaining SO2 by heating and then let the syrup cool down: The
syrup becomes more viscous. No crystals appear. Even after 2 days no crystals
appear.
Carefully add some H2SO4 (96% by weight) (appr. 1 part of H2SO4 to 7 parts of
green syrup): The acid dissolves, the syrup remains viscous, the color remains
dark green and no crystals appear.
Add acetone to the green syrup (the idea behind this was to precipitate the
solid from the liquid). The acetone is added in a ratio of 2 volumes of
acetone to 1 volume of green syrup: The acetone does not mix with the green
syrup. The green syrup remains under the mobile acetone layer as a oil-like
layer. The acetone layer is affected by the green syrup. It becomes a little
turbid and it becomes light brown/yellow. The color of the syrup does not
change.
Add much water (approximately two times the volume of the acetone and green
syrup together): Now all liquids mix. The result is a dark green liquid,
which looks a little turbid. This liquid is not viscous anymore.
Add much more water (volume becomes 5-fold): The resulting liquid becomes
completely clear and fairly dark green.