Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium bisulfite : NaHSO3
sulphuric acid : H2SO4
potassium dichromate : K2Cr2O7
sulphuric acid : H2SO4
chrome alum : KCr(SO4)2 . 12H2O
sodium nitrite : NaNO2
Class:
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elem=Cr
coordination
Summary:
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Chromium (III) builds a coordination complex with nitrite of a fairly
intense purple color.
Description:
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Experiment 1:
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Add an excess amount of sodium nitrite to a solution of chrome alum: The
greyish purple solution becomes green, while the sodium nitrite dissolves.
After a minute or so, however, the color changes from green to a fairly
intense purple. The solution remains clear.
Add a little amount of dilute sulphuric acid (appr. 1 mol/l): The color of
the liquid does not change. Some gas is evolved and the gas mixture above
the liquid becomes pale brown.
Heat the liquid: A fairly vigorous evolution of gas is observed. The gas
is colorless, but when it goes out of the test tube it becomes dark brown.
This shows that the gas mainly is NO. When the liquid is kept boiling for
a while, then the evolution of gas ceases and plain water vapour escapes
from the liquid. The liquid becomes bluish green (a fairly common color
for a chromium (III) solution).
Experiment 2:
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Create an acidic chromium (III) solution from potassium dichromate, sodium
bisulfite (slightly excess amount) and sulphuric acid and drive off all excess
SO2 by boiling and leave liquid stand for several weeks. The resulting liquid
is purplish blue/green.
Add some solid NaNO2 to the purplish blue/green solution: The solid dissolves
and the color of the gas mixture in the test tube becomes pale brown. The
liquid becomes dark purple.