Description of experiment
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Needed compounds:
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nitric acid : HNO3
sulphuric acid : H2SO4
potassium dichromate : K2Cr2O7
sodium nitrite : NaNO2
Class:
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elem=Cr
redox
Summary:
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Dichromates are not capable of oxidizing nitrite in neutral environments.
When some acid is added, then the oxidation proceeds, but a fairly low
pH is needed for a fast and complete reaction.
Description:
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Add an excess amount of NaNO2 to some solid K2Cr2O7 and add some water: Both
solids dissolve, the NaNO2 dissolves much easier. When both solids have
dissolved, then the liquid is orange and clear. Apparently there is no
reaction.
Add a small amount of dilute sulphuric acid (appr. 1 mol/l): The liquid
slowly changes color. It shifts from bright orange to brown/orange.
Add more dilute sulphuric acid (appr. 1 mol/l): The liquid becomes blueish
grey when viewed under white TL-light. The change of color is not
instantaneous, it takes a few tens of seconds in order to complete.
Besides the reduction of the dichromate, some NO/NO2 is formed as well,
the gas mixture above the liquid becomes pale brown.
Repeat the same experiment with nitric acid instead of sulphuric acid
(dilute nitric acid, concentration is approximately 2 mol/l): On addition
of the first small amount of dilute acid, the dichromate/nitrite mixture
slowly changes color to brown/orange, lateron somewhat greyish orange. Also
some gas is evolved slowly. On addition of more acid, the liquid slowly
turns purple and more of a colorless gas is produced. On contact with air,
the colorless gas becomes brown. The purple color, obtained here, is more
purple than a plain aqueous Cr (III) solution.