Description of experiment
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Needed compounds:
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sulphuric acid : H2SO4
sodium nitrite : NaNO2
sodium sulfide : Na2S . 3H2O
Class:
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elem=N,S
redox
Summary:
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When sodium sulfide and sodium nitrite are mixed as solids, or when mixed in
aqueous solution, no reaction occurs. Sulfide ion and nitrite ion can coexist
in solution.
When a very small amount of acid is added, a bright yellow/ochre solid compound
is formed, which slowly turns lighter. When more acid is added, then the solid
becomes almost white.
The very light yellow, almost white compound most likely is very finely divided
elemental sulphur. The much brighter yellow/ochre definitely is not sulphur,
its color is too strong and also differs quite a lot from the kind of yellow of
sulphur.
Description:
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Take a tiny amount of solid sodium sulfide and add some solid sodium nitrite,
assuring that the nitrite is in large excess: When the two solids are mixed,
nothing happens.
Add some water: Both solids dissolve and the solution becomes colorless.
Nothing special seems to happen. It seems that nitrite and sulfide ion can
coexist in non-acidic solution.
Add a single drop of dilute sulphuric acid: The liquid at once becomes turbid.
The precipitate has a bright yellow/ochre color, which differs a lot from the
color of elemental sulphur, which is lighter yellow and also is purely yellow
with no brown/orange hue.
On shaking, the color becomes somewhat lighter, more like the well-known light
yellow color of very finely divided sulphur.
Add more acid: Bubbles of gas are formed and the air above the liquid becomes
light brown. The gas is nitrogen monoxide, the decomposition product of nitrite
when it is strongly acidified. The very finely divided light yellow solid
clumps together and the liquid becomes almost white and somewhat larger yellow
particles float around in the liquid. These larger particles now most likely
are sulphur. They have the same color as elemental sulphur (flowers of sulphur.
Remark:
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The final reaction is that sulfide is oxidized to free sulphur by the nitrous
acid formed on acidification of the sulfide/nitrite solution, but the nature of
the intermediate bright yellow/ochre compound is unclear.