Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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acetic acid : CH3COOH
sulphuric acid : H2SO4
sodium nitrite : NaNO2
sodium thiosulfate : Na2S2O3 . 5H2O
Class:
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elem=N,S
acid/base
redox
Summary:
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When thiosulfate ion and nitrite ion are brought together in solution, then
nothing happens. When the solution is acidified, then a remarkable reaction
occurs. The liquid then quickly turns dark brown and remains clear. Immediately
after that, a colorless gas is produced and the color becomes lighter. The
color goes from dark brown through brown/orange and yellow to lime green. The
liquid remains clear and lime green for a few minutes and then it slowly
becomes turbid.
Description:
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Add a solution of sodium thiosulfate to a solution of sodium nitrite: The two
liquids mix, no visible reaction occurs. The liquid remains colorless.
Add a few drops of dilute sulphuric acid: The liquid turns dark brown but it
remains clear. After a few seconds, evolution of a colorless gas sets in and
the color of the liquid fades. In the test tube, the gas mix above the liquid
becomes brown, so the colorless gas most likely is NO, it at least contains
some NO.
Wait for a while: The liquid changes color further. Its color goes from dark
brown through brown orange, orange/yellow to yellow. After that, the color
slowly changes further to a greenish yellow (pale limegreen). All the time, gas
is evolved, but not as quickly as in the start of the experiment. After a few
minutes, the liquid becomes turbid and finely divided sulphur starts to
precipitate.
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When the experiment is repeated with 80% acetic acid instead of dilute
sulphuric acid, then the same reactions occur, but the final coloration to
limegreen does not happen. The color sticks at yellow. When to this yellow
liquid a drop of dilute sulphuric acid is added, then it quickly becomes
limegreen and after a while it becomes turbid.
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What is this brown compound? And the yellow and green compound? It must be some
(little known) sulphur/nitrogen/oxygen compound.
The changes of color are not very critical. The experiment was performed with
analytical grade lab reagent chemicals but also with chemicals from a raw
photography supplier. Both experiments showed the same result.